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). For a more in-depth
discussion on identifiers, see "Identifier parsing". 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
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are denoted by '@', which works
much as the word "these" or "those" does in English, in
that it indicates multiple values are expected.
@days # ($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.)
Identifier parsing¶
Up until Perl 5.18, the actual rules of what a valid identifier was were a bit
fuzzy. However, in general, anything defined here should work on previous
versions of Perl, while the opposite -- edge cases that work in previous
versions, but aren't defined here -- probably won't work on newer versions. As
an important side note, please note that the following only applies to
bareword identifiers as found in Perl source code, not identifiers introduced
through symbolic references, which have much fewer restrictions. If working
under the effect of the "use utf8;" pragma, the following rules
apply:
/ (?[ ( \p{Word} & \p{XID_Start} ) + [_] ])
(?[ ( \p{Word} & \p{XID_Continue} ) ]) * /x
That is, a "start" character followed by any number of
"continue" characters. Perl requires every character in an
identifier to also match "\w" (this prevents some problematic
cases); and Perl additionally accepts identfier names beginning with an
underscore.
If not under "use utf8", the source is treated as ASCII + 128 extra
controls, and identifiers should match
/ (?aa) (?!\d) \w+ /x
That is, any word character in the ASCII range, as long as the first character
is not a digit.
There are two package separators in Perl: A double colon ("::") and a
single quote ("'"). Normal identifiers can start or end with a
double colon, and can contain several parts delimited by double colons. Single
quotes have similar rules, but with the exception that they are not legal at
the end of an identifier: That is, "$'foo" and "$foo'bar"
are legal, but "$foo'bar'" is not.
Additionally, if the identifier is preceded by a sigil -- that is, if the
identifier is part of a variable name -- it may optionally be enclosed in
braces.
While you can mix double colons with singles quotes, the quotes must come after
the colons: "$::::'foo" and "$foo::'bar" are legal, but
"$::'::foo" and "$foo'::bar" are not.
Put together, a grammar to match a basic identifier becomes
/
(?(DEFINE)
(?<variable>
(?&sigil)
(?:
(?&normal_identifier)
| \{ \s* (?&normal_identifier) \s* \}
)
)
(?<normal_identifier>
(?: :: )* '?
(?&basic_identifier)
(?: (?= (?: :: )+ '? | (?: :: )* ' ) (?&normal_identifier) )?
(?: :: )*
)
(?<basic_identifier>
# is use utf8 on?
(?(?{ (caller(0))[8] & $utf8::hint_bits })
(?&Perl_XIDS) (?&Perl_XIDC)*
| (?aa) (?!\d) \w+
)
)
(?<sigil> [&*\$\@\%])
(?<Perl_XIDS> (?[ ( \p{Word} & \p{XID_Start} ) + [_] ]) )
(?<Perl_XIDC> (?[ \p{Word} & \p{XID_Continue} ]) )
)
/x
Meanwhile, special identifiers don't follow the above rules; For the most part,
all of the identifiers in this category have a special meaning given by Perl.
Because they have special parsing rules, these generally can't be
fully-qualified. They come in four forms:
- A sigil, followed solely by digits matching \p{POSIX_Digit}, like $0, $1,
or $10000.
- A sigil, followed by either a caret and a single POSIX uppercase letter,
like $^V or $^W, or a sigil followed by a literal control character matching
the "\p{POSIX_Cntrl}" property. Due to a historical oddity, if not
running under "use utf8", the 128 extra controls in the
"[0x80-0xff]" range may also be used in length one variables. The
use of a literal control character is deprecated. Support for this form will
be removed in a future version of perl.
- Similar to the above, a sigil, followed by bareword text in brackets,
where the first character is either a caret followed by an uppercase letter,
or a literal control, like "${^GLOBAL_PHASE}" or
"${\7LOBAL_PHASE}". The use of a literal control character is
deprecated. Support for this form will be removed in a future version of
perl.
- A sigil followed by a single character matching the
"\p{POSIX_Punct}" property, like $! or "%+".
Note that as of Perl 5.20, literal control characters in variable names are
deprecated.
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 right-hand 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 FALSE in the Boolean sense if it is undefined,
the null string or the number 0 (or its string equivalent, "0"), and
TRUE if it is anything else. 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.
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;
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. If a tied hash is evaluated in scalar
context, the "SCALAR" method is called (with a fallback to
"FIRSTKEY").
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 (but not multiple underscores in a row:
"23__500" is not legal; "23_500" is). 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.
By default floating point numbers substituted inside strings use the dot
(".") as the decimal separator. If "use locale" is in
effect, and
POSIX::setlocale() has been called, the character used for
the decimal separator is affected by the LC_NUMERIC locale. See perllocale and
POSIX.
