perldata.pod

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=head1 NAME

perldata - Perl data types

=head1 DESCRIPTION

=head2 Variable names
X<variable, name> X<variable name> X<data type> X<type>

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 L<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 I<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 C<::> (or by the slightly
archaic C<'>); all but the last are interpreted as names of packages,
to locate the namespace in which to look up the final identifier
(see L<perlmod/Packages> 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 L<perlref>.
X<identifier>

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 C<$> (see L<perlop> and L<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 L<perlvar>.
X<variable, built-in>

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.
X<scalar>

    $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.
X<array>

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

Entire hashes are denoted by '%':
X<hash>

    %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 $foo
and @foo are two different variables.  It also means that C<$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.
X<namespace>

Because variable references always start with '$', '@', or '%', the
"reserved" words aren't in fact reserved with respect to variable
names.  They I<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
C<open(LOG,'logfile')> rather than C<open(log,'logfile')>.  Using
uppercase filehandles also improves readability and protects you
from conflict with future reserved words.  Case I<is> significant--"FOO",
"Foo", and "foo" are all different names.  Names that start with a
letter or underscore may also contain digits and underscores.
X<identifier, case sensitivity>
X<case>

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 L<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.,  C<$%> or
C<$$>.  (Most of these one character names have a predefined
significance to Perl.  For instance, C<$$> is the current process
id.)

=head2 Context
X<context> X<scalar context> X<list context>

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 C<use warnings> pragma or Perl's B<-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 C<"fred";> or C<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 L<perlfunc/wantarray>
for how you would dynamically discern your function's calling
context.

=head2 Scalar values
X<scalar> X<number> X<string> X<reference>

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.
X<boolean> X<bool> X<true> X<false> X<truth>

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 C<"">.
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 L<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.
X<defined> X<undefined> X<undef> X<null> X<string, null>

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 B<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 C<NaN> or C<Infinity> properly.  At other times, you
might prefer to determine whether string data can be used numerically
by calling the POSIX::strtod() function or by inspecting your string
with a regular expression (as documented in L<perlre>).

    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 C<$#days>, as in B<csh>.  However, this
isn't the length of the array; it's the subscript of the last element,
which is a different value since there is ordinarily a 0th element.
Assigning to C<$#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.)
X<$#> X<array, length>

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:
X<array, length>

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

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

    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 C<"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.  If a tied hash
is evaluated in scalar context, a fatal error will result, since this
bucket usage information is currently not available for tied hashes.
X<hash, scalar context> X<hash, bucket> X<bucket>

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 buckets

=head2 Scalar value constructors
X<scalar, literal> X<scalar, constant>