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NAME    [Toc]    [Back]

       perldata - Perl data types

DESCRIPTION    [Toc]    [Back]

       Variable names

       Perl has three built-in data types: scalars, arrays of
       scalars, and associative arrays of scalars, known as
       "hashes".  A scalar is a single string (of any size, limited
 only by the available memory), number, or a reference
       to something (which will be discussed in perlref).  Normal
       arrays are ordered lists of scalars indexed by number,
       starting with 0.  Hashes are unordered collections of
       scalar values indexed by their associated string key.

       Values are usually referred to by name, or through a named
       reference.  The first character of the name tells you to
       what sort of data structure it refers.  The rest of the
       name tells you the particular value to which it refers.
       Usually this name is a single identifier, that is, a
       string beginning with a letter or underscore, and containing
 letters, underscores, and digits.  In some cases, it
       may be a chain of identifiers, separated by "::" (or by
       the slightly archaic "'"); all but the last are interpreted
 as names of packages, to locate the namespace in
       which to look up the final identifier (see "Packages" in
       perlmod for details).  It's possible to substitute for a
       simple identifier, an expression that produces a reference
       to the value at runtime.   This is described in more
       detail below and in perlref.

       Perl also has its own built-in variables whose names don't
       follow these rules.  They have strange names so they don't
       accidentally collide with one of your normal variables.
       Strings that match parenthesized parts of a regular
       expression are saved under names containing only digits
       after the "$" (see perlop and perlre).  In addition, several
 special variables that provide windows into the inner
       working of Perl have names containing punctuation characters
 and control characters.  These are documented in perlvar.

       Scalar values are always named with '$', even when referring
 to a scalar that is part of an array or a hash.  The
       '$' symbol works semantically like the English word "the"
       in that it indicates a single value is expected.

           $days               # the simple scalar value "days"
           $days[28]           # the 29th element of array @days
           $days{'Feb'}        # the 'Feb' value from hash %days
           $#days              # the last index of array @days

       Entire arrays (and slices of arrays and hashes) are
       denoted by '@', which works much like the word "these" or
       "those" does in English, in that it indicates multiple
       values are expected.

           @days                  #    ($days[0],    $days[1],...
           @days[3,4,5]                #         same          as
           @days{'a','c'}      # same as ($days{'a'},$days{'c'})

       Entire hashes are denoted by '%':

           %days               # (key1, val1, key2, val2 ...)

       In addition, subroutines are named with an initial '&',
       though this is optional when unambiguous, just as the word
       "do" is often redundant in English.  Symbol table entries
       can be named with an initial '*', but you don't really
       care about that yet (if ever :-).

       Every variable type has its own namespace, as do several
       non-variable identifiers.  This means that you can, without
 fear of conflict, use the same name for a scalar variable,
 an array, or a hash--or, for that matter, for a
       filehandle, a directory handle, a subroutine name, a format
 name, or a label.  This means that $foo and @foo are
       two different variables.  It also means that $foo[1] is a
       part of @foo, not a part of $foo.  This may seem a bit
       weird, but that's okay, because it is weird.

       Because variable references always start with '$', '@', or
       '%', the "reserved" words aren't in fact reserved with
       respect to variable names.  They are reserved with respect
       to labels and filehandles, however, which don't have an
       initial special character.  You can't have a filehandle
       named "log", for instance.  Hint: you could say
       "open(LOG,'logfile')" rather than "open(log,'logfile')".
       Using uppercase filehandles also improves readability and
       protects you from conflict with future reserved words.
       Case is significant--"FOO", "Foo", and "foo" are all different
 names.  Names that start with a letter or underscore
 may also contain digits and underscores.

       It is possible to replace such an alphanumeric name with
       an expression that returns a reference to the appropriate
       type.  For a description of this, see perlref.

       Names that start with a digit may contain only more digits.
  Names that do not start with a letter, underscore,
       digit or a caret (i.e.  a control character) are limited
       to one character, e.g.,  $% or $$.  (Most of these one
       character names have a predefined significance to Perl.
       For instance, $$ is the current process id.)

