3 perldata - Perl data types
9 Perl has three data structures: scalars, arrays of scalars, and
10 associative arrays of scalars, known as "hashes". Normal arrays are
11 indexed by number, starting with 0. (Negative subscripts count from
12 the end.) Hash arrays are indexed by string.
14 Scalar values are always named with '$', even when referring to a scalar
15 that is part of an array. It works like the English word "the". Thus
18 $days # the simple scalar value "days"
19 $days[28] # the 29th element of array @days
20 $days{'Feb'} # the 'Feb' value from hash %days
21 $#days # the last index of array @days
23 but entire arrays or array slices are denoted by '@', which works much like
24 the word "these" or "those":
26 @days # ($days[0], $days[1],... $days[n])
27 @days[3,4,5] # same as @days[3..5]
28 @days{'a','c'} # same as ($days{'a'},$days{'c'})
30 and entire hashes are denoted by '%':
32 %days # (key1, val1, key2, val2 ...)
34 In addition, subroutines are named with an initial '&', though this is
35 optional when it's otherwise unambiguous (just as "do" is often
36 redundant in English). Symbol table entries can be named with an
37 initial '*', but you don't really care about that yet.
39 Every variable type has its own namespace. You can, without fear of
40 conflict, use the same name for a scalar variable, an array, or a hash
41 (or, for that matter, a filehandle, a subroutine name, or a label).
42 This means that $foo and @foo are two different variables. It also
43 means that C<$foo[1]> is a part of @foo, not a part of $foo. This may
44 seem a bit weird, but that's okay, because it is weird.
46 Since variable and array references always start with '$', '@', or '%',
47 the "reserved" words aren't in fact reserved with respect to variable
48 names. (They ARE reserved with respect to labels and filehandles,
49 however, which don't have an initial special character. You can't have
50 a filehandle named "log", for instance. Hint: you could say
51 C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase
52 filehandles also improves readability and protects you from conflict
53 with future reserved words.) Case I<IS> significant--"FOO", "Foo" and
54 "foo" are all different names. Names that start with a letter or
55 underscore may also contain digits and underscores.
57 It is possible to replace such an alphanumeric name with an expression
58 that returns a reference to an object of that type. For a description
59 of this, see L<perlref>.
61 Names that start with a digit may only contain more digits. Names
62 which do not start with a letter, underscore, or digit are limited to
63 one character, e.g. C<$%> or C<$$>. (Most of these one character names
64 have a predefined significance to Perl. For instance, C<$$> is the
69 The interpretation of operations and values in Perl sometimes depends
70 on the requirements of the context around the operation or value.
71 There are two major contexts: scalar and list. Certain operations
72 return list values in contexts wanting a list, and scalar values
73 otherwise. (If this is true of an operation it will be mentioned in
74 the documentation for that operation.) In other words, Perl overloads
75 certain operations based on whether the expected return value is
76 singular or plural. (Some words in English work this way, like "fish"
79 In a reciprocal fashion, an operation provides either a scalar or a
80 list context to each of its arguments. For example, if you say
84 the integer operation provides a scalar context for the <STDIN>
85 operator, which responds by reading one line from STDIN and passing it
86 back to the integer operation, which will then find the integer value
87 of that line and return that. If, on the other hand, you say
91 then the sort operation provides a list context for <STDIN>, which
92 will proceed to read every line available up to the end of file, and
93 pass that list of lines back to the sort routine, which will then
94 sort those lines and return them as a list to whatever the context
97 Assignment is a little bit special in that it uses its left argument to
98 determine the context for the right argument. Assignment to a scalar
99 evaluates the righthand side in a scalar context, while assignment to
100 an array or array slice evaluates the righthand side in a list
101 context. Assignment to a list also evaluates the righthand side in a
104 User defined subroutines may choose to care whether they are being
105 called in a scalar or list context, but most subroutines do not
106 need to care, because scalars are automatically interpolated into
107 lists. See L<perlfunc/wantarray>.
111 All data in Perl is a scalar or an array of scalars or a hash of scalars.
112 Scalar variables may contain various kinds of singular data, such as
113 numbers, strings, and references. In general, conversion from one form to
114 another is transparent. (A scalar may not contain multiple values, but
115 may contain a reference to an array or hash containing multiple values.)
116 Because of the automatic conversion of scalars, operations and functions
117 that return scalars don't need to care (and, in fact, can't care) whether
118 the context is looking for a string or a number.
