1 | =head1 NAME
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2 |
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3 | perldata - Perl data types
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4 |
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5 | =head1 DESCRIPTION
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6 |
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7 | =head2 Variable names
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8 | X<variable, name> X<variable name> X<data type> X<type>
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9 |
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10 | Perl has three built-in data types: scalars, arrays of scalars, and
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11 | associative arrays of scalars, known as "hashes". A scalar is a
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12 | single string (of any size, limited only by the available memory),
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13 | number, or a reference to something (which will be discussed
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14 | in L<perlref>). Normal arrays are ordered lists of scalars indexed
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15 | by number, starting with 0. Hashes are unordered collections of scalar
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16 | values indexed by their associated string key.
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17 |
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18 | Values are usually referred to by name, or through a named reference.
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19 | The first character of the name tells you to what sort of data
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20 | structure it refers. The rest of the name tells you the particular
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21 | value to which it refers. Usually this name is a single I<identifier>,
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22 | that is, a string beginning with a letter or underscore, and
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23 | containing letters, underscores, and digits. In some cases, it may
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24 | be a chain of identifiers, separated by C<::> (or by the slightly
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25 | archaic C<'>); all but the last are interpreted as names of packages,
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26 | to locate the namespace in which to look up the final identifier
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27 | (see L<perlmod/Packages> for details). It's possible to substitute
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28 | for a simple identifier, an expression that produces a reference
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29 | to the value at runtime. This is described in more detail below
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30 | and in L<perlref>.
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31 | X<identifier>
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32 |
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33 | Perl also has its own built-in variables whose names don't follow
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34 | these rules. They have strange names so they don't accidentally
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35 | collide with one of your normal variables. Strings that match
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36 | parenthesized parts of a regular expression are saved under names
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37 | containing only digits after the C<$> (see L<perlop> and L<perlre>).
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38 | In addition, several special variables that provide windows into
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39 | the inner working of Perl have names containing punctuation characters
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40 | and control characters. These are documented in L<perlvar>.
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41 | X<variable, built-in>
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42 |
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43 | Scalar values are always named with '$', even when referring to a
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44 | scalar that is part of an array or a hash. The '$' symbol works
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45 | semantically like the English word "the" in that it indicates a
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46 | single value is expected.
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47 | X<scalar>
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48 |
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49 | $days # the simple scalar value "days"
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50 | $days[28] # the 29th element of array @days
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51 | $days{'Feb'} # the 'Feb' value from hash %days
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52 | $#days # the last index of array @days
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53 |
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54 | Entire arrays (and slices of arrays and hashes) are denoted by '@',
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55 | which works much like the word "these" or "those" does in English,
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56 | in that it indicates multiple values are expected.
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57 | X<array>
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58 |
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59 | @days # ($days[0], $days[1],... $days[n])
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60 | @days[3,4,5] # same as ($days[3],$days[4],$days[5])
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61 | @days{'a','c'} # same as ($days{'a'},$days{'c'})
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62 |
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63 | Entire hashes are denoted by '%':
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64 | X<hash>
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65 |
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66 | %days # (key1, val1, key2, val2 ...)
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67 |
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68 | In addition, subroutines are named with an initial '&', though this
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69 | is optional when unambiguous, just as the word "do" is often redundant
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70 | in English. Symbol table entries can be named with an initial '*',
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71 | but you don't really care about that yet (if ever :-).
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72 |
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73 | Every variable type has its own namespace, as do several
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74 | non-variable identifiers. This means that you can, without fear
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75 | of conflict, use the same name for a scalar variable, an array, or
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76 | a hash--or, for that matter, for a filehandle, a directory handle, a
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77 | subroutine name, a format name, or a label. This means that $foo
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78 | and @foo are two different variables. It also means that C<$foo[1]>
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79 | is a part of @foo, not a part of $foo. This may seem a bit weird,
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80 | but that's okay, because it is weird.
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81 | X<namespace>
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82 |
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83 | Because variable references always start with '$', '@', or '%', the
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84 | "reserved" words aren't in fact reserved with respect to variable
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85 | names. They I<are> reserved with respect to labels and filehandles,
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86 | however, which don't have an initial special character. You can't
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87 | have a filehandle named "log", for instance. Hint: you could say
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88 | C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using
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89 | uppercase filehandles also improves readability and protects you
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90 | from conflict with future reserved words. Case I<is> significant--"FOO",
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91 | "Foo", and "foo" are all different names. Names that start with a
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92 | letter or underscore may also contain digits and underscores.
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93 | X<identifier, case sensitivity>
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94 | X<case>
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95 |
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96 | It is possible to replace such an alphanumeric name with an expression
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97 | that returns a reference to the appropriate type. For a description
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98 | of this, see L<perlref>.
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99 |
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100 | Names that start with a digit may contain only more digits. Names
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101 | that do not start with a letter, underscore, digit or a caret (i.e.
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102 | a control character) are limited to one character, e.g., C<$%> or
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103 | C<$$>. (Most of these one character names have a predefined
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104 | significance to Perl. For instance, C<$$> is the current process
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105 | id.)
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106 |
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107 | =head2 Context
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108 | X<context> X<scalar context> X<list context>
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109 |
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110 | The interpretation of operations and values in Perl sometimes depends
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111 | on the requirements of the context around the operation or value.
