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  13 <big><strong><span class="block">&nbsp;perldata - Perl data types</span></strong></big>
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  17 <p><a name="__index__"></a></p>
  18 <!-- INDEX BEGIN -->
  19
  20 <ul>
  21
  22         <li><a href="#name">NAME</a></li>
  23         <li><a href="#description">DESCRIPTION</a></li>
  24         <ul>
  25
  26                 <li><a href="#variable_names">Variable names</a></li>
  27                 <li><a href="#context">Context</a></li>
  28                 <li><a href="#scalar_values">Scalar values</a></li>
  29                 <li><a href="#scalar_value_constructors">Scalar value constructors</a></li>
  30                 <ul>
  31
  32                         <li><a href="#version_strings">Version Strings</a></li>
  33                         <li><a href="#special_literals">Special Literals</a></li>
  34                         <li><a href="#barewords">Barewords</a></li>
  35                         <li><a href="#array_joining_delimiter">Array Joining Delimiter</a></li>
  36                 </ul>
  37
  38                 <li><a href="#list_value_constructors">List value constructors</a></li>
  39                 <li><a href="#subscripts">Subscripts</a></li>
  40                 <li><a href="#slices">Slices</a></li>
  41                 <li><a href="#typeglobs_and_filehandles">Typeglobs and Filehandles</a></li>
  42         </ul>
  43
  44         <li><a href="#see_also">SEE ALSO</a></li>
  45 </ul>
  46 <!-- INDEX END -->
  47
  48 <hr />
  49 <p>
  50 </p>
  51 <h1><a name="name">NAME</a></h1>
  52 <p>perldata - Perl data types</p>
  53 <p>
  54 </p>
  55 <hr />
  56 <h1><a name="description">DESCRIPTION</a></h1>
  57 <p>
  58 </p>
  59 <h2><a name="variable_names_x_variable__name__x_variable_name__x_data_type__x_type_">Variable names
  60    </a></h2>
  61 <p>Perl has three built-in data types: scalars, arrays of scalars, and
  62 associative arrays of scalars, known as ``hashes''.  A scalar is a
  63 single string (of any size, limited only by the available memory),
  64 number, or a reference to something (which will be discussed
  65 in <a href="file://C|\msysgit\mingw\html/pod/perlref.html">the perlref manpage</a>).  Normal arrays are ordered lists of scalars indexed
  66 by number, starting with 0.  Hashes are unordered collections of scalar
  67 values indexed by their associated string key.</p>
  68 <p>Values are usually referred to by name, or through a named reference.
  69 The first character of the name tells you to what sort of data
  70 structure it refers.  The rest of the name tells you the particular
  71 value to which it refers.  Usually this name is a single <em>identifier</em>,
  72 that is, a string beginning with a letter or underscore, and
  73 containing letters, underscores, and digits.  In some cases, it may
  74 be a chain of identifiers, separated by <code>::</code> (or by the slightly
  75 archaic <code>'</code>); all but the last are interpreted as names of packages,
  76 to locate the namespace in which to look up the final identifier
  77 (see <a href="file://C|\msysgit\mingw\html/pod/perlmod.html#packages">Packages in the perlmod manpage</a> for details).  It's possible to substitute
  78 for a simple identifier, an expression that produces a reference
  79 to the value at runtime.   This is described in more detail below
  80 and in <a href="file://C|\msysgit\mingw\html/pod/perlref.html">the perlref manpage</a>.
  81 </p>
  82 <p>Perl also has its own built-in variables whose names don't follow
  83 these rules.  They have strange names so they don't accidentally
  84 collide with one of your normal variables.  Strings that match
  85 parenthesized parts of a regular expression are saved under names
  86 containing only digits after the <code>$</code> (see <a href="file://C|\msysgit\mingw\html/pod/perlop.html">the perlop manpage</a> and <a href="file://C|\msysgit\mingw\html/pod/perlre.html">the perlre manpage</a>).
  87 In addition, several special variables that provide windows into
  88 the inner working of Perl have names containing punctuation characters
  89 and control characters.  These are documented in <a href="file://C|\msysgit\mingw\html/pod/perlvar.html">the perlvar manpage</a>.
  90 </p>
  91 <p>Scalar values are always named with '$', even when referring to a
  92 scalar that is part of an array or a hash.  The '$' symbol works
  93 semantically like the English word ``the'' in that it indicates a
  94 single value is expected.
  95 </p>
  96 <pre>
  97     $days               # the simple scalar value &quot;days&quot;
  98     $days[28]           # the 29th element of array @days
  99     $days{'Feb'}        # the 'Feb' value from hash %days
 100     $#days              # the last index of array @days</pre>
 101 <p>Entire arrays (and slices of arrays and hashes) are denoted by '@',
 102 which works much like the word ``these'' or ``those'' does in English,
 103 in that it indicates multiple values are expected.
 104 </p>
 105 <pre>
 106     @days               # ($days[0], $days[1],... $days[n])
 107     @days[3,4,5]        # same as ($days[3],$days[4],$days[5])
 108     @days{'a','c'}      # same as ($days{'a'},$days{'c'})</pre>
 109 <p>Entire hashes are denoted by '%':
 110 </p>
 111 <pre>
 112     %days               # (key1, val1, key2, val2 ...)</pre>
 113 <p>In addition, subroutines are named with an initial '&amp;', though this
 114 is optional when unambiguous, just as the word ``do'' is often redundant
 115 in English.  Symbol table entries can be named with an initial '*',
 116 but you don't really care about that yet (if ever :-).</p>
 117 <p>Every variable type has its own namespace, as do several
 118 non-variable identifiers.  This means that you can, without fear
 119 of conflict, use the same name for a scalar variable, an array, or
 120 a hash--or, for that matter, for a filehandle, a directory handle, a
 121 subroutine name, a format name, or a label.  This means that $foo
 122 and @foo are two different variables.  It also means that <code>$foo[1]</code>
 123 is a part of @foo, not a part of $foo.  This may seem a bit weird,
 124 but that's okay, because it is weird.
