perldata - Perl data types
Variable names
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 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 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
the slightly archaic "'"); all but the last are interpreted
as names of packages, to locate the namespace in
which to look up the final identifier (see "Packages" in
perlmod 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 perlref.
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 "$" (see perlop and perlre). In addition, several
special variables that provide windows into the inner
working of Perl have names containing punctuation characters
and control characters. These are documented in perlvar.
Scalar values are always named with '$', 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.
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$days{'Feb'} # the 'Feb' value from hash %days
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are
denoted by '@', which works much like the word "these" or
"those" does in English, in that it indicates multiple
values are expected.
@days # ($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 '%':
%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 $foo[1] is a
part of @foo, not a part of $foo. This may seem a bit
weird, but that's okay, because it is weird.
Because variable references always start with '$', '@', 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 the appropriate
type. For a description of this, see 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., $% 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:
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 "use warnings" pragma or Perl's -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 ""fred";" or "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 "wantarray" in perlfunc for how you would
dynamically discern your function's calling context.
Scalar values [Toc] [Back]
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.
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 "". 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 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.
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 awk:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
}
That method may be best because otherwise you won't treat
IEEE notations like "NaN" or "Infinity" properly. At
other times, you might prefer to determine whether string
data can be used numerically by calling the POSIX::str-
tod() function or by inspecting your string with a regular
expression (as documented in perlre).
warn "has nondigits" if / warn
"not a natural number" unless /^+$/; # rejects -3
warn "not an integer" unless /^-?+$/;
# rejects +3
warn "not an integer" unless /^[+-]?+$/;
warn "not a decimal number" unless /^-?+.?*$/; #
rejects .2
warn "not a decimal number" unless
/^-?(?:+(?:.*)?|.+)$/;
warn "not a C float"
unless /^([+-]?)(?=|.)*(.*)?([Ee]([+-]?+))?$/;
The length of an array is a scalar value. You may find
the length of array @days by evaluating $#days, as in 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
$#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.)
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:
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
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 "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 buckets to
the next power of two:
keys(%users) = 1000; # allocate 1024
buckets
Scalar value constructors [Toc] [Back]
Numeric literals are specified in any of the following
floating point or integer formats:
12345
12345.67
.23E-10 # a very small number
3.14_15_92 # a very important number
4_294_967_296 # underscore for legibility
0xff # hex
0xdead_beef # more hex
0377 # octal
0b011011 # binary
You are allowed to use underscores (underbars) in numeric
literals between digits for legibility. You could, for
example, group binary digits by threes (as for a Unixstyle
mode argument such as 0b110_100_100) or by fours (to
represent nibbles, as in 0b1010_0110) or in other groups.
String literals are usually delimited by either single or
double quotes. They work much like quotes in the standard
Unix shells: double-quoted string literals are subject to
backslash and variable substitution; single-quoted strings
are not (except for "'" and "\"). The usual C-style
backslash rules apply for making characters such as newline,
tab, etc., as well as some more exotic forms. See
"Quote and Quote-like Operators" in perlop for a list.
Hexadecimal, octal, or binary, representations in string
literals (e.g. '0xff') are not automatically converted to
their integer representation. The hex() and oct() functions
make these conversions for you. See "hex" in perlfunc
and "oct" in perlfunc 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 or hash
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 interpolated
print "The price is $Price.0; # interpolated
There is no double interpolation in Perl, so the $100 is
left as is.
As in some shells, you can enclose the variable name in
braces to disambiguate it from following alphanumerics
(and underscores). You must also do this when interpolating
a variable into a string to separate the variable name
from a following double-colon or an apostrophe, since
these would be otherwise treated as a package separator:
$who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl0;
print "We use ${who}speak when ${who}'s here.0;
Without the braces, Perl would have looked for a $whospeak,
a $who::0, and a $who's variable. The last two
would be the $0 and the $s variables in the (presumably)
non-existent package "who".
In fact, an identifier within such curlies is forced to be
a string, as is any simple identifier within a hash subscript.
Neither need quoting. 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.
Version Strings
Note: Version Strings (v-strings) have been deprecated.
They will not be available after Perl 5.8. The marginal
benefits of v-strings were greatly outweighed by the
potential for Surprise and Confusion.
A literal of the form "v1.20.300.4000" is parsed as a
string composed of characters with the specified ordinals.
