PERLDATA(1) Perl Programmers Reference Guide PERLDATA(1)NAMEperldata - 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 begin-
ning 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 run-
time. 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 expres-
sion are saved under names containing only digits after the
"$" (see perlop and perlre). In addition, several special
variables that provide windows into the inner working of
Perl have names containing punctuation characters and con-
trol characters. These are documented in perlvar.
Scalar values are always named with '$', 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
$#days # the last index of array @days
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Entire arrays (and slices of arrays and hashes) are denoted
by '@', which works much like the word "these" or "those"
does in English, in that it indicates multiple values are
expected.
@days # ($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 ini-
tial 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 char-
acter, e.g., $% or $$. (Most of these one character names
have a predefined significance to Perl. For instance, $$ is
the current process id.)
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Context
The interpretation of operations and values in Perl some-
times 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 documenta-
tion 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 evalu-
ates 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 sub-
routines do not need to bother, though. That's because both
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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 tran-
sparent. 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 con-
sidered 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 unde-
fined 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
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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. How-
ever, 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;
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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 exam-
ple, 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 (only numbers, begins with 0)
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.
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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 $days{Feb} and the quotes will be assumed
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automatically. But anything more complicated in the sub-
script will be interpreted as an expression. This means for
example that "$version{2.0}++" is equivalent to "$ver-
sion{2}++", not to "$version{'2.0'}++".
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 poten-
tial 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 some-
what less readable interpolation form
"\x{1}\x{14}\x{12c}\x{fa0}". This is useful for represent-
ing 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 $^V special variable also con-
tains the running Perl interpreter's version in this form.
See "$^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.
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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 warn-
ings" 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 subrou-
tine call produces a compile-time error instead. The res-
triction 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 $temp";
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system "echo @ARGV";
Within search patterns (which also undergo double-quotish
substitution) there is an unfortunate ambiguity: Is
"/$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 pre-
cedence 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 con-
text 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,
);
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To use a here-document to assign an array, one line per ele-
ment, 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 interpo-
lated 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 ele-
ments 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
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# 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 ele-
ments 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 func-
tion 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 = $string =~ /\d+/g;
would not have worked, since a pattern match in scalar con-
text will only return true or false, rather than a count of
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matches.
The final element of a list assignment may be an array or a
hash:
($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 interchange-
able, 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 parame-
ters 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',
};
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or for using call-by-named-parameter to complicated func-
tions:
$field = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
labels => \%labels
);
Note that just because a hash is initialized in that order
doesn't mean that it comes out in that order. See "sort" in
perlfunc for examples of how to arrange for an output order-
ing.
Subscripts
An array is subscripted by specifying a dollar sign ("$"),
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, $myar-
ray[-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 ele-
ment at a time. You can also subscript a list to get a sin-
gle element from it.
$whoami = $ENV{"USER"}; # one element from the hash
$parent = $ISA[0]; # one element from the array
$dir = (getpwnam("daemon"))[7]; # likewise, but with list
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A slice accesses several elements of a list, an array, or a
hash simultaneously using a list of subscripts. It's more
convenient than writing out the individual elements as a
list of separate scalar values.
($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$1/g; # "titlecase" words
}
A slice of an empty list is still an empty list. Thus:
@a = ()[1,0]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
@c = (0,1)[2,3]; # @c has no elements
But:
@a = (1)[1,0]; # @a has two elements
@b = (1,undef)[1,0,2]; # @b has three elements
This makes it easy to write loops that terminate when a null
list is returned:
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while ( ($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 sel-
dom 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:
$fh = *STDOUT;
or perhaps as a real reference, like this:
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$fh = \*STDOUT;
See perlsub for examples of using these as indirect filehan-
dles 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'"
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forbids such practice.
Another way to create anonymous filehandles is with the Sym-
bol 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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