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112 | There are two major contexts: list and scalar. Certain operations
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113 | return list values in contexts wanting a list, and scalar values
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114 | otherwise. If this is true of an operation it will be mentioned in
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115 | the documentation for that operation. In other words, Perl overloads
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116 | certain operations based on whether the expected return value is
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117 | singular or plural. Some words in English work this way, like "fish"
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118 | and "sheep".
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119 |
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120 | In a reciprocal fashion, an operation provides either a scalar or a
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121 | list context to each of its arguments. For example, if you say
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122 |
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123 | int( <STDIN> )
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124 |
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125 | the integer operation provides scalar context for the <>
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126 | operator, which responds by reading one line from STDIN and passing it
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127 | back to the integer operation, which will then find the integer value
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128 | of that line and return that. If, on the other hand, you say
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129 |
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130 | sort( <STDIN> )
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131 |
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132 | then the sort operation provides list context for <>, which
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133 | will proceed to read every line available up to the end of file, and
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134 | pass that list of lines back to the sort routine, which will then
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135 | sort those lines and return them as a list to whatever the context
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136 | of the sort was.
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137 |
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138 | Assignment is a little bit special in that it uses its left argument
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139 | to determine the context for the right argument. Assignment to a
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140 | scalar evaluates the right-hand side in scalar context, while
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141 | assignment to an array or hash evaluates the righthand side in list
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142 | context. Assignment to a list (or slice, which is just a list
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143 | anyway) also evaluates the righthand side in list context.
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144 |
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145 | When you use the C<use warnings> pragma or Perl's B<-w> command-line
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146 | option, you may see warnings
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147 | about useless uses of constants or functions in "void context".
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148 | Void context just means the value has been discarded, such as a
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149 | statement containing only C<"fred";> or C<getpwuid(0);>. It still
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150 | counts as scalar context for functions that care whether or not
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151 | they're being called in list context.
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152 |
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153 | User-defined subroutines may choose to care whether they are being
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154 | called in a void, scalar, or list context. Most subroutines do not
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155 | need to bother, though. That's because both scalars and lists are
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156 | automatically interpolated into lists. See L<perlfunc/wantarray>
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157 | for how you would dynamically discern your function's calling
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158 | context.
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159 |
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160 | =head2 Scalar values
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161 | X<scalar> X<number> X<string> X<reference>
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162 |
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163 | All data in Perl is a scalar, an array of scalars, or a hash of
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164 | scalars. A scalar may contain one single value in any of three
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165 | different flavors: a number, a string, or a reference. In general,
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166 | conversion from one form to another is transparent. Although a
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167 | scalar may not directly hold multiple values, it may contain a
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168 | reference to an array or hash which in turn contains multiple values.
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169 |
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170 | Scalars aren't necessarily one thing or another. There's no place
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171 | to declare a scalar variable to be of type "string", type "number",
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172 | type "reference", or anything else. Because of the automatic
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173 | conversion of scalars, operations that return scalars don't need
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174 | to care (and in fact, cannot care) whether their caller is looking
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175 | for a string, a number, or a reference. Perl is a contextually
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176 | polymorphic language whose scalars can be strings, numbers, or
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177 | references (which includes objects). Although strings and numbers
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178 | are considered pretty much the same thing for nearly all purposes,
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179 | references are strongly-typed, uncastable pointers with builtin
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180 | reference-counting and destructor invocation.
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181 |
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182 | A scalar value is interpreted as TRUE in the Boolean sense if it is not
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183 | the null string or the number 0 (or its string equivalent, "0"). The
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184 | Boolean context is just a special kind of scalar context where no
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185 | conversion to a string or a number is ever performed.
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186 | X<boolean> X<bool> X<true> X<false> X<truth>
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187 |
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188 | There are actually two varieties of null strings (sometimes referred
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189 | to as "empty" strings), a defined one and an undefined one. The
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190 | defined version is just a string of length zero, such as C<"">.
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191 | The undefined version is the value that indicates that there is
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192 | no real value for something, such as when there was an error, or
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193 | at end of file, or when you refer to an uninitialized variable or
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194 | element of an array or hash. Although in early versions of Perl,
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195 | an undefined scalar could become defined when first used in a
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196 | place expecting a defined value, this no longer happens except for
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197 | rare cases of autovivification as explained in L<perlref>. You can
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198 | use the defined() operator to determine whether a scalar value is
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199 | defined (this has no meaning on arrays or hashes), and the undef()
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200 | operator to produce an undefined value.
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201 | X<defined> X<undefined> X<undef> X<null> X<string, null>
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202 |
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203 | To find out whether a given string is a valid non-zero number, it's
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204 | sometimes enough to test it against both numeric 0 and also lexical
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205 | "0" (although this will cause noises if warnings are on). That's
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206 | because strings that aren't numbers count as 0, just as they do in B<awk>:
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207 |
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208 | if ($str == 0 && $str ne "0") {
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209 | warn "That doesn't look like a number";
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210 | }
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211 |
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212 | That method may be best because otherwise you won't treat IEEE
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213 | notations like C<NaN> or C<Infinity> properly. At other times, you
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214 | might prefer to determine whether string data can be used numerically
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215 | by calling the POSIX::strtod() function or by inspecting your string
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216 | with a regular expression (as documented in L<perlre>).