120 Scalars aren't necessarily one thing or another. There's no place to
121 declare a scalar variable to be of type "string", or of type "number", or
122 type "filehandle", or anything else. Perl is a contextually polymorphic
123 language whose scalars can be strings, numbers, or references (which
124 includes objects). While strings and numbers are considered pretty
125 much same thing for nearly all purposes, references are strongly-typed
126 uncastable pointers with built-in reference-counting and destructor
129 A scalar value is interpreted as TRUE in the Boolean sense if it is not
130 the null string or the number 0 (or its string equivalent, "0"). The
131 Boolean context is just a special kind of scalar context.
133 There are actually two varieties of null scalars: defined and
134 undefined. Undefined null scalars are returned when there is no real
135 value for something, such as when there was an error, or at end of
136 file, or when you refer to an uninitialized variable or element of an
137 array. An undefined null scalar may become defined the first time you
138 use it as if it were defined, but prior to that you can use the
139 defined() operator to determine whether the value is defined or not.
141 To find out whether a given string is a valid non-zero number, it's usually
142 enough to test it against both numeric 0 and also lexical "0" (although
143 this will cause B<-w> noises). That's because strings that aren't
144 numbers count as 0, just as the do in I<awk>:
146 if ($str == 0 && $str ne "0") {
147 warn "That doesn't look like a number";
150 That's usually preferable because otherwise you won't treat IEEE notations
151 like C<NaN> or C<Infinity> properly. At other times you might prefer to
152 use a regular expression to check whether data is numeric. See L<perlre>
153 for details on regular expressions.
155 warn "has nondigits" if /\D/;
156 warn "not a whole number" unless /^\d+$/;
157 warn "not an integer" unless /^[+-]?\d+$/
158 warn "not a decimal number" unless /^[+-]?\d+\.?\d*$/
160 unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
162 The length of an array is a scalar value. You may find the length of
163 array @days by evaluating C<$#days>, as in B<csh>. (Actually, it's not
164 the length of the array, it's the subscript of the last element, since
165 there is (ordinarily) a 0th element.) Assigning to C<$#days> changes the
166 length of the array. Shortening an array by this method destroys
167 intervening values. Lengthening an array that was previously shortened
168 I<NO LONGER> recovers the values that were in those elements. (It used to
169 in Perl 4, but we had to break this make to make sure destructors were
170 called when expected.) You can also gain some measure of efficiency by
171 preextending an array that is going to get big. (You can also extend
172 an array by assigning to an element that is off the end of the array.)
173 You can truncate an array down to nothing by assigning the null list ()
174 to it. The following are equivalent:
179 If you evaluate a named array in a scalar context, it returns the length of
180 the array. (Note that this is not true of lists, which return the
181 last value, like the C comma operator.) The following is always true:
183 scalar(@whatever) == $#whatever - $[ + 1;
185 Version 5 of Perl changed the semantics of $[: files that don't set
186 the value of $[ no longer need to worry about whether another
187 file changed its value. (In other words, use of $[ is deprecated.)
188 So in general you can just assume that
190 scalar(@whatever) == $#whatever + 1;
192 Some programmers choose to use an explicit conversion so nothing's
195 $element_count = scalar(@whatever);
197 If you evaluate a hash in a scalar context, it returns a value which is
198 true if and only if the hash contains any key/value pairs. (If there
199 are any key/value pairs, the value returned is a string consisting of
200 the number of used buckets and the number of allocated buckets, separated
201 by a slash. This is pretty much only useful to find out whether Perl's
202 (compiled in) hashing algorithm is performing poorly on your data set.
203 For example, you stick 10,000 things in a hash, but evaluating %HASH in
204 scalar context reveals "1/16", which means only one out of sixteen buckets
205 has been touched, and presumably contains all 10,000 of your items. This
206 isn't supposed to happen.)
208 =head2 Scalar value constructors
210 Numeric literals are specified in any of the customary floating point or
218 4_294_967_296 # underline for legibility
220 String literals are usually delimited by either single or double quotes. They
221 work much like shell quotes: double-quoted string literals are subject
222 to backslash and variable substitution; single-quoted strings are not
223 (except for "C<\'>" and "C<\\>"). The usual Unix backslash rules apply for making
224 characters such as newline, tab, etc., as well as some more exotic
225 forms. See L<perlop/qq> for a list.