       The interpretation of operations and values in Perl sometimes
 depends on the requirements of the context around
       the operation or value.  There are two major contexts:
       list and scalar.  Certain operations return list values in
       contexts wanting a list, and scalar values otherwise.  If
       this is true of an operation it will be mentioned in the
       documentation for that operation.  In other words, Perl
       overloads certain operations based on whether the expected
       return value is singular or plural.  Some words in English
       work this way, like "fish" and "sheep".

       In a reciprocal fashion, an operation provides either a
       scalar or a list context to each of its arguments.  For
       example, if you say

           int( <STDIN> )

       the integer operation provides scalar context for the <>
       operator, which responds by reading one line from STDIN
       and passing it back to the integer operation, which will
       then find the integer value of that line and return  that.
       If, on the other hand, you say

           sort( <STDIN> )

       then the sort operation provides list context for <>,
       which will proceed to read every line available up to the
       end of file, and pass that list of lines back to the sort
       routine, which will then sort those lines and return them
       as a list to whatever the context of the sort was.

       Assignment is a little bit special in that it uses its
       left argument to determine the context for the right argument.
  Assignment to a scalar evaluates the right-hand
       side in scalar context, while assignment to an array or
       hash evaluates the righthand side in list context.
       Assignment to a list (or slice, which is just a list anyway)
 also evaluates the righthand side in list context.

       When you use the "use warnings" pragma or Perl's -w command-line
 option, you may see warnings about useless uses
       of constants or functions in "void context".  Void context
       just means the value has been discarded, such as a statement
 containing only ""fred";" or "getpwuid(0);".  It
       still counts as scalar context for functions that care
       whether or not they're being called in list context.

       User-defined subroutines may choose to care whether they
       are being called in a void, scalar, or list context.  Most
       subroutines do not need to bother, though.  That's because
       both scalars and lists are automatically interpolated into
       lists.  See "wantarray" in perlfunc for how you would
       dynamically discern your function's calling context.

       Scalar values    [Toc]    [Back]

       All data in Perl is a scalar, an array of scalars, or a
       hash of scalars.  A scalar may contain one single value in
       any of three different flavors: a number, a string, or a
       reference.  In general, conversion from one form to
       another is transparent.  Although a scalar may not
       directly hold multiple values, it may contain a reference
       to an array or hash which in turn contains multiple values.

       Scalars aren't necessarily one thing or another.  There's
       no place to declare a scalar variable to be of type
       "string", type "number", type "reference", or anything
       else.  Because of the automatic conversion of scalars,
       operations that return scalars don't need to care (and in
       fact, cannot care) whether their caller is looking for a
       string, a number, or a reference.  Perl is a contextually
       polymorphic language whose scalars can be strings, numbers,
 or references (which includes objects).  Although
       strings and numbers are considered pretty much the same
       thing for nearly all purposes, references are
       strongly-typed, uncastable pointers with builtin reference-counting
 and destructor invocation.

       A scalar value is interpreted as TRUE in the Boolean sense
       if it is not the null string or the number 0 (or its
       string equivalent, "0").  The Boolean context is just a
       special kind of scalar context where no conversion to a
       string or a number is ever performed.

       There are actually two varieties of null strings (sometimes
 referred to as "empty" strings), a defined one and
       an undefined one.  The defined version is just a string of
       length zero, such as "".  The undefined version is the
       value that indicates that there is no real value for something,
 such as when there was an error, or at end of file,
       or when you refer to an uninitialized variable or element
       of an array or hash.  Although in early versions of Perl,
       an undefined scalar could become defined when first used
       in a place expecting a defined value, this no longer happens
 except for rare cases of autovivification as
       explained in perlref.  You can use the defined() operator
       to determine whether a scalar value is defined (this has
       no meaning on arrays or hashes), and the undef() operator
       to produce an undefined value.