 125 </p>
 126 <p>Because variable references always start with '$', '@', or '%', the
 127 ``reserved'' words aren't in fact reserved with respect to variable
 128 names.  They <em>are</em> reserved with respect to labels and filehandles,
 129 however, which don't have an initial special character.  You can't
 130 have a filehandle named ``log'', for instance.  Hint: you could say
 131 <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open"><code>open(LOG,'logfile')</code></a> rather than <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open"><code>open(log,'logfile')</code></a>.  Using
 132 uppercase filehandles also improves readability and protects you
 133 from conflict with future reserved words.  Case <em>is</em> significant--``FOO'',
 134 ``Foo'', and ``foo'' are all different names.  Names that start with a
 135 letter or underscore may also contain digits and underscores.
 136
 137 </p>
 138 <p>It is possible to replace such an alphanumeric name with an expression
 139 that returns a reference to the appropriate type.  For a description
 140 of this, see <a href="file://C|\msysgit\mingw\html/pod/perlref.html">the perlref manpage</a>.</p>
 141 <p>Names that start with a digit may contain only more digits.  Names
 142 that do not start with a letter, underscore, digit or a caret (i.e.
 143 a control character) are limited to one character, e.g.,  <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>$%</code></a> or
 144 <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>$$</code></a>.  (Most of these one character names have a predefined
 145 significance to Perl.  For instance, <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>$$</code></a> is the current process
 146 id.)</p>
 147 <p>
 148 </p>
 149 <h2><a name="context_x_context__x_scalar_context__x_list_context_">Context
 150   </a></h2>
 151 <p>The interpretation of operations and values in Perl sometimes depends
 152 on the requirements of the context around the operation or value.
 153 There are two major contexts: list and scalar.  Certain operations
 154 return list values in contexts wanting a list, and scalar values
 155 otherwise.  If this is true of an operation it will be mentioned in
 156 the documentation for that operation.  In other words, Perl overloads
 157 certain operations based on whether the expected return value is
 158 singular or plural.  Some words in English work this way, like ``fish''
 159 and ``sheep''.</p>
 160 <p>In a reciprocal fashion, an operation provides either a scalar or a
 161 list context to each of its arguments.  For example, if you say</p>
 162 <pre>
 163     int( &lt;STDIN&gt; )</pre>
 164 <p>the integer operation provides scalar context for the &lt;&gt;
 165 operator, which responds by reading one line from STDIN and passing it
 166 back to the integer operation, which will then find the integer value
 167 of that line and return that.  If, on the other hand, you say</p>
 168 <pre>
 169     sort( &lt;STDIN&gt; )</pre>
 170 <p>then the sort operation provides list context for &lt;&gt;, which
 171 will proceed to read every line available up to the end of file, and
 172 pass that list of lines back to the sort routine, which will then
 173 sort those lines and return them as a list to whatever the context
 174 of the sort was.</p>
 175 <p>Assignment is a little bit special in that it uses its left argument
 176 to determine the context for the right argument.  Assignment to a
 177 scalar evaluates the right-hand side in scalar context, while
 178 assignment to an array or hash evaluates the righthand side in list
 179 context.  Assignment to a list (or slice, which is just a list
 180 anyway) also evaluates the righthand side in list context.</p>
 181 <p>When you use the <code>use warnings</code> pragma or Perl's <strong>-w</strong> command-line
 182 option, you may see warnings
 183 about useless uses of constants or functions in ``void context''.
 184 Void context just means the value has been discarded, such as a
 185 statement containing only <code>&quot;fred&quot;;</code> or <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_getpwuid"><code>getpwuid(0);</code></a>.  It still
 186 counts as scalar context for functions that care whether or not
 187 they're being called in list context.</p>
 188 <p>User-defined subroutines may choose to care whether they are being
 189 called in a void, scalar, or list context.  Most subroutines do not
 190 need to bother, though.  That's because both scalars and lists are
 191 automatically interpolated into lists.  See <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#wantarray">wantarray in the perlfunc manpage</a>
 192 for how you would dynamically discern your function's calling
 193 context.</p>
 194 <p>
 195 </p>
 196 <h2><a name="scalar_values_x_scalar__x_number__x_string__x_reference_">Scalar values
 197    </a></h2>
 198 <p>All data in Perl is a scalar, an array of scalars, or a hash of
 199 scalars.  A scalar may contain one single value in any of three
 200 different flavors: a number, a string, or a reference.  In general,
 201 conversion from one form to another is transparent.  Although a
 202 scalar may not directly hold multiple values, it may contain a
 203 reference to an array or hash which in turn contains multiple values.</p>
 204 <p>Scalars aren't necessarily one thing or another.  There's no place
 205 to declare a scalar variable to be of type ``string'', type ``number'',
 206 type ``reference'', or anything else.  Because of the automatic
 207 conversion of scalars, operations that return scalars don't need
 208 to care (and in fact, cannot care) whether their caller is looking
 209 for a string, a number, or a reference.  Perl is a contextually
 210 polymorphic language whose scalars can be strings, numbers, or
 211 references (which includes objects).  Although strings and numbers
 212 are considered pretty much the same thing for nearly all purposes,
 213 references are strongly-typed, uncastable pointers with builtin
 214 reference-counting and destructor invocation.</p>
 215 <p>A scalar value is interpreted as TRUE in the Boolean sense if it is not
 216 the null string or the number 0 (or its string equivalent, ``0'').  The
 217 Boolean context is just a special kind of scalar context where no
 218 conversion to a string or a number is ever performed.