227 You can also embed newlines directly in your strings, i.e. they can end
228 on a different line than they begin. This is nice, but if you forget
229 your trailing quote, the error will not be reported until Perl finds
230 another line containing the quote character, which may be much further
231 on in the script. Variable substitution inside strings is limited to
232 scalar variables, arrays, and array slices. (In other words,
233 identifiers beginning with $ or @, followed by an optional bracketed
234 expression as a subscript.) The following code segment prints out "The
237 $Price = '$100'; # not interpreted
238 print "The price is $Price.\n"; # interpreted
240 As in some shells, you can put curly brackets around the identifier to
241 delimit it from following alphanumerics. In fact, an identifier
242 within such curlies is forced to be a string, as is any single
243 identifier within a hash subscript. Our earlier example,
251 and the quotes will be assumed automatically. But anything more complicated
252 in the subscript will be interpreted as an expression.
255 single-quoted string must be separated from a preceding word by a
256 space, since single quote is a valid (though deprecated) character in
257 an identifier (see L<perlmod/Packages>).
259 Two special literals are __LINE__ and __FILE__, which represent the
260 current line number and filename at that point in your program. They
261 may only be used as separate tokens; they will not be interpolated into
262 strings. In addition, the token __END__ may be used to indicate the
263 logical end of the script before the actual end of file. Any following
264 text is ignored, but may be read via the DATA filehandle. (The DATA
265 filehandle may read data only from the main script, but not from any
266 required file or evaluated string.) The two control characters ^D and
267 ^Z are synonyms for __END__ (or __DATA__ in a module; see L<SelfLoader> for
268 details on __DATA__).
270 A word that has no other interpretation in the grammar will
271 be treated as if it were a quoted string. These are known as
272 "barewords". As with filehandles and labels, a bareword that consists
273 entirely of lowercase letters risks conflict with future reserved
274 words, and if you use the B<-w> switch, Perl will warn you about any
275 such words. Some people may wish to outlaw barewords entirely. If you
280 then any bareword that would NOT be interpreted as a subroutine call
281 produces a compile-time error instead. The restriction lasts to the
282 end of the enclosing block. An inner block may countermand this
283 by saying C<no strict 'subs'>.
285 Array variables are interpolated into double-quoted strings by joining all
286 the elements of the array with the delimiter specified in the C<$">
287 variable ($LIST_SEPARATOR in English), space by default. The following
290 $temp = join($",@ARGV);
295 Within search patterns (which also undergo double-quotish substitution)
296 there is a bad ambiguity: Is C</$foo[bar]/> to be interpreted as
297 C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
298 expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
299 @foo)? If @foo doesn't otherwise exist, then it's obviously a
300 character class. If @foo exists, Perl takes a good guess about C<[bar]>,
301 and is almost always right. If it does guess wrong, or if you're just
302 plain paranoid, you can force the correct interpretation with curly
305 A line-oriented form of quoting is based on the shell "here-doc" syntax.
306 Following a C<E<lt>E<lt>> you specify a string to terminate the quoted material,
307 and all lines following the current line down to the terminating string
308 are the value of the item. The terminating string may be either an
309 identifier (a word), or some quoted text. If quoted, the type of
310 quotes you use determines the treatment of the text, just as in regular
311 quoting. An unquoted identifier works like double quotes. There must
312 be no space between the C<E<lt>E<lt>> and the identifier. (If you put a space it
313 will be treated as a null identifier, which is valid, and matches the
314 first blank line.) The terminating string must appear by itself
315 (unquoted and with no surrounding whitespace) on the terminating line.
321 print <<"EOF"; # same as above
325 print <<`EOC`; # execute commands
330 print <<"foo", <<"bar"; # you can stack them
336 myfunc(<<"THIS", 23, <<'THAT');
343 Just don't forget that you have to put a semicolon on the end
344 to finish the statement, as Perl doesn't know you're not going to
353 =head2 List value constructors
355 List values are denoted by separating individual values by commas
356 (and enclosing the list in parentheses where precedence requires it):
360 In a context not requiring a list value, the value of the list
361 literal is the value of the final element, as with the C comma operator.