       To find out whether a given string is a valid non-zero
       number, it's sometimes enough to test it against both
       numeric 0 and also lexical "0" (although this will cause
       noises if warnings are on).  That's because strings that
       aren't numbers count as 0, just as they do in awk:
           if ($str == 0 && $str ne "0")  {
               warn "That doesn't look like a number";

       That method may be best because otherwise you won't treat
       IEEE notations like "NaN" or "Infinity" properly.  At
       other times, you might prefer to determine whether string
       data can be used numerically by calling the POSIX::str-
       tod() function or by inspecting your string with a regular
       expression (as documented in perlre).

           warn "has nondigits"         if      /            warn
"not a natural number" unless /^+$/;             # rejects -3
           warn    "not   an   integer"         unless   /^-?+$/;
# rejects +3
           warn "not an integer"       unless /^[+-]?+$/;
           warn "not a decimal number" unless  /^-?+.?*$/;      #
rejects .2
           warn     "not     a     decimal     number"     unless
           warn "not a C float"
               unless /^([+-]?)(?=|.)*(.*)?([Ee]([+-]?+))?$/;

       The length of an array is a scalar value.  You may find
       the length of array @days by evaluating $#days, as in csh.
       However, this isn't the length of the array; it's the subscript
 of the last element, which is a different value
       since there is ordinarily a 0th element.  Assigning to
       $#days actually changes the length of the array.  Shortening
 an array this way destroys intervening values.
       Lengthening an array that was previously shortened does
       not recover values that were in those elements.  (It used
       to do so in Perl 4, but we had to break this to make sure
       destructors were called when expected.)

       You can also gain some minuscule measure of efficiency by
       pre-extending an array that is going to get big.  You can
       also extend an array by assigning to an element that is
       off the end of the array.  You can truncate an array down
       to nothing by assigning the null list () to it.  The following
 are equivalent:

           @whatever = ();
           $#whatever = -1;

       If you evaluate an array in scalar context, it returns the
       length of the array.  (Note that this is not true of
       lists, which return the last value, like the C comma operator,
 nor of built-in functions, which return whatever
       they feel like returning.)  The following is always true:

           scalar(@whatever) == $#whatever - $[ + 1;

       Version 5 of Perl changed the semantics of $[: files that
       don't set the value of $[ no longer need to worry about
       whether another file changed its value.  (In other words,
       use of $[ is deprecated.)  So in general you can assume

           scalar(@whatever) == $#whatever + 1;

       Some programmers choose to use an explicit conversion so
       as to leave nothing to doubt:

           $element_count = scalar(@whatever);

       If you evaluate a hash in scalar context, it returns false
       if the hash is empty.  If there are any key/value pairs,
       it returns true; more precisely, the value returned is a
       string consisting of the number of used buckets and the
       number of allocated buckets, separated by a slash.  This
       is pretty much useful only to find out whether Perl's
       internal hashing algorithm is performing poorly on your
       data set.  For example, you stick 10,000 things in a hash,
       but evaluating %HASH in scalar context reveals "1/16",
       which means only one out of sixteen buckets has been
       touched, and presumably contains all 10,000 of your items.
       This isn't supposed to happen.

       You can preallocate space for a hash by assigning to the
       keys() function.  This rounds up the allocated buckets to
       the next power of two:

           keys(%users) = 1000;                 #  allocate  1024

       Scalar value constructors    [Toc]    [Back]

       Numeric literals are specified in any of the following
       floating point or integer formats:

           .23E-10             # a very small number
           3.14_15_92          # a very important number
           4_294_967_296       # underscore for legibility
           0xff                # hex
           0xdead_beef         # more hex
           0377                # octal
           0b011011            # binary

       You are allowed to use underscores (underbars) in numeric
       literals between digits for legibility.  You could, for
       example, group binary digits by threes (as for a Unixstyle
 mode argument such as 0b110_100_100) or by fours (to
       represent nibbles, as in 0b1010_0110) or in other  groups.