 219     </p>
 220 <p>There are actually two varieties of null strings (sometimes referred
 221 to as ``empty'' strings), a defined one and an undefined one.  The
 222 defined version is just a string of length zero, such as <code>&quot;&quot;</code>.
 223 The undefined version is the value that indicates that there is
 224 no real value for something, such as when there was an error, or
 225 at end of file, or when you refer to an uninitialized variable or
 226 element of an array or hash.  Although in early versions of Perl,
 227 an undefined scalar could become defined when first used in a
 228 place expecting a defined value, this no longer happens except for
 229 rare cases of autovivification as explained in <a href="file://C|\msysgit\mingw\html/pod/perlref.html">the perlref manpage</a>.  You can
 230 use the <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_defined"><code>defined()</code></a> operator to determine whether a scalar value is
 231 defined (this has no meaning on arrays or hashes), and the <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_undef"><code>undef()</code></a>
 232 operator to produce an undefined value.
 233     </p>
 234 <p>To find out whether a given string is a valid non-zero number, it's
 235 sometimes enough to test it against both numeric 0 and also lexical
 236 ``0'' (although this will cause noises if warnings are on).  That's
 237 because strings that aren't numbers count as 0, just as they do in <strong>awk</strong>:</p>
 238 <pre>
 239     if ($str == 0 &amp;&amp; $str ne &quot;0&quot;)  {
 240         warn &quot;That doesn't look like a number&quot;;
 241     }</pre>
 242 <p>That method may be best because otherwise you won't treat IEEE
 243 notations like <code>NaN</code> or <code>Infinity</code> properly.  At other times, you
 244 might prefer to determine whether string data can be used numerically
 245 by calling the POSIX::strtod() function or by inspecting your string
 246 with a regular expression (as documented in <a href="file://C|\msysgit\mingw\html/pod/perlre.html">the perlre manpage</a>).</p>
 247 <pre>
 248     warn &quot;has nondigits&quot;        if     /\D/;
 249     warn &quot;not a natural number&quot; unless /^\d+$/;             # rejects -3
 250     warn &quot;not an integer&quot;       unless /^-?\d+$/;           # rejects +3
 251     warn &quot;not an integer&quot;       unless /^[+-]?\d+$/;
 252     warn &quot;not a decimal number&quot; unless /^-?\d+\.?\d*$/;     # rejects .2
 253     warn &quot;not a decimal number&quot; unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
 254     warn &quot;not a C float&quot;
 255         unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;</pre>
 256 <p>The length of an array is a scalar value.  You may find the length
 257 of array @days by evaluating <code>$#days</code>, as in <strong>csh</strong>.  However, this
 258 isn't the length of the array; it's the subscript of the last element,
 259 which is a different value since there is ordinarily a 0th element.
 260 Assigning to <code>$#days</code> actually changes the length of the array.
 261 Shortening an array this way destroys intervening values.  Lengthening
 262 an array that was previously shortened does not recover values
 263 that were in those elements.  (It used to do so in Perl 4, but we
 264 had to break this to make sure destructors were called when expected.)
 265  </p>
 266 <p>You can also gain some minuscule measure of efficiency by pre-extending
 267 an array that is going to get big.  You can also extend an array
 268 by assigning to an element that is off the end of the array.  You
 269 can truncate an array down to nothing by assigning the null list
 270 () to it.  The following are equivalent:</p>
 271 <pre>
 272     @whatever = ();
 273     $#whatever = -1;</pre>
 274 <p>If you evaluate an array in scalar context, it returns the length
 275 of the array.  (Note that this is not true of lists, which return
 276 the last value, like the C comma operator, nor of built-in functions,
 277 which return whatever they feel like returning.)  The following is
 278 always true:
 279 </p>
 280 <pre>
 281     scalar(@whatever) == $#whatever - $[ + 1;</pre>
 282 <p>Version 5 of Perl changed the semantics of <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>$[</code></a>: files that don't set
 283 the value of <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>$[</code></a> no longer need to worry about whether another
 284 file changed its value.  (In other words, use of <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___"><code>$[</code></a> is deprecated.)
 285 So in general you can assume that
 286 </p>
 287 <pre>
 288     scalar(@whatever) == $#whatever + 1;</pre>
 289 <p>Some programmers choose to use an explicit conversion so as to
 290 leave nothing to doubt:</p>
 291 <pre>
 292     $element_count = scalar(@whatever);</pre>
 293 <p>If you evaluate a hash in scalar context, it returns false if the
 294 hash is empty.  If there are any key/value pairs, it returns true;
 295 more precisely, the value returned is a string consisting of the
 296 number of used buckets and the number of allocated buckets, separated
 297 by a slash.  This is pretty much useful only to find out whether
 298 Perl's internal hashing algorithm is performing poorly on your data
 299 set.  For example, you stick 10,000 things in a hash, but evaluating
 300 %HASH in scalar context reveals <code>&quot;1/16&quot;</code>, which means only one out
 301 of sixteen buckets has been touched, and presumably contains all
 302 10,000 of your items.  This isn't supposed to happen.
 303   </p>
 304 <p>You can preallocate space for a hash by assigning to the <code>keys()</code> function.
