PERLDATA(1) Perl Programmers Reference Guide 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 val-ues values
ues 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 particu-lar particular
lar 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 ini-tial initial
tial '*', 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 upper-case uppercase
case 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 con-text context
text around the operation or value. There are two major contexts: list and scalar. Certain opera-tions operations
tions 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 argu-ments. arguments.
ments. 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, con-version conversion
version 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 con-version conversion
version 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 poly-morphic polymorphic
morphic 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, ref-erences references
erences are strongly-typed, uncastable pointers with builtin reference-counting and destructor invo-cation. invocation.
cation.
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 pro-duce produce
duce 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 ele-ment, element,
ment, which is a different value since there is ordinarily a 0th element. Assigning to $#days actu-ally actually
ally 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 ele-ments. elements.
ments. (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 equiva-lent: equivalent:
lent:
@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 allo-cated allocated
cated 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.
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 sub-stitution; substitution;
stitution; 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 expres-sion expression
sion 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
$days{Feb} and the quotes will be assumed automatically. But anything more complicated in the sub-script subscript
script will be interpreted as an expression. This means for example that "$version{2.0}++" is equiv-alent equivalent
alent to "$version{2}++", not to "$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 potential for Surprise and Confu-sion. Confusion.
sion.
A literal of the form "v1.20.300.4000" is parsed as a string composed of characters with the speci-fied specified
fied ordinals. This form, known as v-strings, provides an alternative, more readable way to con-struct construct
struct 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 ver-sion version
sion "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 inter-preted interpreted
preted 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 con-tinue continue
tinue 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;" direc-tive), directive),
tive), __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 point-ing pointing
ing 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 delim-iter delimiter
iter specified in the $" variable ($LIST_SEPARATOR if "use English;" is specified), space by default.
The following are equivalent:
$temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish substitution) there is an unfortunate ambi-guity: ambiguity:
guity: Is "/$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 paren-theses parentheses
theses 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 ini-tializing initializing
tializing 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 = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
labels => \%labels
);
Note that just because a hash is initialized in that order doesn't mean that it comes out in that
order. See "sort" in perlfunc for examples of how to arrange for an output ordering.
Subscripts
An array is subscripted by specifying a dollar sign ("$"), 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 exam-ple, example,
ple, $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
$dir = (getpwnam("daemon"))[7]; # likewise, but with list
A slice accesses several elements of a list, an array, or a hash simultaneously using a list of sub-scripts. subscripts.
scripts. It's more convenient than writing out the individual elements as a list of separate scalar
values.
($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 manipula-tions, manipulations,
tions, 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 automati-cally automatically
cally 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 exam-ple: example:
ple:
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.
perl v5.8.8 2006-01-07 PERLDATA(1)
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