PERLDATA(1) PERLDATA(1)
NAME
perldata - Perl data types
DESCRIPTION
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
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 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
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 '%':
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%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
id.)
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: 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,
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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
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
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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 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.) 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
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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
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 $100."
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$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 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'.
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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
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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;
List value constructors
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
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individual element were a member of LIST. Thus arrays lose their
identity in a LIST--the list
(@foo,@bar,&SomeSub)
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];
# 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);
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 $x to f()'s return count
This is very handy when you want to do a list assignment in a Boolean
context, because most list functions return a null list when finished,
which when assigned produces a 0, which is interpreted as FALSE.
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The final element may be an array or a hash:
($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
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 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
);
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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
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.
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