 305 This rounds up the allocated buckets to the next power of two:</p>
 306 <pre>
 307     keys(%users) = 1000;                # allocate 1024 buckets</pre>
 308 <p>
 309 </p>
 310 <h2><a name="scalar_value_constructors_x_scalar__literal__x_scalar__constant_">Scalar value constructors
 311  </a></h2>
 312 <p>Numeric literals are specified in any of the following floating point or
 313 integer formats:</p>
 314 <pre>
 315     12345
 316     12345.67
 317     .23E-10             # a very small number
 318     3.14_15_92          # a very important number
 319     4_294_967_296       # underscore for legibility
 320     0xff                # hex
 321     0xdead_beef         # more hex
 322     0377                # octal (only numbers, begins with 0)
 323     0b011011            # binary</pre>
 324 <p>You are allowed to use underscores (underbars) in numeric literals
 325 between digits for legibility.  You could, for example, group binary
 326 digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
 327 or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
 328 </p>
 329 <p>String literals are usually delimited by either single or double
 330 quotes.  They work much like quotes in the standard Unix shells:
 331 double-quoted string literals are subject to backslash and variable
 332 substitution; single-quoted strings are not (except for <code>\'</code> and
 333 <code>\\</code>).  The usual C-style backslash rules apply for making
 334 characters such as newline, tab, etc., as well as some more exotic
 335 forms.  See <a href="file://C|\msysgit\mingw\html/pod/perlop.html#quote_and_quotelike_operators">Quote and Quote-like Operators in the perlop manpage</a> for a list.
 336 </p>
 337 <p>Hexadecimal, octal, or binary, representations in string literals
 338 (e.g. '0xff') are not automatically converted to their integer
 339 representation.  The <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_hex"><code>hex()</code></a> and <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_oct"><code>oct()</code></a> functions make these conversions
 340 for you.  See <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_hex">hex in the perlfunc manpage</a> and <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_oct">oct in the perlfunc manpage</a> for more details.</p>
 341 <p>You can also embed newlines directly in your strings, i.e., they can end
 342 on a different line than they begin.  This is nice, but if you forget
 343 your trailing quote, the error will not be reported until Perl finds
 344 another line containing the quote character, which may be much further
 345 on in the script.  Variable substitution inside strings is limited to
 346 scalar variables, arrays, and array or hash slices.  (In other words,
 347 names beginning with $ or @, followed by an optional bracketed
 348 expression as a subscript.)  The following code segment prints out ``The
 349 price is $100.''
 350 </p>
 351 <pre>
 352     $Price = '$100';    # not interpolated
 353     print &quot;The price is $Price.\n&quot;;     # interpolated</pre>
 354 <p>There is no double interpolation in Perl, so the <code>$100</code> is left as is.</p>
 355 <p>As in some shells, you can enclose the variable name in braces to
 356 disambiguate it from following alphanumerics (and underscores).
 357 You must also do
 358 this when interpolating a variable into a string to separate the
 359 variable name from a following double-colon or an apostrophe, since
 360 these would be otherwise treated as a package separator:
 361 </p>
 362 <pre>
 363     $who = &quot;Larry&quot;;
 364     print PASSWD &quot;${who}::0:0:Superuser:/:/bin/perl\n&quot;;
 365     print &quot;We use ${who}speak when ${who}'s here.\n&quot;;</pre>
 366 <p>Without the braces, Perl would have looked for a $whospeak, a
 367 <code>$who::0</code>, and a <code>$who's</code> variable.  The last two would be the
 368 $0 and the $s variables in the (presumably) non-existent package
 369 <code>who</code>.</p>
 370 <p>In fact, an identifier within such curlies is forced to be a string,
 371 as is any simple identifier within a hash subscript.  Neither need
 372 quoting.  Our earlier example, <code>$days{'Feb'}</code> can be written as
 373 <code>$days{Feb}</code> and the quotes will be assumed automatically.  But
 374 anything more complicated in the subscript will be interpreted as an
 375 expression.  This means for example that <code>$version{2.0}++</code> is
 376 equivalent to <code>$version{2}++</code>, not to <code>$version{'2.0'}++</code>.</p>
 377 <p>
 378 </p>
 379 <h3><a name="version_strings_x_version_string__x_vstring__x_vstring_">Version Strings
 380   </a></h3>
 381 <p><strong>Note:</strong> Version Strings (v-strings) have been deprecated.  They will
 382 not be available after Perl 5.8.  The marginal benefits of v-strings
 383 were greatly outweighed by the potential for Surprise and Confusion.</p>
 384 <p>A literal of the form <code>v1.20.300.4000</code> is parsed as a string composed
 385 of characters with the specified ordinals.  This form, known as
 386 v-strings, provides an alternative, more readable way to construct
 387 strings, rather than use the somewhat less readable interpolation form
 388 <code>&quot;\x{1}\x{14}\x{12c}\x{fa0}&quot;</code>.  This is useful for representing
 389 Unicode strings, and for comparing version ``numbers'' using the string
 390 comparison operators, <code>cmp</code>, <code>gt</code>, <code>lt</code> etc.  If there are two or
 391 more dots in the literal, the leading <code>v</code> may be omitted.</p>
 392 <pre>
 393     print v9786;              # prints UTF-8 encoded SMILEY, &quot;\x{263a}&quot;
 394     print v102.111.111;       # prints &quot;foo&quot;
 395     print 102.111.111;        # same</pre>
 396 <p>Such literals are accepted by both <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_require"><code>require</code></a> and <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_use"><code>use</code></a> for
 397 doing a version check.  The <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___v"><code>$^V</code></a> special variable also contains the
 398 running Perl interpreter's version in this form.  See <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___v">$^V in the perlvar manpage</a>.
 399 Note that using the v-strings for IPv4 addresses is not portable unless
 400 you also use the <code>inet_aton()/inet_ntoa()</code> routines of the Socket package.</p>
 401 <p>Note that since Perl 5.8.1 the single-number v-strings (like <code>v65</code>)
 402 are not v-strings before the <code>=&gt;</code> operator (which is usually used
 403 to separate a hash key from a hash value), instead they are interpreted
 404 as literal strings ('v65').  They were v-strings from Perl 5.6.0 to
 405 Perl 5.8.0, but that caused more confusion and breakage than good.
 406 Multi-number v-strings like <code>v65.66</code> and <code>65.66.67</code> continue to
 407 be v-strings always.</p>
 408 <p>
 409 </p>
 410 <h3><a name="special_literals_x_special_literal__x___end____x___data____x_end__x_data__x_end__x_data__x__d__x__z_">Special Literals
 411
 412    </a></h3>
 413 <p>The special literals __FILE__, __LINE__, and __PACKAGE__
 414 represent the current filename, line number, and package name at that
 415 point in your program.  They may be used only as separate tokens; they
 416 will not be interpolated into strings.  If there is no current package
 417 (due to an empty <code>package;</code> directive), __PACKAGE__ is the undefined
 418 value.
