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PERLDATA(1)							   PERLDATA(1)

NAME    [Toc]    [Back]

     perldata -	Perl data types

DESCRIPTION    [Toc]    [Back]

     Variable names

     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 which produces a reference to the	value at
     runtime; this is described	in more	detail below, and in the perlref

     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 which 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 which
     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

	 $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 '%':

									Page 1

PERLDATA(1)							   PERLDATA(1)

	 %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	which
     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

     Context    [Toc]    [Back]

     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,

									Page 2

PERLDATA(1)							   PERLDATA(1)

     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.

     Scalar values    [Toc]    [Back]

     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

     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

									Page 3

PERLDATA(1)							   PERLDATA(1)

     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 -w noises).  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's usually preferable because otherwise you won't treat IEEE
     notations like NaN	or Infinity properly.  At other	times you might	prefer
     to	use 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 whole number"   unless /^\d+$/;
	 warn "not an integer"	     unless /^[+-]?\d+$/
	 warn "not a decimal number" unless /^[+-]?\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 csh.  (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 measure of	efficiency by preextending
 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.)  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

									Page 4

PERLDATA(1)							   PERLDATA(1)

	 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 which 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.)

     Scalar value constructors    [Toc]    [Back]

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

	 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

     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."

									Page 5

PERLDATA(1)							   PERLDATA(1)

	 $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,


     can be written as


     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 a 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 -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 'subs'.

									Page 6

PERLDATA(1)							   PERLDATA(1)

     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

     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.

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

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

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

									Page 7

PERLDATA(1)							   PERLDATA(1)

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

     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
	     + 20;

     List value	constructors

     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 the list literal is
     the value of the final element, as	with the C comma operator.  For

	 @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 =	(

     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

									Page 8

PERLDATA(1)							   PERLDATA(1)

     individual	element	were a member of LIST.	Thus arrays 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
     when it's called in a list	context.  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];

	 $time = stat($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);

     Array 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.

									Page 9

PERLDATA(1)							   PERLDATA(1)

     The final element may be an array or a hash:

	 ($a, $b, @rest) = split;
	 local($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

	 # 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 which 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

								       Page 10

PERLDATA(1)							   PERLDATA(1)

     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.

     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.	It also	used to	be the preferred way to	pass
     filehandles into a	function, but now that we have the *foo{THING}
     notation it isn't often needed for	that, either.  It is still needed to
     pass new filehandles into functions (*HANDLE{IO} only works if HANDLE has
     already been used).

     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;

     This is also a way	to create a local filehandle.  For example:

	 sub newopen {
	     my	$path =	shift;
	     local *FH;	 # not my!
	     open (FH, $path) || return	undef;
	     return *FH;
	 $fh = newopen('/etc/passwd');

     Another way to create local filehandles is	with IO::Handle	and its	ilk,
     see the bottom of the open() entry	in the perlfunc	manpage.

     See the perlref manpage, the perlsub manpage, and the section on Symbol
     Tables in the perlmod manpage for more discussion on typeglobs.

								       PPPPaaaaggggeeee 11111111
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