364 @foo = ('cc', '-E', $bar);
366 assigns the entire list value to array foo, but
368 $foo = ('cc', '-E', $bar);
370 assigns the value of variable bar to variable foo. Note that the value
371 of an actual array in a scalar context is the length of the array; the
372 following assigns to $foo the value 3:
374 @foo = ('cc', '-E', $bar);
375 $foo = @foo; # $foo gets 3
377 You may have an optional comma before the closing parenthesis of an
378 list literal, so that you can say:
386 LISTs do automatic interpolation of sublists. That is, when a LIST is
387 evaluated, each element of the list is evaluated in a list context, and
388 the resulting list value is interpolated into LIST just as if each
389 individual element were a member of LIST. Thus arrays lose their
390 identity in a LIST--the list
394 contains all the elements of @foo followed by all the elements of @bar,
395 followed by all the elements returned by the subroutine named SomeSub when
396 it's called in a list context.
397 To make a list reference that does I<NOT> interpolate, see L<perlref>.
399 The null list is represented by (). Interpolating it in a list
400 has no effect. Thus ((),(),()) is equivalent to (). Similarly,
401 interpolating an array with no elements is the same as if no
402 array had been interpolated at that point.
404 A list value may also be subscripted like a normal array. You must
405 put the list in parentheses to avoid ambiguity. Examples:
407 # Stat returns list value.
408 $time = (stat($file))[8];
411 $time = stat($file)[8]; # OOPS, FORGOT PARENS
414 $hexdigit = ('a','b','c','d','e','f')[$digit-10];
416 # A "reverse comma operator".
417 return (pop(@foo),pop(@foo))[0];
419 Lists may be assigned to if and only if each element of the list
420 is legal to assign to:
422 ($a, $b, $c) = (1, 2, 3);
424 ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
426 Array assignment in a scalar context returns the number of elements
427 produced by the expression on the right side of the assignment:
429 $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
430 $x = (($foo,$bar) = f()); # set $x to f()'s return count
432 This is very handy when you want to do a list assignment in a Boolean
433 context, since most list functions return a null list when finished,
434 which when assigned produces a 0, which is interpreted as FALSE.
436 The final element may be an array or a hash:
438 ($a, $b, @rest) = split;
439 local($a, $b, %rest) = @_;
441 You can actually put an array or hash anywhere in the list, but the first one
442 in the list will soak up all the values, and anything after it will get
443 a null value. This may be useful in a local() or my().
445 A hash literal contains pairs of values to be interpreted
446 as a key and a value:
448 # same as map assignment above
449 %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
451 While literal lists and named arrays are usually interchangeable, that's
452 not the case for hashes. Just because you can subscript a list value like
453 a normal array does not mean that you can subscript a list value as a
454 hash. Likewise, hashes included as parts of other lists (including
455 parameters lists and return lists from functions) always flatten out into
456 key/value pairs. That's why it's good to use references sometimes.
458 It is often more readable to use the C<=E<gt>> operator between key/value
459 pairs. The C<=E<gt>> operator is mostly just a more visually distinctive
460 synonym for a comma, but it also quotes its left-hand operand, which makes
461 it nice for initializing hashes:
469 or for initializing hash references to be used as records:
472 witch => 'Mable the Merciless',
473 cat => 'Fluffy the Ferocious',
474 date => '10/31/1776',
477 or for using call-by-named-parameter to complicated functions:
479 $field = $query->radio_group(
480 name => 'group_name',
481 values => ['eenie','meenie','minie'],
487 Note that just because a hash is initialized in that order doesn't
488 mean that it comes out in that order. See L<perlfunc/sort> for examples
489 of how to arrange for an output ordering.
491 =head2 Typeglobs and FileHandles
493 Perl uses an internal type called a I<typeglob> to hold an entire
494 symbol table entry. The type prefix of a typeglob is a C<*>, because
495 it represents all types. This used to be the preferred way to
496 pass arrays and hashes by reference into a function, but now that
497 we have real references, this is seldom needed.
499 One place where you still use typeglobs (or references thereto)
500 is for passing or storing filehandles. If you want to save away
501 a filehandle, do it this way:
505 or perhaps as a real reference, like this:
509 This is also the way to create a local filehandle. For example:
514 open (FH, $path) || return undef;
517 $fh = newopen('/etc/passwd');
519 See L<perlref>, L<perlsub>, and L<perlmod/"Symbols Tables"> for more
520 discussion on typeglobs. See L<perlfunc/open> for other ways of
521 generating filehandles.