       String literals are usually delimited by either single or
       double quotes.  They work much like quotes in the standard
       Unix shells: double-quoted string literals are subject to
       backslash and variable substitution; single-quoted strings
       are not (except for "'" and "\").  The usual C-style
       backslash rules apply for making characters such as newline,
 tab, etc., as well as some more exotic forms.  See
       "Quote and Quote-like Operators" in perlop for a list.

       Hexadecimal, octal, or binary, representations in string
       literals (e.g. '0xff') are not automatically converted to
       their integer representation.  The hex() and oct() functions
 make these conversions for you.  See "hex" in perlfunc
 and "oct" in perlfunc for more details.

       You can also embed newlines directly in your strings,
       i.e., they can end on a different line than they begin.
       This is nice, but if you forget your trailing quote, the
       error will not be reported until Perl finds another line
       containing the quote character, which may be much further
       on in the script.  Variable substitution inside strings is
       limited to scalar variables, arrays, and array or hash
       slices.  (In other words, names beginning with $ or @,
       followed by an optional bracketed expression as a subscript.)
  The following code segment prints out "The price
       is $100."

           $Price = '$100';    # not interpolated
           print "The price is $Price.0;     # interpolated

       There is no double interpolation in Perl, so the $100 is
       left as is.

       As in some shells, you can enclose the variable name in
       braces to disambiguate it from following alphanumerics
       (and underscores).  You must also do this when interpolating
 a variable into a string to separate the variable name
       from a following double-colon or an apostrophe, since
       these would be otherwise treated as a package separator:

           $who = "Larry";
           print PASSWD "${who}::0:0:Superuser:/:/bin/perl0;
           print "We use ${who}speak when ${who}'s here.0;

       Without the braces, Perl would have looked for a $whospeak,
 a $who::0, and a $who's variable.  The last two
       would be the $0 and the $s variables in the (presumably)
       non-existent package "who".

       In fact, an identifier within such curlies is forced to be
       a string, as is any simple identifier within a hash subscript.
  Neither need quoting.  Our earlier example,
       $days{'Feb'} can be written as $days{Feb} and the quotes
       will be assumed automatically.  But anything more complicated
 in the subscript will be interpreted as an expression.

       Version Strings
       Note: Version Strings (v-strings) have been deprecated.
       They will not be available after Perl 5.8.  The marginal
       benefits of v-strings were greatly outweighed by the
       potential for Surprise and Confusion.

       A literal of the form "v1.20.300.4000" is parsed as a
       string composed of characters with the specified ordinals.
       This form, known as v-strings, provides an alternative,
       more readable way to construct strings, rather than use
       the somewhat less readable interpolation form
       "a0}".  This is useful for repre

       senting Unicode strings, and for comparing version "numbers"
 using the string comparison operators, "cmp", "gt",
       "lt" etc.  If there are two or more dots in the literal,
       the leading "v" may be omitted.

           print  v9786;              # prints UTF-8 encoded SMILEY, "}"

           print v102.111.111;       # prints "foo"
           print 102.111.111;        # same

       Such literals are accepted by both "require" and "use" for
       doing a version check.  The $^V special variable also contains
 the running Perl interpreter's version in this form.
       See "$^V" in perlvar.  Note that using the v-strings for
       IPv4 addresses is not portable unless you also use the
       inet_aton()/inet_ntoa() routines of the Socket package.

       Note that since Perl 5.8.1 the single-number v-strings
       (like "v65") are not v-strings before the "=>" operator
       (which is usually used to separate a hash key from a hash
       value), instead they are interpreted as literal strings
       ('v65').  They were v-strings from Perl 5.6.0 to Perl
       5.8.0, but that caused more confusion and breakage than
       good.  Multi-number v-strings like "v65.66" and 65.66.67
       continue to be v-strings always.