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217 |
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218 | warn "has nondigits" if /\D/;
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219 | warn "not a natural number" unless /^\d+$/; # rejects -3
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220 | warn "not an integer" unless /^-?\d+$/; # rejects +3
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221 | warn "not an integer" unless /^[+-]?\d+$/;
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222 | warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
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223 | warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
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224 | warn "not a C float"
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225 | unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
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226 |
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227 | The length of an array is a scalar value. You may find the length
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228 | of array @days by evaluating C<$#days>, as in B<csh>. However, this
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229 | isn't the length of the array; it's the subscript of the last element,
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230 | which is a different value since there is ordinarily a 0th element.
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231 | Assigning to C<$#days> actually changes the length of the array.
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232 | Shortening an array this way destroys intervening values. Lengthening
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233 | an array that was previously shortened does not recover values
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234 | that were in those elements. (It used to do so in Perl 4, but we
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235 | had to break this to make sure destructors were called when expected.)
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236 | X<$#> X<array, length>
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237 |
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238 | You can also gain some minuscule measure of efficiency by pre-extending
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239 | an array that is going to get big. You can also extend an array
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240 | by assigning to an element that is off the end of the array. You
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241 | can truncate an array down to nothing by assigning the null list
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242 | () to it. The following are equivalent:
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243 |
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244 | @whatever = ();
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245 | $#whatever = -1;
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246 |
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247 | If you evaluate an array in scalar context, it returns the length
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248 | of the array. (Note that this is not true of lists, which return
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249 | the last value, like the C comma operator, nor of built-in functions,
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250 | which return whatever they feel like returning.) The following is
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251 | always true:
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252 | X<array, length>
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253 |
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254 | scalar(@whatever) == $#whatever - $[ + 1;
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255 |
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256 | Version 5 of Perl changed the semantics of C<$[>: files that don't set
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257 | the value of C<$[> no longer need to worry about whether another
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258 | file changed its value. (In other words, use of C<$[> is deprecated.)
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259 | So in general you can assume that
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260 | X<$[>
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261 |
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262 | scalar(@whatever) == $#whatever + 1;
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263 |
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264 | Some programmers choose to use an explicit conversion so as to
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265 | leave nothing to doubt:
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266 |
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267 | $element_count = scalar(@whatever);
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268 |
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269 | If you evaluate a hash in scalar context, it returns false if the
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270 | hash is empty. If there are any key/value pairs, it returns true;
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271 | more precisely, the value returned is a string consisting of the
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272 | number of used buckets and the number of allocated buckets, separated
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273 | by a slash. This is pretty much useful only to find out whether
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274 | Perl's internal hashing algorithm is performing poorly on your data
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275 | set. For example, you stick 10,000 things in a hash, but evaluating
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276 | %HASH in scalar context reveals C<"1/16">, which means only one out
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277 | of sixteen buckets has been touched, and presumably contains all
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278 | 10,000 of your items. This isn't supposed to happen.
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279 | X<hash, scalar context> X<hash, bucket> X<bucket>
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280 |
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281 | You can preallocate space for a hash by assigning to the keys() function.
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282 | This rounds up the allocated buckets to the next power of two:
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283 |
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284 | keys(%users) = 1000; # allocate 1024 buckets
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285 |
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286 | =head2 Scalar value constructors
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287 | X<scalar, literal> X<scalar, constant>
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288 |
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289 | Numeric literals are specified in any of the following floating point or
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290 | integer formats:
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291 |
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292 | 12345
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293 | 12345.67
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294 | .23E-10 # a very small number
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295 | 3.14_15_92 # a very important number
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296 | 4_294_967_296 # underscore for legibility
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297 | 0xff # hex
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298 | 0xdead_beef # more hex
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299 | 0377 # octal (only numbers, begins with 0)
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300 | 0b011011 # binary
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301 |
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302 | You are allowed to use underscores (underbars) in numeric literals
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303 | between digits for legibility. You could, for example, group binary
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304 | digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
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305 | or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
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306 | X<number, literal>
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307 |
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308 | String literals are usually delimited by either single or double
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309 | quotes. They work much like quotes in the standard Unix shells:
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310 | double-quoted string literals are subject to backslash and variable
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311 | substitution; single-quoted strings are not (except for C<\'> and
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312 | C<\\>). The usual C-style backslash rules apply for making
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313 | characters such as newline, tab, etc., as well as some more exotic
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314 | forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
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315 | X<string, literal>
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316 |
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317 | Hexadecimal, octal, or binary, representations in string literals
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318 | (e.g. '0xff') are not automatically converted to their integer
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319 | representation. The hex() and oct() functions make these conversions
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320 | for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
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321 |
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322 | You can also embed newlines directly in your strings, i.e., they can end
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323 | on a different line than they begin. This is nice, but if you forget
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324 | your trailing quote, the error will not be reported until Perl finds
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325 | another line containing the quote character, which may be much further
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326 | on in the script. Variable substitution inside strings is limited to
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327 | scalar variables, arrays, and array or hash slices. (In other words,
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328 | names beginning with $ or @, followed by an optional bracketed
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329 | expression as a subscript.) The following code segment prints out "The
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330 | price is $Z<>100."
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331 | X<interpolation>
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332 |
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333 | $Price = '$100'; # not interpolated
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334 | print "The price is $Price.\n"; # interpolated
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335 |
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336 | There is no double interpolation in Perl, so the C<$100> is left as is.