This form, known as v-strings, provides an alternative,
more readable way to construct strings, rather than use
the somewhat less readable interpolation form
"a0}". This is useful for repre
senting Unicode strings, and for comparing version "numbers"
using the string comparison operators, "cmp", "gt",
"lt" etc. If there are two or more dots in the literal,
the leading "v" may be omitted.
print v9786; # prints UTF-8 encoded SMILEY, "}"
print v102.111.111; # prints "foo"
print 102.111.111; # same
Such literals are accepted by both "require" and "use" for
doing a version check. The $^V special variable also contains
the running Perl interpreter's version in this form.
See "$^V" in perlvar. Note that using the v-strings for
IPv4 addresses is not portable unless you also use the
inet_aton()/inet_ntoa() routines of the Socket package.
Note that since Perl 5.8.1 the single-number v-strings
(like "v65") are not v-strings before the "=>" operator
(which is usually used to separate a hash key from a hash
value), instead they are interpreted as literal strings
('v65'). They were v-strings from Perl 5.6.0 to Perl
5.8.0, but that caused more confusion and breakage than
good. Multi-number v-strings like "v65.66" and 65.66.67
continue to be v-strings always.
Special Literals
The special literals __FILE__, __LINE__, and __PACKAGE__
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 two control characters ^D and ^Z, and 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.
Text after __DATA__ but may be read via the filehandle
"PACKNAME::DATA", where "PACKNAME" is the package that was
current when the __DATA__ token was encountered. The
filehandle is left open pointing to the contents after
__DATA__. It is the program's responsibility to "close
DATA" when it is done reading from it. For compatibility
with older scripts written before __DATA__ was introduced,
__END__ behaves like __DATA__ in the toplevel script (but
not in files loaded with "require" or "do") and leaves the
remaining contents of the file accessible via
"main::DATA".
See SelfLoader 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.
Barewords
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
"use warnings" pragma or 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 Joining Delimiter
Arrays and slices are interpolated into double-quoted
strings by joining the elements with the delimiter specified
in the $" variable ($LIST_SEPARATOR if "use English;"
is specified), space by default. The following are equivalent:
$temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish
substitution) there is an unfortunate ambiguity: Is
"/$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 braces as
above.
If you're looking for the information on how to use
here-documents, which used to be here, that's been moved
to "Quote and Quote-like Operators" in perlop.
List value constructors [Toc] [Back]
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 what
appears to be a list literal is simply 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 the scalar variable
$foo. Note that the value of an actual array in 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,
);
To use a here-document to assign an array, one line per
element, you might use an approach like this:
@sauces = <<End_Lines =~ m/(*/g;
normal tomato
spicy tomato
green chile
pesto
white wine
End_Lines
LISTs do automatic interpolation of sublists. That is,
when a LIST is evaluated, each element of the list is
evaluated in 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 list context, followed
by the key/value pairs of %glarch. To make a list
reference that does NOT interpolate, see perlref.
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.
This interpolation combines with the facts that the opening
and closing parentheses are optional (except when necessary
for precedence) and lists may end with an optional
comma to mean that multiple commas within lists are legal
syntax. The list "1,,3" is a concatenation of two lists,
"1," and 3, the first of which ends with that optional
comma. "1,,3" is "(1,),(3)" is "1,3" (And similarly for
"1,,,3" is "(1,),(,),3" is "1,3" and so on.) Not that
we'd advise you to use this obfuscation.
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];
Lists may be assigned to only when each element of the
list is itself legal to assign to:
($a, $b, $c) = (1, 2, 3);
($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f,
0x0f0, 0xf00);
An exception to this is that you may assign to "undef" in
a list. This is useful for throwing away some of the
return values of a function:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
List assignment in 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 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.
It's also the source of a useful idiom for executing a
function or performing an operation in list context and
then counting the number of return values, by assigning to
an empty list and then using that assignment in scalar
context. For example, this code:
$count = () = $string =~ /+/g;
will place into $count the number of digit groups found in
$string. This happens because the pattern match is in
list context (since it is being assigned to the empty
list), and will therefore return a list of all matching
parts of the string. The list assignment in scalar context
will translate that into the number of elements (here, the
number of times the pattern matched) and assign that to
$count. Note that simply using
$count = $string =~ /+/g;
would not have worked, since a pattern match in scalar
context will only return true or false, rather than a
count of matches.
The final element of a list assignment may be an array or
a hash:
($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 become undefined. This
may be useful in a my() or local().
A hash can be initialized using a literal list holding
pairs of items 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 often 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 simple
identifier ("=>" doesn't quote compound identifiers, that
contain double colons). 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 = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
labels => labels
);
Note that just because a hash is initialized in that order
doesn't mean that it comes out in that order. See "sort"
in perlfunc for examples of how to arrange for an output
ordering.
