PERLDATA(1) Perl Programmers Reference Guide PERLDATA(1)NAMEperldata - 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 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'})
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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.)
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
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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.
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.
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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
-w noises). That's because strings that aren't numbers
count as 0, just as they do in awk:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
}
That's usually preferable because otherwise you won't
treat IEEE notations like NaN or Infinity properly. At
other times you might prefer to use the POSIX::strtod
function or a regular expression to check whether data is
numeric. See the perlre manpage for details on regular
expressions.
warn "has nondigits" if /\D/;
warn "not a natural number" unless /^\d+$/; # 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 csh.
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(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
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the next power of two:
keys(%users) = 1000; # allocate 1024 buckets
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."
$Price = '$100'; # not interpreted
print "The price is $Price.\n"; # interpreted
As in some shells, you can put curly brackets around the
name to delimit it from following alphanumerics. In fact,
an identifier within such curlies is forced to be a
string, as is any single identifier within a hash
subscript. Our earlier example,
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$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 -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'.
Array variables are interpolated into double-quoted
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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
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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 $x to f()'s return count
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This is very handy when you want to do a list assignment
in a Boolean context, because most list functions return a
null list when finished, which when assigned produces a 0,
which is interpreted as FALSE.
The final element may be an array or a hash:
($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:
$rec = {
witch => 'Mable the Merciless',
cat => 'Fluffy the Ferocious',
date => '10/31/1776',
};
or for using call-by-named-parameter to complicated
functions:
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$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.
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.
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
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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.
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