       Special Literals

       The special literals __FILE__, __LINE__, and __PACKAGE__
       represent the current filename, line number, and package
       name at that point in your program.  They may be used only
       as separate tokens; they will not be interpolated into
       strings.  If there is no current package (due to an empty
       "package;" directive), __PACKAGE__ is the undefined value.

       The two control characters ^D and ^Z, and the tokens
       __END__ and __DATA__ may be used to indicate the logical
       end of the script before the actual end of file.  Any following
 text is ignored.

       Text after __DATA__ but may be read via the filehandle
       "PACKNAME::DATA", where "PACKNAME" is the package that was
       current when the __DATA__ token was encountered.  The
       filehandle is left open pointing to the contents after
       __DATA__.  It is the program's responsibility to "close
       DATA" when it is done reading from it.  For compatibility
       with older scripts written before __DATA__ was introduced,
       __END__ behaves like __DATA__ in the toplevel script (but
       not in files loaded with "require" or "do") and leaves the
       remaining contents of the file accessible via

       See SelfLoader for more description of __DATA__, and an
       example of its use.  Note that you cannot read from the
       DATA filehandle in a BEGIN block: the BEGIN block is executed
 as soon as it is seen (during compilation), at which
       point the corresponding __DATA__ (or __END__) token has
       not yet been seen.


       A word that has no other interpretation in the grammar
       will be treated as if it were a quoted string.  These are
       known as "barewords".  As with filehandles and labels, a
       bareword that consists entirely of lowercase letters risks
       conflict with future reserved words, and if you use the
       "use warnings" pragma or the -w switch, Perl will warn you
       about any such words.  Some people may wish to outlaw
       barewords entirely.  If you say

           use strict 'subs';

       then any bareword that would NOT be interpreted as a subroutine
 call produces a compile-time error instead.  The
       restriction lasts to the end of the enclosing block.  An
       inner block may countermand this by saying "no strict

       Array Joining Delimiter

       Arrays and slices are interpolated into double-quoted
       strings by joining the elements with the delimiter specified
 in the $" variable ($LIST_SEPARATOR if "use English;"
       is specified), space by default.  The following are equivalent:

           $temp = join($", @ARGV);
           system "echo $temp";

           system "echo @ARGV";

       Within search patterns (which also undergo double-quotish
       substitution) there is an unfortunate ambiguity:  Is
       "/$foo[bar]/" to be interpreted as "/${foo}[bar]/" (where
       "[bar]" is a character class for the regular expression)
       or as "/${foo[bar]}/" (where "[bar]" is the subscript to
       array @foo)?  If @foo doesn't otherwise exist, then it's
       obviously a character class.  If @foo exists, Perl takes a
       good guess about "[bar]", and is almost always right.  If
       it does guess wrong, or if you're just plain paranoid, you
       can force the correct interpretation with curly braces as

       If you're looking for the information on how to use
       here-documents, which used to be here, that's been moved
       to "Quote and Quote-like Operators" in perlop.

       List value constructors    [Toc]    [Back]

       List values are denoted by separating individual values by
       commas (and enclosing the list in parentheses where precedence
 requires it):


       In a context not requiring a list value, the value of what
       appears to be a list literal is simply the value of the
       final element, as with the C comma operator.  For example,

           @foo = ('cc', '-E', $bar);

       assigns the entire list value to array @foo, but

           $foo = ('cc', '-E', $bar);

       assigns the value of variable $bar to the scalar variable
       $foo.  Note that the value of an actual array in scalar
       context is the length of the array; the following assigns
       the value 3 to $foo:

           @foo = ('cc', '-E', $bar);
           $foo = @foo;                # $foo gets 3

       You may have an optional comma before the closing parenthesis
 of a list literal, so that you can say:

           @foo = (

       To use a here-document to assign an array, one line per
       element, you might use an approach like this:

           @sauces = <<End_Lines =~ m/(*/g;
               normal tomato
               spicy tomato
               green chile
               white wine
       LISTs do automatic interpolation of sublists.  That is,
       when a LIST is evaluated, each element of the list is
       evaluated in list context, and the resulting list value is
       interpolated into LIST just as if each individual element
       were a member of LIST.  Thus arrays and hashes lose their
       identity in a LIST--the list


       contains all the elements of @foo followed by all the elements
 of @bar, followed by all the elements returned by
       the subroutine named SomeSub called in list context, followed
 by the key/value pairs of %glarch.  To make a list
       reference that does NOT interpolate, see perlref.