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337 |
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338 | As in some shells, you can enclose the variable name in braces to
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339 | disambiguate it from following alphanumerics (and underscores).
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340 | You must also do
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341 | this when interpolating a variable into a string to separate the
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342 | variable name from a following double-colon or an apostrophe, since
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343 | these would be otherwise treated as a package separator:
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344 | X<interpolation>
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345 |
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346 | $who = "Larry";
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347 | print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
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348 | print "We use ${who}speak when ${who}'s here.\n";
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349 |
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350 | Without the braces, Perl would have looked for a $whospeak, a
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351 | C<$who::0>, and a C<$who's> variable. The last two would be the
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352 | $0 and the $s variables in the (presumably) non-existent package
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353 | C<who>.
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354 |
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355 | In fact, an identifier within such curlies is forced to be a string,
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356 | as is any simple identifier within a hash subscript. Neither need
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357 | quoting. Our earlier example, C<$days{'Feb'}> can be written as
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358 | C<$days{Feb}> and the quotes will be assumed automatically. But
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359 | anything more complicated in the subscript will be interpreted as an
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360 | expression. This means for example that C<$version{2.0}++> is
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361 | equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
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362 |
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363 | =head3 Version Strings
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364 | X<version string> X<vstring> X<v-string>
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365 |
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366 | B<Note:> Version Strings (v-strings) have been deprecated. They will
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367 | not be available after Perl 5.8. The marginal benefits of v-strings
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368 | were greatly outweighed by the potential for Surprise and Confusion.
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369 |
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370 | A literal of the form C<v1.20.300.4000> is parsed as a string composed
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371 | of characters with the specified ordinals. This form, known as
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372 | v-strings, provides an alternative, more readable way to construct
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373 | strings, rather than use the somewhat less readable interpolation form
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374 | C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
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375 | Unicode strings, and for comparing version "numbers" using the string
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376 | comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
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377 | more dots in the literal, the leading C<v> may be omitted.
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378 |
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379 | print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
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380 | print v102.111.111; # prints "foo"
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381 | print 102.111.111; # same
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382 |
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383 | Such literals are accepted by both C<require> and C<use> for
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384 | doing a version check. The C<$^V> special variable also contains the
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385 | running Perl interpreter's version in this form. See L<perlvar/$^V>.
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386 | Note that using the v-strings for IPv4 addresses is not portable unless
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387 | you also use the inet_aton()/inet_ntoa() routines of the Socket package.
|
---|
388 |
|
---|
389 | Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
|
---|
390 | are not v-strings before the C<< => >> operator (which is usually used
|
---|
391 | to separate a hash key from a hash value), instead they are interpreted
|
---|
392 | as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
|
---|
393 | Perl 5.8.0, but that caused more confusion and breakage than good.
|
---|
394 | Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
|
---|
395 | be v-strings always.
|
---|
396 |
|
---|
397 | =head3 Special Literals
|
---|
398 | X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
|
---|
399 | X<end> X<data> X<^D> X<^Z>
|
---|
400 |
|
---|
401 | The special literals __FILE__, __LINE__, and __PACKAGE__
|
---|
402 | represent the current filename, line number, and package name at that
|
---|
403 | point in your program. They may be used only as separate tokens; they
|
---|
404 | will not be interpolated into strings. If there is no current package
|
---|
405 | (due to an empty C<package;> directive), __PACKAGE__ is the undefined
|
---|
406 | value.
|
---|
407 | X<__FILE__> X<__LINE__> X<__PACKAGE__> X<line> X<file> X<package>
|
---|
408 |
|
---|
409 | The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
|
---|
410 | may be used to indicate the logical end of the script before the actual
|
---|
411 | end of file. Any following text is ignored.
|
---|
412 |
|
---|
413 | Text after __DATA__ but may be read via the filehandle C<PACKNAME::DATA>,
|
---|
414 | where C<PACKNAME> is the package that was current when the __DATA__
|
---|
415 | token was encountered. The filehandle is left open pointing to the
|
---|
416 | contents after __DATA__. It is the program's responsibility to
|
---|
417 | C<close DATA> when it is done reading from it. For compatibility with
|
---|
418 | older scripts written before __DATA__ was introduced, __END__ behaves
|
---|
419 | like __DATA__ in the toplevel script (but not in files loaded with
|
---|
420 | C<require> or C<do>) and leaves the remaining contents of the
|
---|
421 | file accessible via C<main::DATA>.
|
---|
422 |
|
---|
423 | See L<SelfLoader> for more description of __DATA__, and
|
---|
424 | an example of its use. Note that you cannot read from the DATA
|
---|
425 | filehandle in a BEGIN block: the BEGIN block is executed as soon
|
---|
426 | as it is seen (during compilation), at which point the corresponding
|
---|
427 | __DATA__ (or __END__) token has not yet been seen.