 419      </p>
 420 <p>The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
 421 may be used to indicate the logical end of the script before the actual
 422 end of file.  Any following text is ignored.</p>
 423 <p>Text after __DATA__ but may be read via the filehandle <code>PACKNAME::DATA</code>,
 424 where <code>PACKNAME</code> is the package that was current when the __DATA__
 425 token was encountered.  The filehandle is left open pointing to the
 426 contents after __DATA__.  It is the program's responsibility to
 427 <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_close"><code>close DATA</code></a> when it is done reading from it.  For compatibility with
 428 older scripts written before __DATA__ was introduced, __END__ behaves
 429 like __DATA__ in the toplevel script (but not in files loaded with
 430 <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_require"><code>require</code></a> or <code>do</code>) and leaves the remaining contents of the
 431 file accessible via <code>main::DATA</code>.</p>
 432 <p>See <a href="file://C|\msysgit\mingw\html/lib/SelfLoader.html">the SelfLoader manpage</a> for more description of __DATA__, and
 433 an example of its use.  Note that you cannot read from the DATA
 434 filehandle in a BEGIN block: the BEGIN block is executed as soon
 435 as it is seen (during compilation), at which point the corresponding
 436 __DATA__ (or __END__) token has not yet been seen.</p>
 437 <p>
 438 </p>
 439 <h3><a name="barewords_x_bareword_">Barewords
 440 </a></h3>
 441 <p>A word that has no other interpretation in the grammar will
 442 be treated as if it were a quoted string.  These are known as
 443 ``barewords''.  As with filehandles and labels, a bareword that consists
 444 entirely of lowercase letters risks conflict with future reserved
 445 words, and if you use the <code>use warnings</code> pragma or the <strong>-w</strong> switch,
 446 Perl will warn you about any
 447 such words.  Some people may wish to outlaw barewords entirely.  If you
 448 say</p>
 449 <pre>
 450     use strict 'subs';</pre>
 451 <p>then any bareword that would NOT be interpreted as a subroutine call
 452 produces a compile-time error instead.  The restriction lasts to the
 453 end of the enclosing block.  An inner block may countermand this
 454 by saying <code>no strict 'subs'</code>.</p>
 455 <p>
 456 </p>
 457 <h3><a name="array_joining_delimiter_x_array__interpolation__x_interpolation__array__x___">Array Joining Delimiter
 458   </a></h3>
 459 <p>Arrays and slices are interpolated into double-quoted strings
 460 by joining the elements with the delimiter specified in the <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item__"><code>$&quot;</code></a>
 461 variable (<a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item__list_separator"><code>$LIST_SEPARATOR</code></a> if ``use English;'' is specified),
 462 space by default.  The following are equivalent:</p>
 463 <pre>
 464     $temp = join($&quot;, @ARGV);
 465     system &quot;echo $temp&quot;;</pre>
 466 <pre>
 467     system &quot;echo @ARGV&quot;;</pre>
 468 <p>Within search patterns (which also undergo double-quotish substitution)
 469 there is an unfortunate ambiguity:  Is <code>/$foo[bar]/</code> to be interpreted as
 470 <code>/${foo}[bar]/</code> (where <code>[bar]</code> is a character class for the regular
 471 expression) or as <code>/${foo[bar]}/</code> (where <code>[bar]</code> is the subscript to array
 472 @foo)?  If @foo doesn't otherwise exist, then it's obviously a
 473 character class.  If @foo exists, Perl takes a good guess about <code>[bar]</code>,
 474 and is almost always right.  If it does guess wrong, or if you're just
 475 plain paranoid, you can force the correct interpretation with curly
 476 braces as above.</p>
 477 <p>If you're looking for the information on how to use here-documents,
 478 which used to be here, that's been moved to
 479 <a href="file://C|\msysgit\mingw\html/pod/perlop.html#quote_and_quotelike_operators">Quote and Quote-like Operators in the perlop manpage</a>.</p>
 480 <p>
 481 </p>
 482 <h2><a name="list_value_constructors_x_list_">List value constructors
 483 </a></h2>
 484 <p>List values are denoted by separating individual values by commas
 485 (and enclosing the list in parentheses where precedence requires it):</p>
 486 <pre>
 487     (LIST)</pre>
 488 <p>In a context not requiring a list value, the value of what appears
 489 to be a list literal is simply the value of the final element, as
 490 with the C comma operator.  For example,</p>
 491 <pre>
 492     @foo = ('cc', '-E', $bar);</pre>
 493 <p>assigns the entire list value to array @foo, but</p>
 494 <pre>
 495     $foo = ('cc', '-E', $bar);</pre>
 496 <p>assigns the value of variable $bar to the scalar variable $foo.
 497 Note that the value of an actual array in scalar context is the
 498 length of the array; the following assigns the value 3 to $foo:</p>
 499 <pre>
 500     @foo = ('cc', '-E', $bar);
 501     $foo = @foo;                # $foo gets 3</pre>
 502 <p>You may have an optional comma before the closing parenthesis of a
 503 list literal, so that you can say:</p>
 504 <pre>
 505     @foo = (
 506         1,
 507         2,
 508         3,
 509     );</pre>
 510 <p>To use a here-document to assign an array, one line per element,
 511 you might use an approach like this:</p>
 512 <pre>
 513     @sauces = &lt;&lt;End_Lines =~ m/(\S.*\S)/g;
 514         normal tomato
 515         spicy tomato
 516         green chile
 517         pesto
 518         white wine
 519     End_Lines</pre>
 520 <p>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</p>
 525 <pre>
 526     (@foo,@bar,&amp;SomeSub,%glarch)</pre>
 527 <p>contains all the elements of @foo followed by all the elements of @bar,
 528 followed by all the elements returned by the subroutine named SomeSub
 529 called in list context, followed by the key/value pairs of %glarch.