       The null list is represented by ().  Interpolating it in a
       list  has no effect.  Thus ((),(),()) is equivalent to ().
       Similarly, interpolating an array with no elements is the
       same as if no array had been interpolated at that point.

       This interpolation combines with the facts that the opening
 and closing parentheses are optional (except when necessary
 for precedence) and lists may end with an optional
       comma to mean that multiple commas within lists are legal
       syntax. The list "1,,3" is a concatenation of two lists,
       "1," and 3, the first of which ends with that optional
       comma.  "1,,3" is "(1,),(3)" is "1,3" (And similarly for
       "1,,,3" is "(1,),(,),3" is "1,3" and so on.)  Not that
       we'd advise you to use this obfuscation.

       A  list value may also be subscripted like a normal array.
       You must put the list in parentheses to avoid ambiguity.
       For example:

           # Stat returns list value.
           $time = (stat($file))[8];

           # SYNTAX ERROR HERE.
           $time = stat($file)[8];  # OOPS, FORGOT PARENTHESES

           # Find a hex digit.
           $hexdigit = ('a','b','c','d','e','f')[$digit-10];

           # A "reverse comma operator".
           return (pop(@foo),pop(@foo))[0];

       Lists may be assigned to only when each element of the
       list is itself legal to assign to:

           ($a, $b, $c) = (1, 2, 3);

           ($map{'red'}, $map{'blue'}, $map{'green'})  =  (0x00f,
0x0f0, 0xf00);

       An exception to this is that you may assign to "undef" in
       a list.  This is useful for throwing away some of the
       return values of a function:

           ($dev, $ino, undef, undef, $uid, $gid) = stat($file);

       List assignment in scalar context returns the number of
       elements produced by the expression on the right side of
       the assignment:

           $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not
           $x = (($foo,$bar) = f());           # set $x to  f()'s
return count

       This is handy when you want to do a list assignment in a
       Boolean context, because most list functions return a null
       list when finished, which when assigned produces a 0,
       which is interpreted as FALSE.

       It's also the source of a useful idiom for executing a
       function or performing an operation in list context and
       then counting the number of return values, by assigning to
       an empty list and then using that assignment in scalar
       context. For example, this code:

           $count = () = $string =~ /+/g;

       will place into $count the number of digit groups found in
       $string.  This happens because the pattern match is in
       list context (since it is being assigned to the empty
       list), and will therefore return a list of all matching
       parts of the string. The list assignment in scalar context
       will translate that into the number of elements (here, the
       number of times the pattern matched) and assign that to
       $count. Note that simply using

           $count = $string =~ /+/g;

       would not have worked, since a pattern match in scalar
       context will only return true or false, rather than a
       count of matches.

       The final element of a list assignment may be an array or
       a hash:

           ($a, $b, @rest) = split;
           my($a, $b, %rest) = @_;

       You can actually put an array or hash anywhere in the
       list, but the first one in the list will soak up all the
       values, and anything after it will become undefined.  This
       may be useful in a my() or local().

       A hash can be initialized using a literal list holding
       pairs of items to be interpreted as a key and a value:
           # same as map assignment above
           %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);

       While literal lists and named arrays are often interchangeable,
 that's not the case for hashes.  Just because
       you can subscript a list value like a normal array does
       not mean that you can subscript a list value as a hash.
       Likewise, hashes included as parts of other lists (including
 parameters lists and return lists from functions)
       always flatten out into key/value pairs.  That's why it's
       good to use references sometimes.