|
---|
428 |
|
---|
429 | =head3 Barewords
|
---|
430 | X<bareword>
|
---|
431 |
|
---|
432 | A word that has no other interpretation in the grammar will
|
---|
433 | be treated as if it were a quoted string. These are known as
|
---|
434 | "barewords". As with filehandles and labels, a bareword that consists
|
---|
435 | entirely of lowercase letters risks conflict with future reserved
|
---|
436 | words, and if you use the C<use warnings> pragma or the B<-w> switch,
|
---|
437 | Perl will warn you about any
|
---|
438 | such words. Some people may wish to outlaw barewords entirely. If you
|
---|
439 | say
|
---|
440 |
|
---|
441 | use strict 'subs';
|
---|
442 |
|
---|
443 | then any bareword that would NOT be interpreted as a subroutine call
|
---|
444 | produces a compile-time error instead. The restriction lasts to the
|
---|
445 | end of the enclosing block. An inner block may countermand this
|
---|
446 | by saying C<no strict 'subs'>.
|
---|
447 |
|
---|
448 | =head3 Array Joining Delimiter
|
---|
449 | X<array, interpolation> X<interpolation, array> X<$">
|
---|
450 |
|
---|
451 | Arrays and slices are interpolated into double-quoted strings
|
---|
452 | by joining the elements with the delimiter specified in the C<$">
|
---|
453 | variable (C<$LIST_SEPARATOR> if "use English;" is specified),
|
---|
454 | space by default. The following are equivalent:
|
---|
455 |
|
---|
456 | $temp = join($", @ARGV);
|
---|
457 | system "echo $temp";
|
---|
458 |
|
---|
459 | system "echo @ARGV";
|
---|
460 |
|
---|
461 | Within search patterns (which also undergo double-quotish substitution)
|
---|
462 | there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
|
---|
463 | C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
|
---|
464 | expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
|
---|
465 | @foo)? If @foo doesn't otherwise exist, then it's obviously a
|
---|
466 | character class. If @foo exists, Perl takes a good guess about C<[bar]>,
|
---|
467 | and is almost always right. If it does guess wrong, or if you're just
|
---|
468 | plain paranoid, you can force the correct interpretation with curly
|
---|
469 | braces as above.
|
---|
470 |
|
---|
471 | If you're looking for the information on how to use here-documents,
|
---|
472 | which used to be here, that's been moved to
|
---|
473 | L<perlop/Quote and Quote-like Operators>.
|
---|
474 |
|
---|
475 | =head2 List value constructors
|
---|
476 | X<list>
|
---|
477 |
|
---|
478 | List values are denoted by separating individual values by commas
|
---|
479 | (and enclosing the list in parentheses where precedence requires it):
|
---|
480 |
|
---|
481 | (LIST)
|
---|
482 |
|
---|
483 | In a context not requiring a list value, the value of what appears
|
---|
484 | to be a list literal is simply the value of the final element, as
|
---|
485 | with the C comma operator. For example,
|
---|
486 |
|
---|
487 | @foo = ('cc', '-E', $bar);
|
---|
488 |
|
---|
489 | assigns the entire list value to array @foo, but
|
---|
490 |
|
---|
491 | $foo = ('cc', '-E', $bar);
|
---|
492 |
|
---|
493 | assigns the value of variable $bar to the scalar variable $foo.
|
---|
494 | Note that the value of an actual array in scalar context is the
|
---|
495 | length of the array; the following assigns the value 3 to $foo:
|
---|
496 |
|
---|
497 | @foo = ('cc', '-E', $bar);
|
---|
498 | $foo = @foo; # $foo gets 3
|
---|
499 |
|
---|
500 | You may have an optional comma before the closing parenthesis of a
|
---|
501 | list literal, so that you can say:
|
---|
502 |
|
---|
503 | @foo = (
|
---|
504 | 1,
|
---|
505 | 2,
|
---|
506 | 3,
|
---|
507 | );
|
---|
508 |
|
---|
509 | To use a here-document to assign an array, one line per element,
|
---|
510 | you might use an approach like this:
|
---|
511 |
|
---|
512 | @sauces = <<End_Lines =~ m/(\S.*\S)/g;
|
---|
513 | normal tomato
|
---|
514 | spicy tomato
|
---|
515 | green chile
|
---|
516 | pesto
|
---|
517 | white wine
|
---|
518 | End_Lines
|
---|
519 |
|
---|
520 | LISTs do automatic interpolation of sublists. That is, when a LIST is
|
---|
521 | evaluated, each element of the list is evaluated in list context, and
|
---|
522 | the resulting list value is interpolated into LIST just as if each
|
---|
523 | individual element were a member of LIST. Thus arrays and hashes lose their
|
---|
524 | identity in a LIST--the list
|
---|
525 |
|
---|
526 | (@foo,@bar,&SomeSub,%glarch)
|
---|
527 |
|
---|
528 | contains all the elements of @foo followed by all the elements of @bar,
|
---|
529 | followed by all the elements returned by the subroutine named SomeSub
|
---|
530 | called in list context, followed by the key/value pairs of %glarch.
|
---|
531 | To make a list reference that does I<NOT> interpolate, see L<perlref>.
|
---|
532 |
|
---|
533 | The null list is represented by (). Interpolating it in a list
|
---|
534 | has no effect. Thus ((),(),()) is equivalent to (). Similarly,
|
---|
535 | interpolating an array with no elements is the same as if no
|
---|
536 | array had been interpolated at that point.
|
---|
537 |
|
---|
538 | This interpolation combines with the facts that the opening
|
---|
539 | and closing parentheses are optional (except when necessary for
|
---|
540 | precedence) and lists may end with an optional comma to mean that
|
---|
541 | multiple commas within lists are legal syntax. The list C<1,,3> is a
|
---|
542 | concatenation of two lists, C<1,> and C<3>, the first of which ends
|
---|
543 | with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
|
---|
544 | similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
|
---|
545 | we'd advise you to use this obfuscation.
