PERLDATA(1)      Perl Programmers Reference Guide     PERLDATA(1)

NNAAMMEE
       perldata - Perl data types

DDEESSCCRRIIPPTTIIOONN
       VVaarriiaabbllee nnaammeess

       Perl has three data structures: scalars, arrays of
       scalars, and associative arrays of scalars, known as
       "hashes".  Normal arrays are indexed by number, starting
       with 0.  (Negative subscripts count from the end.)  Hash
       arrays are indexed by string.

       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.
       Most often, it consists of 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 ', but that's deprecated); all but the last are
       interpreted as names of packages, to locate the namespace
       in which to look up the final identifier (see the Packages
       entry in the perlmod manpage 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 the perlref
       manpage.

       There are also special variables whose names don't follow
       these rules, so that 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 the
       perlop manpage and the perlre manpage).  In addition,
       several special variables that provide windows into the
       inner working of Perl have names containing punctuation
       characters (see the perlvar manpage).

       Scalar values are always named with '$', even when
       referring to a scalar that is part of an array.  It works
       like the English word "the".  Thus we have:

           $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

       but entire arrays or array slices are denoted by '@',
       which works much like the word "these" or "those":

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

       and entire hashes are denoted by '%':

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

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

       Every variable type has its own namespace.  You can,
       without fear of conflict, use the same name for a scalar
       variable, an array, or a hash (or, for that matter, a
       filehandle, a subroutine 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 and array 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 an object of
       that type.  For a description of this, see the perlref
       manpage.

       Names that start with a digit may contain only more
       digits.  Names that do not start with a letter,
       underscore, or digit 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.)

       CCoonntteexxtt

       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: scalar and list.  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 a scalar context for the
       <STDIN> 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 a list context for
       <STDIN>, 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 righthand
       side in a scalar context, while assignment to an array or
       array slice evaluates the righthand side in a list
       context.  Assignment to a list also evaluates the
       righthand side in a list context.

       User defined subroutines may choose to care whether they
       are being called in a scalar or list context, but most
       subroutines do not need to care, because scalars are
       automatically interpolated into lists.  See the wantarray
       entry in the perlfunc manpage.

       SSccaallaarr vvaalluueess

       All data in Perl is a scalar or an array of scalars or a
       hash of scalars.  Scalar variables may contain various
       kinds of singular data, such as numbers, strings, and
       references.  In general, conversion from one form to
       another is transparent.  (A scalar may not contain
       multiple values, but may contain a reference to an array
       or hash containing multiple values.)  Because of the
       automatic conversion of scalars, operations, and functions
       that return scalars don't need to care (and, in fact,
       can't care) whether the context is looking for a string or
       a number.

       Scalars aren't necessarily one thing or another.  There's
       no place to declare a scalar variable to be of type
       "string", or of type "number", or type "filehandle", or
       anything else.  Perl is a contextually polymorphic
       language whose scalars can be strings, numbers, or
       references (which includes objects).  While 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.

       There are actually two varieties of null scalars: defined
       and undefined.  Undefined null scalars are returned when
       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.  An
       undefined null scalar may become defined the first time
       you use it as if it were defined, but prior to that you
       can use the defined() operator to determine whether the
       value is defined or not.

       To find out whether a given string is a valid nonzero
       number, it's usually enough to test it against both
       numeric 0 and also lexical "0" (although this will cause
       --ww noises).  That's because strings that aren't numbers
       count as 0, just as they do in aawwkk:

           if ($str == 0 && $str ne "0")  {
               warn "That doesn't look like a number";
           }

       That's usually preferable because otherwise you won't
       treat IEEE notations like NaN or Infinity properly.  At
       other times you might prefer to use the POSIX::strtod
       function or a regular expression to check whether data is
       numeric.  See the perlre manpage for details on regular
       expressions.

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

       The length of an array is a scalar value.  You may find
       the length of array @days by evaluating $#days, as in ccsshh.
       (Actually, it's not the length of the array, it's the
       subscript of the last element, because there is
       (ordinarily) a 0th element.)  Assigning to $#days changes
       the length of the array.  Shortening an array by this
       method destroys intervening values.  Lengthening an array
       that was previously shortened NO LONGER recovers the
       values that were in those elements.  (It used to in Perl
       4, but we had to break this to make sure destructors were
       called when expected.)  You can also gain some miniscule
       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 a named array in a 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
       that

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

       Some programmers choose to use an explicit conversion so
       nothing's left to doubt:

           $element_count = scalar(@whatever);