 530 To make a list reference that does <em>NOT</em> interpolate, see <a href="file://C|\msysgit\mingw\html/pod/perlref.html">the perlref manpage</a>.</p>
 531 <p>The null list is represented by ().  Interpolating it in a list
 532 has no effect.  Thus ((),(),()) is equivalent to ().  Similarly,
 533 interpolating an array with no elements is the same as if no
 534 array had been interpolated at that point.</p>
 535 <p>This interpolation combines with the facts that the opening
 536 and closing parentheses are optional (except when necessary for
 537 precedence) and lists may end with an optional comma to mean that
 538 multiple commas within lists are legal syntax. The list <code>1,,3</code> is a
 539 concatenation of two lists, <code>1,</code> and <code>3</code>, the first of which ends
 540 with that optional comma.  <code>1,,3</code> is <code>(1,),(3)</code> is <code>1,3</code> (And
 541 similarly for <code>1,,,3</code> is <code>(1,),(,),3</code> is <code>1,3</code> and so on.)  Not that
 542 we'd advise you to use this obfuscation.</p>
 543 <p>A list value may also be subscripted like a normal array.  You must
 544 put the list in parentheses to avoid ambiguity.  For example:</p>
 545 <pre>
 546     # Stat returns list value.
 547     $time = (stat($file))[8];</pre>
 548 <pre>
 549     # SYNTAX ERROR HERE.
 550     $time = stat($file)[8];  # OOPS, FORGOT PARENTHESES</pre>
 551 <pre>
 552     # Find a hex digit.
 553     $hexdigit = ('a','b','c','d','e','f')[$digit-10];</pre>
 554 <pre>
 555     # A &quot;reverse comma operator&quot;.
 556     return (pop(@foo),pop(@foo))[0];</pre>
 557 <p>Lists may be assigned to only when each element of the list
 558 is itself legal to assign to:</p>
 559 <pre>
 560     ($a, $b, $c) = (1, 2, 3);</pre>
 561 <pre>
 562     ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);</pre>
 563 <p>An exception to this is that you may assign to <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_undef"><code>undef</code></a> in a list.
 564 This is useful for throwing away some of the return values of a
 565 function:</p>
 566 <pre>
 567     ($dev, $ino, undef, undef, $uid, $gid) = stat($file);</pre>
 568 <p>List assignment in scalar context returns the number of elements
 569 produced by the expression on the right side of the assignment:</p>
 570 <pre>
 571     $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2
 572     $x = (($foo,$bar) = f());           # set $x to f()'s return count</pre>
 573 <p>This is handy when you want to do a list assignment in a Boolean
 574 context, because most list functions return a null list when finished,
 575 which when assigned produces a 0, which is interpreted as FALSE.</p>
 576 <p>It's also the source of a useful idiom for executing a function or
 577 performing an operation in list context and then counting the number of
 578 return values, by assigning to an empty list and then using that
 579 assignment in scalar context. For example, this code:</p>
 580 <pre>
 581     $count = () = $string =~ /\d+/g;</pre>
 582 <p>will place into $count the number of digit groups found in $string.
 583 This happens because the pattern match is in list context (since it
 584 is being assigned to the empty list), and will therefore return a list
 585 of all matching parts of the string. The list assignment in scalar
 586 context will translate that into the number of elements (here, the
 587 number of times the pattern matched) and assign that to $count. Note
 588 that simply using</p>
 589 <pre>
 590     $count = $string =~ /\d+/g;</pre>
 591 <p>would not have worked, since a pattern match in scalar context will
 592 only return true or false, rather than a count of matches.</p>
 593 <p>The final element of a list assignment may be an array or a hash:</p>
 594 <pre>
 595     ($a, $b, @rest) = split;
 596     my($a, $b, %rest) = @_;</pre>
 597 <p>You can actually put an array or hash anywhere in the list, but the first one
 598 in the list will soak up all the values, and anything after it will become
 599 undefined.  This may be useful in a <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_my"><code>my()</code></a> or local().</p>
 600 <p>A hash can be initialized using a literal list holding pairs of
 601 items to be interpreted as a key and a value:</p>
 602 <pre>
 603     # same as map assignment above
 604     %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);</pre>
 605 <p>While literal lists and named arrays are often interchangeable, that's
 606 not the case for hashes.  Just because you can subscript a list value like
 607 a normal array does not mean that you can subscript a list value as a
 608 hash.  Likewise, hashes included as parts of other lists (including
 609 parameters lists and return lists from functions) always flatten out into
 610 key/value pairs.  That's why it's good to use references sometimes.</p>
 611 <p>It is often more readable to use the <code>=&gt;</code> operator between key/value
 612 pairs.  The <code>=&gt;</code> operator is mostly just a more visually distinctive
 613 synonym for a comma, but it also arranges for its left-hand operand to be
 614 interpreted as a string -- if it's a bareword that would be a legal simple
 615 identifier (<code>=&gt;</code> doesn't quote compound identifiers, that contain
 616 double colons). This makes it nice for initializing hashes:</p>
 617 <pre>
 618     %map = (
 619                  red   =&gt; 0x00f,
 620                  blue  =&gt; 0x0f0,