Subscripts
An array is subscripted by specifying a dollary sign
("$"), then the name of the array (without the leading
"@"), then the subscript inside square brackets. For
example:
@myarray = (5, 50, 500, 5000);
print "Element Number 2 is", $myarray[2], "0;
The array indices start with 0. A negative subscript
retrieves its value from the end. In our example,
$myarray[-1] would have been 5000, and $myarray[-2] would
have been 500.
Hash subscripts are similar, only instead of square brackets
curly brackets are used. For example:
%scientists =
(
"Newton" => "Isaac",
"Einstein" => "Albert",
"Darwin" => "Charles",
"Feynman" => "Richard",
);
print "Darwin's First Name is ", $scientists{"Darwin"}, "0;
Slices [Toc] [Back]
A common way to access an array or a hash is one scalar
element at a time. You can also subscript a list to get a
single element from it.
$whoami = $ENV{"USER"}; # one element from
the hash
$parent = $ISA[0]; # one element from
the array
$dir = (getpwnam("daemon"))[7]; # likewise, but
with list
A slice accesses several elements of a list, an array, or
a hash simultaneously using a list of subscripts. It's
more convenient than writing out the individual elements
as a list of separate scalar values.
($him, $her) = @folks[0,-1]; # array
slice
@them = @folks[0 .. 3]; # array
slice
($who, $home) = @ENV{"USER", "HOME"}; # hash
slice
($uid, $dir) = (getpwnam("daemon"))[2,7]; # list
slice
Since you can assign to a list of variables, you can also
assign to an array or hash slice.
@days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
= (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
($colors{'red'}, $colors{'blue'}, $colors{'green'})
= (0xff0000, 0x0000ff, 0x00ff00);
($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
Since changing a slice changes the original array or hash
that it's slicing, a "foreach" construct will alter
some--or even all--of the values of the array or hash.
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
foreach (@hash{qw[key1 key2]}) {
s/^/; # trim leading whitespace
s/; # trim trailing whitespace
s/(600
}
A slice of an empty list is still an empty list. Thus:
@a = ()[1,0]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
@c = (0,1)[2,3]; # @c has no elements
But:
@a = (1)[1,0]; # @a has two elements
@b = (1,undef)[1,0,2]; # @b has three elements
This makes it easy to write loops that terminate when a
null list is returned:
while ( ($home, $user) = (getpwent)[7,0]) {
printf "%-8s %s0, $user, $home;
}
As noted earlier in this document, the scalar sense of
list assignment is the number of elements on the righthand
side of the assignment. The null list contains no
elements, so when the password file is exhausted, the
result is 0, not 2.
If you're confused about why you use an '@' there on a
hash slice instead of a '%', think of it like this. The
type of bracket (square or curly) governs whether it's an
array or a hash being looked at. On the other hand, the
leading symbol ('$' or '@') on the array or hash indicates
whether you are getting back a singular value (a scalar)
or a plural one (a list).
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.
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 = here::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 "Symbol
Tables" in perlmod 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 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 = TDOUT;
See perlsub 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 must be used to create new
symbol table entries; *foo{THING} cannot. When in doubt,
use *FH.
All functions that are capable of creating filehandles
(open(), opendir(), pipe(), socketpair(), sysopen(),
socket(), and accept()) automatically create an anonymous
filehandle if the handle passed to them is an uninitialized
scalar variable. This allows the constructs such as
"open(my $fh, ...)" and "open(local $fh,...)" to be used
to create filehandles that will conveniently be closed
automatically when the scope ends, provided there are no
other references to them. This largely eliminates the need
for typeglobs when opening filehandles that must be passed
around, as in the following example:
sub myopen {
open my $fh, "@_"
or die "Can't open '@_': $!";
return $fh;
}
{
my $f = myopen("</etc/motd");
print <$f>;
# $f implicitly closed here
}
Note that if an initialized scalar variable is used
instead the result is different: "my $fh='zzz'; open($fh,
...)" is equivalent to "open( *{'zzz'}, ...)". "use
strict 'refs'" forbids such practice.
Another way to create anonymous filehandles is with the
Symbol module or 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 "open()" in perlfunc for an example.
See perlvar for a description of Perl's built-in variables
and a discussion of legal variable names. See perlref,
perlsub, and "Symbol Tables" in perlmod for more discussion
on typeglobs and the *foo{THING} syntax.
perl v5.8.5 2002-11-06 17 [ Back ] |