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, a simple identifier within such curlies is forced to be a string, and
likewise within a hash subscript. Neither need quoting. Our earlier example,
$days{'Feb'} can be written as $days{Feb} and the quotes will be assumed
automatically. But anything more complicated in the subscript will be
interpreted as an expression. This means for example that
"$version{2.0}++" is equivalent to "$version{2}++", not to
"$version{'2.0'}++".
Version Strings
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 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. 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. __SUB__
gives a reference to the current subroutine. 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. (But the empty "package;" is no longer supported,
as of version 5.10.) Outside of a subroutine, __SUB__ is the undefined value.
__SUB__ is only available in 5.16 or higher, and only with a "use
v5.16" or "use feature "current_sub"" declaration.
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__ 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 line after
__DATA__. The program should "close DATA" when it is done reading
from it. (Leaving it open leaks filehandles if the module is reloaded for any
reason, so it's a safer practice to close it.) For compatibility with older
scripts written before __DATA__ was introduced, __END__ behaves like __DATA__
in the top level 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. Perl limits barewords (like identifiers) to about 250 characters.
Future versions of Perl are likely to eliminate these arbitrary limitations.
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 Interpolation
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 = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
labels => \%labels
);
Note that just because a hash is initialized in that order doesn't mean that it
comes out in that order. See "sort" in perlfunc for examples of how
to arrange for an output ordering.
If a key appears more than once in the initializer list of a hash, the last
occurrence wins:
%circle = (
center => [5, 10],
center => [27, 9],
radius => 100,
color => [0xDF, 0xFF, 0x00],
radius => 54,
);
# same as
%circle = (
center => [27, 9],
color => [0xDF, 0xFF, 0x00],
radius => 54,
);
This can be used to provide overridable configuration defaults:
# values in %args take priority over %config_defaults
%config = (%config_defaults, %args);
Subscripts¶
An array can be accessed one scalar at a time by specifying a dollar sign
("$"), then the name of the array (without the leading
"@"), then the subscript inside square brackets. For example:
@myarray = (5, 50, 500, 5000);
print "The Third Element 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";
You can also subscript a list to get a single element from it:
$dir = (getpwnam("daemon"))[7];
Multi-dimensional array emulation¶
Multidimensional arrays may be emulated by subscripting a hash with a list. The
elements of the list are joined with the subscript separator (see
"$;" in perlvar).
$foo{$a,$b,$c}
is equivalent to
$foo{join($;, $a, $b, $c)}
The default subscript separator is "\034", the same as SUBSEP in
awk.
Slices¶
A slice accesses several elements of a list, an array, or a hash simultaneously
using a list of subscripts. It's more convenient than writing out the
individual elements as a list of separate scalar values.
($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$1/g; # "titlecase" words
}
A slice of an empty list is still an empty list. Thus:
@a = ()[1,0]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
But:
@a = (1)[1,0]; # @a has two elements
@b = (1,undef)[1,0,2]; # @b has three elements
More generally, a slice yields the empty list if it indexes only beyond the end
of a list:
@a = (1)[ 1,2]; # @a has no elements
@b = (1)[0,1,2]; # @b has three elements
This makes it easy to write loops that terminate when a null list is returned:
while ( ($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.
Slices in scalar context return the last item of the slice.
@a = qw/first second third/;
%h = (first => 'A', second => 'B');
$t = @a[0, 1]; # $t is now 'second'
$u = @h{'first', 'second'}; # $u is now 'B'
If you're confused about why you use an '@' there on a hash slice instead of a
'%', think of it like this. The type of bracket (square or curly) governs
whether it's an array or a hash being looked at. On the other hand, the
leading symbol ('$' or '@') on the array or hash indicates whether you are
getting back a singular value (a scalar) or a plural one (a list).
Key/Value Hash Slices
Starting in Perl 5.20, a hash slice operation with the % symbol is a variant of
slice operation returning a list of key/value pairs rather than just values:
%h = (blonk => 2, foo => 3, squink => 5, bar => 8);
%subset = %h{'foo', 'bar'}; # key/value hash slice
# %subset is now (foo => 3, bar => 8)
However, the result of such a slice cannot be localized, deleted or used in
assignment. These are otherwise very much consistent with hash slices using
the @ symbol.
Index/Value Array Slices
Similar to key/value hash slices (and also introduced in Perl 5.20), the % array
slice syntax returns a list of index/value pairs:
@a = "a".."z";
@list = %a[3,4,6];
# @list is now (3, "d", 4, "e", 6, "g")
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.