 621                  green =&gt; 0xf00,
 622    );</pre>
 623 <p>or for initializing hash references to be used as records:</p>
 624 <pre>
 625     $rec = {
 626                 witch =&gt; 'Mable the Merciless',
 627                 cat   =&gt; 'Fluffy the Ferocious',
 628                 date  =&gt; '10/31/1776',
 629     };</pre>
 630 <p>or for using call-by-named-parameter to complicated functions:</p>
 631 <pre>
 632    $field = $query-&gt;radio_group(
 633                name      =&gt; 'group_name',
 634                values    =&gt; ['eenie','meenie','minie'],
 635                default   =&gt; 'meenie',
 636                linebreak =&gt; 'true',
 637                labels    =&gt; \%labels
 638    );</pre>
 639 <p>Note that just because a hash is initialized in that order doesn't
 640 mean that it comes out in that order.  See <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_sort">sort in the perlfunc manpage</a> for examples
 641 of how to arrange for an output ordering.</p>
 642 <p>
 643 </p>
 644 <h2><a name="subscripts">Subscripts</a></h2>
 645 <p>An array is subscripted by specifying a dollar sign (<code>$</code>), then the
 646 name of the array (without the leading <code>@</code>), then the subscript inside
 647 square brackets.  For example:</p>
 648 <pre>
 649     @myarray = (5, 50, 500, 5000);
 650     print &quot;Element Number 2 is&quot;, $myarray[2], &quot;\n&quot;;</pre>
 651 <p>The array indices start with 0. A negative subscript retrieves its
 652 value from the end.  In our example, <code>$myarray[-1]</code> would have been
 653 5000, and <code>$myarray[-2]</code> would have been 500.</p>
 654 <p>Hash subscripts are similar, only instead of square brackets curly brackets
 655 are used. For example:</p>
 656 <pre>
 657     %scientists =
 658     (
 659         &quot;Newton&quot; =&gt; &quot;Isaac&quot;,
 660         &quot;Einstein&quot; =&gt; &quot;Albert&quot;,
 661         &quot;Darwin&quot; =&gt; &quot;Charles&quot;,
 662         &quot;Feynman&quot; =&gt; &quot;Richard&quot;,
 663     );</pre>
 664 <pre>
 665     print &quot;Darwin's First Name is &quot;, $scientists{&quot;Darwin&quot;}, &quot;\n&quot;;</pre>
 666 <p>
 667 </p>
 668 <h2><a name="slices_x_slice__x_array__slice__x_hash__slice_">Slices
 669   </a></h2>
 670 <p>A common way to access an array or a hash is one scalar element at a
 671 time.  You can also subscript a list to get a single element from it.</p>
 672 <pre>
 673     $whoami = $ENV{&quot;USER&quot;};             # one element from the hash
 674     $parent = $ISA[0];                  # one element from the array
 675     $dir    = (getpwnam(&quot;daemon&quot;))[7];  # likewise, but with list</pre>
 676 <p>A slice accesses several elements of a list, an array, or a hash
 677 simultaneously using a list of subscripts.  It's more convenient
 678 than writing out the individual elements as a list of separate
 679 scalar values.</p>
 680 <pre>
 681     ($him, $her)   = @folks[0,-1];              # array slice
 682     @them          = @folks[0 .. 3];            # array slice
 683     ($who, $home)  = @ENV{&quot;USER&quot;, &quot;HOME&quot;};      # hash slice
 684     ($uid, $dir)   = (getpwnam(&quot;daemon&quot;))[2,7]; # list slice</pre>
 685 <p>Since you can assign to a list of variables, you can also assign to
 686 an array or hash slice.</p>
 687 <pre>
 688     @days[3..5]    = qw/Wed Thu Fri/;
 689     @colors{'red','blue','green'}
 690                    = (0xff0000, 0x0000ff, 0x00ff00);
 691     @folks[0, -1]  = @folks[-1, 0];</pre>
 692 <p>The previous assignments are exactly equivalent to</p>
 693 <pre>
 694     ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
 695     ($colors{'red'}, $colors{'blue'}, $colors{'green'})
 696                    = (0xff0000, 0x0000ff, 0x00ff00);
 697     ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);</pre>
 698 <p>Since changing a slice changes the original array or hash that it's
 699 slicing, a <code>foreach</code> construct will alter some--or even all--of the
 700 values of the array or hash.</p>
 701 <pre>
 702     foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }</pre>
 703 <pre>
 704     foreach (@hash{qw[key1 key2]}) {
 705         s/^\s+//;           # trim leading whitespace
 706         s/\s+$//;           # trim trailing whitespace
 707         s/(\w+)/\u\L$1/g;   # &quot;titlecase&quot; words
 708     }</pre>
 709 <p>A slice of an empty list is still an empty list.  Thus:</p>
 710 <pre>
 711     @a = ()[1,0];           # @a has no elements
 712     @b = (@a)[0,1];         # @b has no elements
 713     @c = (0,1)[2,3];        # @c has no elements</pre>
 714 <p>But:</p>
 715 <pre>
 716     @a = (1)[1,0];          # @a has two elements
 717     @b = (1,undef)[1,0,2];  # @b has three elements</pre>
 718 <p>This makes it easy to write loops that terminate when a null list
 719 is returned:</p>
 720 <pre>
 721     while ( ($home, $user) = (getpwent)[7,0]) {
 722         printf &quot;%-8s %s\n&quot;, $user, $home;
 723     }</pre>
 724 <p>As noted earlier in this document, the scalar sense of list assignment
 725 is the number of elements on the right-hand side of the assignment.
 726 The null list contains no elements, so when the password file is
 727 exhausted, the result is 0, not 2.</p>
 728 <p>If you're confused about why you use an '@' there on a hash slice
 729 instead of a '%', think of it like this.  The type of bracket (square
 730 or curly) governs whether it's an array or a hash being looked at.