|
---|
546 |
|
---|
547 | A list value may also be subscripted like a normal array. You must
|
---|
548 | put the list in parentheses to avoid ambiguity. For example:
|
---|
549 |
|
---|
550 | # Stat returns list value.
|
---|
551 | $time = (stat($file))[8];
|
---|
552 |
|
---|
553 | # SYNTAX ERROR HERE.
|
---|
554 | $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
|
---|
555 |
|
---|
556 | # Find a hex digit.
|
---|
557 | $hexdigit = ('a','b','c','d','e','f')[$digit-10];
|
---|
558 |
|
---|
559 | # A "reverse comma operator".
|
---|
560 | return (pop(@foo),pop(@foo))[0];
|
---|
561 |
|
---|
562 | Lists may be assigned to only when each element of the list
|
---|
563 | is itself legal to assign to:
|
---|
564 |
|
---|
565 | ($a, $b, $c) = (1, 2, 3);
|
---|
566 |
|
---|
567 | ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
|
---|
568 |
|
---|
569 | An exception to this is that you may assign to C<undef> in a list.
|
---|
570 | This is useful for throwing away some of the return values of a
|
---|
571 | function:
|
---|
572 |
|
---|
573 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
|
---|
574 |
|
---|
575 | List assignment in scalar context returns the number of elements
|
---|
576 | produced by the expression on the right side of the assignment:
|
---|
577 |
|
---|
578 | $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
|
---|
579 | $x = (($foo,$bar) = f()); # set $x to f()'s return count
|
---|
580 |
|
---|
581 | This is handy when you want to do a list assignment in a Boolean
|
---|
582 | context, because most list functions return a null list when finished,
|
---|
583 | which when assigned produces a 0, which is interpreted as FALSE.
|
---|
584 |
|
---|
585 | It's also the source of a useful idiom for executing a function or
|
---|
586 | performing an operation in list context and then counting the number of
|
---|
587 | return values, by assigning to an empty list and then using that
|
---|
588 | assignment in scalar context. For example, this code:
|
---|
589 |
|
---|
590 | $count = () = $string =~ /\d+/g;
|
---|
591 |
|
---|
592 | will place into $count the number of digit groups found in $string.
|
---|
593 | This happens because the pattern match is in list context (since it
|
---|
594 | is being assigned to the empty list), and will therefore return a list
|
---|
595 | of all matching parts of the string. The list assignment in scalar
|
---|
596 | context will translate that into the number of elements (here, the
|
---|
597 | number of times the pattern matched) and assign that to $count. Note
|
---|
598 | that simply using
|
---|
599 |
|
---|
600 | $count = $string =~ /\d+/g;
|
---|
601 |
|
---|
602 | would not have worked, since a pattern match in scalar context will
|
---|
603 | only return true or false, rather than a count of matches.
|
---|
604 |
|
---|
605 | The final element of a list assignment may be an array or a hash:
|
---|
606 |
|
---|
607 | ($a, $b, @rest) = split;
|
---|
608 | my($a, $b, %rest) = @_;
|
---|
609 |
|
---|
610 | You can actually put an array or hash anywhere in the list, but the first one
|
---|
611 | in the list will soak up all the values, and anything after it will become
|
---|
612 | undefined. This may be useful in a my() or local().
|
---|
613 |
|
---|
614 | A hash can be initialized using a literal list holding pairs of
|
---|
615 | items to be interpreted as a key and a value:
|
---|
616 |
|
---|
617 | # same as map assignment above
|
---|
618 | %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
|
---|
619 |
|
---|
620 | While literal lists and named arrays are often interchangeable, that's
|
---|
621 | not the case for hashes. Just because you can subscript a list value like
|
---|
622 | a normal array does not mean that you can subscript a list value as a
|
---|
623 | hash. Likewise, hashes included as parts of other lists (including
|
---|
624 | parameters lists and return lists from functions) always flatten out into
|
---|
625 | key/value pairs. That's why it's good to use references sometimes.
|
---|
626 |
|
---|
627 | It is often more readable to use the C<< => >> operator between key/value
|
---|
628 | pairs. The C<< => >> operator is mostly just a more visually distinctive
|
---|
629 | synonym for a comma, but it also arranges for its left-hand operand to be
|
---|
630 | interpreted as a string -- if it's a bareword that would be a legal simple
|
---|
631 | identifier (C<< => >> doesn't quote compound identifiers, that contain
|
---|
632 | double colons). This makes it nice for initializing hashes:
|
---|
633 |
|
---|
634 | %map = (
|
---|
635 | red => 0x00f,
|
---|
636 | blue => 0x0f0,
|
---|
637 | green => 0xf00,
|
---|
638 | );
|
---|
639 |
|
---|
640 | or for initializing hash references to be used as records:
|
---|
641 |
|
---|
642 | $rec = {
|
---|
643 | witch => 'Mable the Merciless',
|
---|
644 | cat => 'Fluffy the Ferocious',
|
---|
645 | date => '10/31/1776',
|
---|
646 | };
|
---|
647 |
|
---|
648 | or for using call-by-named-parameter to complicated functions:
|
---|
649 |
|
---|
650 | $field = $query->radio_group(
|
---|
651 | name => 'group_name',
|
---|
652 | values => ['eenie','meenie','minie'],
|
---|
653 | default => 'meenie',
|
---|
654 | linebreak => 'true',
|
---|
655 | labels => \%labels
|
---|
656 | );
|
---|
657 |
|
---|
658 | Note that just because a hash is initialized in that order doesn't
|
---|
659 | mean that it comes out in that order. See L<perlfunc/sort> for examples
|
---|
660 | of how to arrange for an output ordering.