       It is often more readable to use the "=>" operator between
       key/value pairs.  The "=>" operator is mostly just a more
       visually distinctive synonym for a comma, but it also
       arranges for its left-hand operand to be interpreted as a
       string -- if it's a bareword that would be a legal simple
       identifier ("=>" doesn't quote compound identifiers, that
       contain double colons). This makes it nice for initializing

           %map = (
                        red   => 0x00f,
                        blue  => 0x0f0,
                        green => 0xf00,

       or for initializing hash references to be used as records:

           $rec = {
                       witch => 'Mable the Merciless',
                       cat   => 'Fluffy the Ferocious',
                       date  => '10/31/1776',

       or for using call-by-named-parameter to complicated functions:

          $field = $query->radio_group(
                      name      => 'group_name',
                      values    => ['eenie','meenie','minie'],
                      default   => 'meenie',
                      linebreak => 'true',
                      labels    => labels

       Note that just because a hash is initialized in that order
       doesn't mean that it comes out in that order.  See "sort"
       in perlfunc for examples of how to arrange for an output

       An array is subscripted by specifying a dollary sign
       ("$"), then the name of the array (without the leading
       "@"), then the subscript inside square brackets.  For

           @myarray = (5, 50, 500, 5000);
           print "Element Number 2 is", $myarray[2], "0;

       The array indices start with 0. A negative subscript
       retrieves its value from the end.  In our example,
       $myarray[-1] would have been 5000, and $myarray[-2] would
       have been 500.

       Hash subscripts are similar, only instead of square brackets
 curly brackets are used. For example:

           %scientists =
               "Newton" => "Isaac",
               "Einstein" => "Albert",
               "Darwin" => "Charles",
               "Feynman" => "Richard",

           print "Darwin's First  Name  is  ",  $scientists{"Darwin"}, "0;

       Slices    [Toc]    [Back]

       A common way to access an array or a hash is one scalar
       element at a time.  You can also subscript a list to get a
       single element from it.

           $whoami = $ENV{"USER"};             # one element from
the hash
           $parent = $ISA[0];                  # one element from
the array
           $dir    = (getpwnam("daemon"))[7];   #  likewise,  but
with list

       A slice accesses several elements of a list, an array, or
       a hash simultaneously using a list of subscripts.  It's
       more convenient than writing out the individual elements
       as a list of separate scalar values.

           ($him,  $her)    =  @folks[0,-1];              # array
           @them          = @folks[0 ..  3];             #  array
           ($who,  $home)   =  @ENV{"USER",  "HOME"};      # hash
           ($uid, $dir)    =  (getpwnam("daemon"))[2,7];  #  list

       Since you can assign to a list of variables, you can also
       assign to an array or hash slice.
           @days[3..5]    = qw/Wed Thu Fri/;
                          = (0xff0000, 0x0000ff, 0x00ff00);
           @folks[0, -1]  = @folks[-1, 0];

       The previous assignments are exactly equivalent to

           ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
           ($colors{'red'}, $colors{'blue'}, $colors{'green'})
                          = (0xff0000, 0x0000ff, 0x00ff00);
           ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);

       Since changing a slice changes the original array or hash
       that it's slicing, a "foreach" construct will alter
       some--or even all--of the values of the array or hash.

           foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }

           foreach (@hash{qw[key1 key2]}) {
               s/^/;           # trim leading whitespace
               s/;           # trim trailing whitespace

       A slice of an empty list is still an empty list.  Thus:

           @a = ()[1,0];           # @a has no elements
           @b = (@a)[0,1];         # @b has no elements
           @c = (0,1)[2,3];        # @c has no elements


           @a = (1)[1,0];          # @a has two elements
           @b = (1,undef)[1,0,2];  # @b has three elements

       This makes it easy to write loops that terminate when a
       null list is returned:

           while ( ($home, $user) = (getpwent)[7,0]) {
               printf "%-8s %s0, $user, $home;

       As noted earlier in this document, the scalar sense of
       list assignment is the number of elements on the righthand
 side of the assignment.  The null list contains no
       elements, so when the password file is exhausted, the
       result is 0, not 2.