       If you evaluate a hash in a scalar context, it returns a
       value that is true if and only if the hash contains any
       key/value pairs.  (If there are any key/value pairs, 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 (compiled in) 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 bucked to
       the next power of two:

           keys(%users) = 1000;                # allocate 1024 buckets

       SSccaallaarr vvaalluuee ccoonnssttrruuccttoorrss

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

           12345
           12345.67
           .23E-10
           0xffff              # hex
           0377                # octal
           4_294_967_296       # underline for legibility

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

       Octal or hex representations in string literals (e.g.
       '0xffff') are not automatically converted to their integer
       representation.  The hex() and oct() functions make these
       conversions for you.  See the hex entry in the perlfunc
       manpage and the oct entry in the perlfunc manpage 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 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 interpreted
           print "The price is $Price.\n";     # interpreted

       As in some shells, you can put curly brackets around the
       name to delimit it from following alphanumerics.  In fact,
       an identifier within such curlies is forced to be a
       string, as is any single identifier within a hash
       subscript.  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.

       Note that a single-quoted string must be separated from a
       preceding word by a space, because single quote is a valid
       (though deprecated) character in a variable name (see the
       Packages entry in the perlmod manpage).

       Three special literals are __FILE__, __LINE__, and
       __PACKAGE__, which 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 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, but may be read via
       a DATA filehandle: main::DATA for __END__, or
       PACKNAME::DATA (where PACKNAME is the current package) for
       __DATA__.  The two control characters ^D and ^Z are
       synonyms for __END__ (or __DATA__ in a module).  See the
       SelfLoader manpage 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 --ww
       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
       'subs'.

       Array variables are interpolated into double-quoted
       strings by joining all the elements of the array with the
       delimiter specified in the $" variable ($LIST_SEPARATOR in
       English), 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 a bad 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 brackets
       as above.

       A line-oriented form of quoting is based on the shell
       "here-doc" syntax.  Following a << you specify a string to
       terminate the quoted material, and all lines following the
       current line down to the terminating string are the value
       of the item.  The terminating string may be either an
       identifier (a word), or some quoted text.  If quoted, the
       type of quotes you use determines the treatment of the
       text, just as in regular quoting.  An unquoted identifier
       works like double quotes.  There must be no space between
       the << and the identifier.  (If you put a space it will be
       treated as a null identifier, which is valid, and matches
       the first empty line.)  The terminating string must appear
       by itself (unquoted and with no surrounding whitespace) on
       the terminating line.

               print <<EOF;
           The price is $Price.
           EOF

               print <<"EOF";  # same as above
           The price is $Price.
           EOF

               print <<`EOC`;  # execute commands
           echo hi there
           echo lo there
           EOC

               print <<"foo", <<"bar"; # you can stack them
           I said foo.
           foo
           I said bar.
           bar

               myfunc(<<"THIS", 23, <<'THAT');
           Here's a line
           or two.
           THIS
           and here's another.
           THAT

       Just don't forget that you have to put a semicolon on the
       end to finish the statement, as Perl doesn't know you're
       not going to try to do this:

               print <<ABC
           179231
           ABC
               + 20;

       LLiisstt vvaalluuee ccoonnssttrruuccttoorrss

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

           (LIST)

       In a context not requiring a list value, the value of the
       list literal is 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 variable foo.  Note
       that the value of an actual array in a 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 = (
               1,
               2,
               3,
           );

       LISTs do automatic interpolation of sublists.  That is,
       when a LIST is evaluated, each element of the list is
       evaluated in a 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

           (@foo,@bar,&SomeSub,%glarch)

       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 a list context,
       followed by the key/value pairs of %glarch.  To make a
       list reference that does NOT interpolate, see the perlref
       manpage.

       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.

       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];

       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);

       Lists may be assigned to if and only if each element of
       the list is legal to assign to:

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

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

       List assignment in a 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 2
           $x = (($foo,$bar) = f());           # set $x to f()'s return count

       This is very 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.

       The final element 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 get a null value.  This
       may be useful in a local() or my().

       A hash literal contains pairs of values 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 usually
       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
       identifier.  This makes it nice for initializing hashes:

           %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 the
       sort entry in the perlfunc manpage for examples of how to
       arrange for an output ordering.

       TTyyppeegglloobbss aanndd FFiilleehhaannddlleess

       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 = \$There::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 the
       section on Symbol Tables in the perlmod manpage 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 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 = \*STDOUT;

       See the perlsub manpage 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 can be used to create new
       symbol table entries, but *foo{THING} cannot.

       Another way to create anonymous filehandles is 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 the open() entry in
       the perlfunc manpage for an example.

       See the perlref manpage, the perlsub manpage, and the
       section on Symbol Tables in the perlmod manpage for more
       discussion on typeglobs and the *foo{THING} syntax.

29/Dec/1998            perl 5.005, patch 03                     1