 731 On the other hand, the leading symbol ('$' or '@') on the array or
 732 hash indicates whether you are getting back a singular value (a
 733 scalar) or a plural one (a list).</p>
 734 <p>
 735 </p>
 736 <h2><a name="typeglobs_and_filehandles_x_typeglob__x_filehandle__x___">Typeglobs and Filehandles
 737   </a></h2>
 738 <p>Perl uses an internal type called a <em>typeglob</em> to hold an entire
 739 symbol table entry.  The type prefix of a typeglob is a <code>*</code>, because
 740 it represents all types.  This used to be the preferred way to
 741 pass arrays and hashes by reference into a function, but now that
 742 we have real references, this is seldom needed.</p>
 743 <p>The main use of typeglobs in modern Perl is create symbol table aliases.
 744 This assignment:</p>
 745 <pre>
 746     *this = *that;</pre>
 747 <p>makes $this an alias for $that, @this an alias for @that, %this an alias
 748 for %that, &amp;this an alias for &amp;that, etc.  Much safer is to use a reference.
 749 This:</p>
 750 <pre>
 751     local *Here::blue = \$There::green;</pre>
 752 <p>temporarily makes $Here::blue an alias for $There::green, but doesn't
 753 make @Here::blue an alias for @There::green, or %Here::blue an alias for
 754 %There::green, etc.  See <a href="file://C|\msysgit\mingw\html/pod/perlmod.html#symbol_tables">Symbol Tables in the perlmod manpage</a> for more examples
 755 of this.  Strange though this may seem, this is the basis for the whole
 756 module import/export system.</p>
 757 <p>Another use for typeglobs is to pass filehandles into a function or
 758 to create new filehandles.  If you need to use a typeglob to save away
 759 a filehandle, do it this way:</p>
 760 <pre>
 761     $fh = *STDOUT;</pre>
 762 <p>or perhaps as a real reference, like this:</p>
 763 <pre>
 764     $fh = \*STDOUT;</pre>
 765 <p>See <a href="file://C|\msysgit\mingw\html/pod/perlsub.html">the perlsub manpage</a> for examples of using these as indirect filehandles
 766 in functions.</p>
 767 <p>Typeglobs are also a way to create a local filehandle using the <code>local()</code>
 768 operator.  These last until their block is exited, but may be passed back.
 769 For example:</p>
 770 <pre>
 771     sub newopen {
 772         my $path = shift;
 773         local  *FH;  # not my!
 774         open   (FH, $path)          or  return undef;
 775         return *FH;
 776     }
 777     $fh = newopen('/etc/passwd');</pre>
 778 <p>Now that we have the <code>*foo{THING}</code> notation, typeglobs aren't used as much
 779 for filehandle manipulations, although they're still needed to pass brand
 780 new file and directory handles into or out of functions. That's because
 781 <code>*HANDLE{IO}</code> only works if HANDLE has already been used as a handle.
 782 In other words, <code>*FH</code> must be used to create new symbol table entries;
 783 <code>*foo{THING}</code> cannot.  When in doubt, use <code>*FH</code>.</p>
 784 <p>All functions that are capable of creating filehandles (open(),
 785 opendir(), pipe(), socketpair(), sysopen(), socket(), and <code>accept())</code>
 786 automatically create an anonymous filehandle if the handle passed to
 787 them is an uninitialized scalar variable. This allows the constructs
 788 such as <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open"><code>open(my $fh, ...)</code></a> and <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open"><code>open(local $fh,...)</code></a> to be used to
 789 create filehandles that will conveniently be closed automatically when
 790 the scope ends, provided there are no other references to them. This
 791 largely eliminates the need for typeglobs when opening filehandles
 792 that must be passed around, as in the following example:</p>
 793 <pre>
 794     sub myopen {
 795         open my $fh, &quot;@_&quot;
 796              or die &quot;Can't open '@_': $!&quot;;
 797         return $fh;
 798     }</pre>
 799 <pre>
 800     {
 801         my $f = myopen(&quot;&lt;/etc/motd&quot;);
 802         print &lt;$f&gt;;
 803         # $f implicitly closed here
 804     }</pre>
 805 <p>Note that if an initialized scalar variable is used instead the
 806 result is different: <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open"><code>my $fh='zzz'; open($fh, ...)</code></a> is equivalent
 807 to <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open"><code>open( *{'zzz'}, ...)</code></a>.
 808 <code>use strict 'refs'</code> forbids such practice.</p>
 809 <p>Another way to create anonymous filehandles is with the Symbol
 810 module or with the IO::Handle module and its ilk.  These modules
 811 have the advantage of not hiding different types of the same name
 812 during the local().  See the bottom of <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open">open() in the perlfunc manpage</a> for an
 813 example.</p>
 814 <p>
 815 </p>
 816 <hr />
 817 <h1><a name="see_also">SEE ALSO</a></h1>
 818 <p>See <a href="file://C|\msysgit\mingw\html/pod/perlvar.html">the perlvar manpage</a> for a description of Perl's built-in variables and
 819 a discussion of legal variable names.  See <a href="file://C|\msysgit\mingw\html/pod/perlref.html">the perlref manpage</a>, <a href="file://C|\msysgit\mingw\html/pod/perlsub.html">the perlsub manpage</a>,
 820 and <a href="file://C|\msysgit\mingw\html/pod/perlmod.html#symbol_tables">Symbol Tables in the perlmod manpage</a> for more discussion on typeglobs and
 821 the <code>*foo{THING}</code> syntax.</p>
 822 <table border="0" width="100%" cellspacing="0" cellpadding="3">
 823 <tr><td class="block" style="background-color: #cccccc" valign="middle">
 824 <big><strong><span class="block">&nbsp;perldata - Perl data types</span></strong></big>
 825 </td></tr>
 826 </table>
 827
 828 </body>
 829
 830 </html>