       If you're confused about why you use an '@' there on a
       hash slice instead of a '%', think of it like this.  The
       type of bracket (square or curly) governs whether it's an
       array or a hash being looked at.  On the other hand, the
       leading symbol ('$' or '@') on the array or hash indicates
       whether you are getting back a singular value (a scalar)
       or a plural one (a list).

       Typeglobs and Filehandles    [Toc]    [Back]

       Perl uses an internal type called a typeglob to hold an
       entire symbol table entry.  The type prefix of a typeglob
       is a "*", because it represents all types.  This used to
       be the preferred way to pass arrays and hashes by reference
 into a function, but now that we have real references,
 this is seldom needed.

       The main use of typeglobs in modern Perl is create symbol
       table aliases.  This assignment:

           *this = *that;

       makes $this an alias for $that, @this an alias for @that,
       %this an alias for %that, &this an alias for &that, etc.
       Much safer is to use a reference.  This:

           local *Here::blue = here::green;

       temporarily makes $Here::blue an alias for $There::green,
       but doesn't make @Here::blue an alias for @There::green,
       or %Here::blue an alias for %There::green, etc.  See "Symbol
 Tables" in perlmod for more examples of this.  Strange
       though this may seem, this is the basis for the whole module
 import/export system.

       Another use for typeglobs is to pass filehandles into a
       function or to create new filehandles.  If you need to use
       a typeglob to save away a filehandle, do it this way:

           $fh = *STDOUT;

       or perhaps as a real reference, like this:

           $fh = TDOUT;

       See perlsub for examples of using these as indirect filehandles
 in functions.

       Typeglobs are also a way to create a local filehandle
       using the local() operator.  These last until their block
       is exited, but may be passed back.  For example:

           sub newopen {
               my $path = shift;
               local  *FH;  # not my!
               open   (FH, $path)          or  return undef;
               return *FH;
           $fh = newopen('/etc/passwd');
       Now that we have the *foo{THING} notation, typeglobs
       aren't used as much for filehandle manipulations, although
       they're still needed to pass brand new file and directory
       handles into or out of functions. That's because *HANDLE{IO}
 only works if HANDLE has already been used as a
       handle.  In other words, *FH must be used to create new
       symbol table entries; *foo{THING} cannot.  When in doubt,
       use *FH.

       All functions that are capable of creating filehandles
       (open(), opendir(), pipe(), socketpair(), sysopen(),
       socket(), and accept()) automatically create an anonymous
       filehandle if the handle passed to them is an uninitialized
 scalar variable. This allows the constructs such as
       "open(my $fh, ...)" and "open(local $fh,...)" to be used
       to create filehandles that will conveniently be closed
       automatically when the scope ends, provided there are no
       other references to them. This largely eliminates the need
       for typeglobs when opening filehandles that must be passed
       around, as in the following example:

           sub myopen {
               open my $fh, "@_"
                    or die "Can't open '@_': $!";
               return $fh;

               my $f = myopen("</etc/motd");
               print <$f>;
               # $f implicitly closed here

       Note that if an initialized scalar variable is used
       instead the result is different: "my $fh='zzz'; open($fh,
       ...)" is equivalent to "open( *{'zzz'}, ...)".  "use
       strict 'refs'" forbids such practice.

       Another way to create anonymous filehandles is with the
       Symbol module or with the IO::Handle module and its ilk.
       These modules have the advantage of not hiding different
       types of the same name during the local().  See the bottom
       of "open()" in perlfunc for an example.

SEE ALSO    [Toc]    [Back]

       See perlvar for a description of Perl's built-in variables
       and a discussion of legal variable names.  See perlref,
       perlsub, and "Symbol Tables" in perlmod for more discussion
 on typeglobs and the *foo{THING} syntax.

perl v5.8.5                 2002-11-06                         17
[ Back ]
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