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PERLDATA(1)
NAME
perldata - Perl data types
DESCRIPTION
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
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 && $str ne "0") {
warn "That doesn't look like a number";
}
That method may be best because otherwise you won't treat IEEE notations
like "NaN" or "Infinity" properly. At other times, you might prefer to
determine whether string data can be used numerically by calling the
POSIX::strtod() function or by inspecting your string with a regular
expression (as documented in perlre).
warn "has nondigits" if /\D/;
warn "not a natural number" unless /^\d+$/; # 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 $#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
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 Unix-style 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.\n"; # 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/perl\n";
print "We use ${who}speak when ${who}'s here.\n";
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
"\x{1}\x{14}\x{12c}\x{fa0}". This is useful for representing 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, "\x{263a}"
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
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/(\S.*\S)/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 =~ /\d+/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 = $string =~ /\d+/g;
would not have worked, since a pattern match in scalar context will only
return true or false, rather than a count of matches.
The final element of a list assignment may be an array or a hash:
($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], "\n";
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"}, "\n";
Slices
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], $folks[0]);
Since changing a slice changes the original array or hash that it's
slicing, a "foreach" construct will alter some--or even all--of the values
of the array or hash.
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
foreach (@hash{qw[key1 key2]}) {
s/^\s+//; # trim leading whitespace
s/\s+$//; # trim trailing whitespace
s/(\w+)/\u\L$1/g; # "titlecase" words
}
A slice of an empty list is still an empty list. Thus:
@a = ()[1,0]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
@c = (0,1)[2,3]; # @c has no elements
But:
@a = (1)[1,0]; # @a has two elements
@b = (1,undef)[1,0,2]; # @b has three elements
This makes it easy to write loops that terminate when a null list is
returned:
while ( ($home, $user) = (getpwent)[7,0]) {
printf "%-8s %s\n", $user, $home;
}
As noted earlier in this document, the scalar sense of list assignment is
the number of elements on the right-hand side of the assignment. The null
list contains no elements, so when the password file is exhausted, the
result is 0, not 2.
If you're confused about why you use an '@' there on a hash slice instead
of a '%', think of it like this. The type of bracket (square or curly)
governs whether it's an array or a hash being looked at. On the other
hand, the leading symbol ('$' 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
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 "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 = \*STDOUT;
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 ALSO
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.
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