|
---|
661 |
|
---|
662 | =head2 Subscripts
|
---|
663 |
|
---|
664 | An array is subscripted by specifying a dollar sign (C<$>), then the
|
---|
665 | name of the array (without the leading C<@>), then the subscript inside
|
---|
666 | square brackets. For example:
|
---|
667 |
|
---|
668 | @myarray = (5, 50, 500, 5000);
|
---|
669 | print "Element Number 2 is", $myarray[2], "\n";
|
---|
670 |
|
---|
671 | The array indices start with 0. A negative subscript retrieves its
|
---|
672 | value from the end. In our example, C<$myarray[-1]> would have been
|
---|
673 | 5000, and C<$myarray[-2]> would have been 500.
|
---|
674 |
|
---|
675 | Hash subscripts are similar, only instead of square brackets curly brackets
|
---|
676 | are used. For example:
|
---|
677 |
|
---|
678 | %scientists =
|
---|
679 | (
|
---|
680 | "Newton" => "Isaac",
|
---|
681 | "Einstein" => "Albert",
|
---|
682 | "Darwin" => "Charles",
|
---|
683 | "Feynman" => "Richard",
|
---|
684 | );
|
---|
685 |
|
---|
686 | print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
|
---|
687 |
|
---|
688 | =head2 Slices
|
---|
689 | X<slice> X<array, slice> X<hash, slice>
|
---|
690 |
|
---|
691 | A common way to access an array or a hash is one scalar element at a
|
---|
692 | time. You can also subscript a list to get a single element from it.
|
---|
693 |
|
---|
694 | $whoami = $ENV{"USER"}; # one element from the hash
|
---|
695 | $parent = $ISA[0]; # one element from the array
|
---|
696 | $dir = (getpwnam("daemon"))[7]; # likewise, but with list
|
---|
697 |
|
---|
698 | A slice accesses several elements of a list, an array, or a hash
|
---|
699 | simultaneously using a list of subscripts. It's more convenient
|
---|
700 | than writing out the individual elements as a list of separate
|
---|
701 | scalar values.
|
---|
702 |
|
---|
703 | ($him, $her) = @folks[0,-1]; # array slice
|
---|
704 | @them = @folks[0 .. 3]; # array slice
|
---|
705 | ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
|
---|
706 | ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
|
---|
707 |
|
---|
708 | Since you can assign to a list of variables, you can also assign to
|
---|
709 | an array or hash slice.
|
---|
710 |
|
---|
711 | @days[3..5] = qw/Wed Thu Fri/;
|
---|
712 | @colors{'red','blue','green'}
|
---|
713 | = (0xff0000, 0x0000ff, 0x00ff00);
|
---|
714 | @folks[0, -1] = @folks[-1, 0];
|
---|
715 |
|
---|
716 | The previous assignments are exactly equivalent to
|
---|
717 |
|
---|
718 | ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
|
---|
719 | ($colors{'red'}, $colors{'blue'}, $colors{'green'})
|
---|
720 | = (0xff0000, 0x0000ff, 0x00ff00);
|
---|
721 | ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
|
---|
722 |
|
---|
723 | Since changing a slice changes the original array or hash that it's
|
---|
724 | slicing, a C<foreach> construct will alter some--or even all--of the
|
---|
725 | values of the array or hash.
|
---|
726 |
|
---|
727 | foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
|
---|
728 |
|
---|
729 | foreach (@hash{qw[key1 key2]}) {
|
---|
730 | s/^\s+//; # trim leading whitespace
|
---|
731 | s/\s+$//; # trim trailing whitespace
|
---|
732 | s/(\w+)/\u\L$1/g; # "titlecase" words
|
---|
733 | }
|
---|
734 |
|
---|
735 | A slice of an empty list is still an empty list. Thus:
|
---|
736 |
|
---|
737 | @a = ()[1,0]; # @a has no elements
|
---|
738 | @b = (@a)[0,1]; # @b has no elements
|
---|
739 | @c = (0,1)[2,3]; # @c has no elements
|
---|
740 |
|
---|
741 | But:
|
---|
742 |
|
---|
743 | @a = (1)[1,0]; # @a has two elements
|
---|
744 | @b = (1,undef)[1,0,2]; # @b has three elements
|
---|
745 |
|
---|
746 | This makes it easy to write loops that terminate when a null list
|
---|
747 | is returned:
|
---|
748 |
|
---|
749 | while ( ($home, $user) = (getpwent)[7,0]) {
|
---|
750 | printf "%-8s %s\n", $user, $home;
|
---|
751 | }
|
---|
752 |
|
---|
753 | As noted earlier in this document, the scalar sense of list assignment
|
---|
754 | is the number of elements on the right-hand side of the assignment.
|
---|
755 | The null list contains no elements, so when the password file is
|
---|
756 | exhausted, the result is 0, not 2.
|
---|
757 |
|
---|
758 | If you're confused about why you use an '@' there on a hash slice
|
---|
759 | instead of a '%', think of it like this. The type of bracket (square
|
---|
760 | or curly) governs whether it's an array or a hash being looked at.
|
---|
761 | On the other hand, the leading symbol ('$' or '@') on the array or
|
---|
762 | hash indicates whether you are getting back a singular value (a
|
---|
763 | scalar) or a plural one (a list).
|
---|
764 |
|
---|
765 | =head2 Typeglobs and Filehandles
|
---|
766 | X<typeglob> X<filehandle> X<*>
|
---|
767 |
|
---|
768 | Perl uses an internal type called a I<typeglob> to hold an entire
|
---|
769 | symbol table entry. The type prefix of a typeglob is a C<*>, because
|
---|
770 | it represents all types. This used to be the preferred way to
|
---|
771 | pass arrays and hashes by reference into a function, but now that
|
---|
772 | we have real references, this is seldom needed.
|
---|
773 |
|
---|
774 | The main use of typeglobs in modern Perl is create symbol table aliases.
|
---|
775 | This assignment:
|
---|
776 |
|
---|
777 | *this = *that;
|
---|
778 |
|
---|
779 | makes $this an alias for $that, @this an alias for @that, %this an alias
|
---|
780 | for %that, &this an alias for &that, etc. Much safer is to use a reference.
|
---|
781 | This:
|
---|
782 |
|
---|
783 | local *Here::blue = \$There::green;
|
---|
784 |
|
---|
785 | temporarily makes $Here::blue an alias for $There::green, but doesn't
|
---|
786 | make @Here::blue an alias for @There::green, or %Here::blue an alias for
|
---|
787 | %There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
|
---|
788 | of this. Strange though this may seem, this is the basis for the whole
|
---|
789 | module import/export system.
|
---|
790 |
|
---|
791 | Another use for typeglobs is to pass filehandles into a function or
|
---|
792 | to create new filehandles. If you need to use a typeglob to save away
|
---|
793 | a filehandle, do it this way:
|
---|
794 |
|
---|
795 | $fh = *STDOUT;
|
---|
796 |
|
---|
797 | or perhaps as a real reference, like this:
|
---|
798 |
|
---|
799 | $fh = \*STDOUT;
|
---|
800 |
|
---|
801 | See L<perlsub> for examples of using these as indirect filehandles
|
---|
802 | in functions.
|
---|
803 |
|
---|
804 | Typeglobs are also a way to create a local filehandle using the local()
|
---|
805 | operator. These last until their block is exited, but may be passed back.
|
---|
806 | For example:
|
---|
807 |
|
---|
808 | sub newopen {
|
---|
809 | my $path = shift;
|
---|
810 | local *FH; # not my!
|
---|
811 | open (FH, $path) or return undef;
|
---|
812 | return *FH;
|
---|
813 | }
|
---|
814 | $fh = newopen('/etc/passwd');
|
---|
815 |
|
---|
816 | Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
|
---|
817 | for filehandle manipulations, although they're still needed to pass brand
|
---|
818 | new file and directory handles into or out of functions. That's because
|
---|
819 | C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
|
---|
820 | In other words, C<*FH> must be used to create new symbol table entries;
|
---|
821 | C<*foo{THING}> cannot. When in doubt, use C<*FH>.
|
---|
822 |
|
---|
823 | All functions that are capable of creating filehandles (open(),
|
---|
824 | opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
|
---|
825 | automatically create an anonymous filehandle if the handle passed to
|
---|
826 | them is an uninitialized scalar variable. This allows the constructs
|
---|
827 | such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
|
---|
828 | create filehandles that will conveniently be closed automatically when
|
---|
829 | the scope ends, provided there are no other references to them. This
|
---|
830 | largely eliminates the need for typeglobs when opening filehandles
|
---|
831 | that must be passed around, as in the following example:
|
---|
832 |
|
---|
833 | sub myopen {
|
---|
834 | open my $fh, "@_"
|
---|
835 | or die "Can't open '@_': $!";
|
---|
836 | return $fh;
|
---|
837 | }
|
---|
838 |
|
---|
839 | {
|
---|
840 | my $f = myopen("</etc/motd");
|
---|
841 | print <$f>;
|
---|
842 | # $f implicitly closed here
|
---|
843 | }
|
---|
844 |
|
---|
845 | Note that if an initialized scalar variable is used instead the
|
---|
846 | result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
|
---|
847 | to C<open( *{'zzz'}, ...)>.
|
---|
848 | C<use strict 'refs'> forbids such practice.
|
---|
849 |
|
---|
850 | Another way to create anonymous filehandles is with the Symbol
|
---|
851 | module or with the IO::Handle module and its ilk. These modules
|
---|
852 | have the advantage of not hiding different types of the same name
|
---|
853 | during the local(). See the bottom of L<perlfunc/open()> for an
|
---|
854 | example.
|
---|
855 |
|
---|
856 | =head1 SEE ALSO
|
---|
857 |
|
---|
858 | See L<perlvar> for a description of Perl's built-in variables and
|
---|
859 | a discussion of legal variable names. See L<perlref>, L<perlsub>,
|
---|
860 | and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
|
---|
861 | the C<*foo{THING}> syntax.
|
---|