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". Normal arrays are ordered lists of scalars
indexed by number, starting with 0 and with negative sub
scripts counting from the end. Hashes are unordered col
lections 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 contain
ing 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 inter
preted as names of packages, to locate the namespace in
which to look up the final identifier (see the Packages
entry in the perlmod manpage 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 the perlref manpage.
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 "$" (see the perlop manpage and the perlre man
page). In addition, several special variables that pro
vide windows into the inner working of Perl have names
containing punctuation characters and control characters.
These are documented in the perlvar manpage.
Scalar values are always named with '$', even when refer
ring to a scalar that is part of an array or a hash. The
'$' symbol works semantically like the English word "the"
in that it indicates a single value is expected.
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$days{'Feb'} # the 'Feb' value from hash %days
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are
denoted by '@', which works much like the word "these" or
"those" does in English, in that it indicates multiple
values are expected.
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as ($days[3],$days[4],$days[5])
@days{'a','c'} # same as ($days{'a'},$days{'c'})
Entire hashes are denoted by '%':
%days # (key1, val1, key2, val2 ...)
In addition, subroutines are named with an initial '&',
though this is optional when unambiguous, just as the word
"do" is often redundant in English. Symbol table entries
can be named with an initial '*', but you don't really
care about that yet (if ever :-).
Every variable type has its own namespace, as do several
non-variable identifiers. This means that you can, with
out fear of conflict, use the same name for a scalar vari
able, an array, or a hash--or, for that matter, for a
filehandle, a directory handle, a subroutine name, a for
mat 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 dif
ferent names. Names that start with a letter or under
score 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 the perlref manpage.
Names that start with a digit may contain only more dig
its. Names that do not start with a letter, underscore,
or digit are limited to one character, e.g., "$%" or
"$$". (Most of these one character names have a prede
fined significance to Perl. For instance, "$$" is the
current process id.)
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
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 argu
ment. 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 any
way) also evaluates the righthand side in list context.
When you use the "use warnings" pragma or Perl's -w com
mand-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 state
ment 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 the wantarray entry in the perlfunc manpage
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 val
ues.
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, num
bers, or references (which includes objects). Although
strings and numbers are considered pretty much the same
thing for nearly all purposes, references are strongly-
typed, uncastable pointers with builtin reference-counting
and destructor invocation.
A scalar value is interpreted as TRUE in the Boolean sense
if it is not the null string or the number 0 (or its
string equivalent, "0"). The Boolean context is just a
special kind of scalar context where no conversion to a
string or a number is ever performed.
There are actually two varieties of null strings (some
times 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 some
thing, 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 hap
pens except for rare cases of autovivification as
explained in the perlref manpage. 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
-w noises). That's because strings that aren't numbers
count as 0, just as they do in awk:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
}
That method may be best because otherwise you won't treat
IEEE notations like "NaN" or "Infinity" properly. At
other times, you might prefer to determine whether string
data can be used numerically by calling the
POSIX:\fIs0:strtod() function or by inspecting your string
with a regular expression (as documented in the perlre
manpage).
warn "has nondigits" if /\D/;
warn "not a natural number" unless /^\d+$/; # rejects -3
warn "not an integer" unless /^-?\d+$/; # rejects +3
warn "not an integer" unless /^[+-]?\d+$/;
warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
warn "not a C float"
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
The length of an array is a scalar value. You may find
the length of array @days by evaluating "$#days", as in
csh. However, this isn't the length of the array; it's
the subscript of the last element, which is a different
value since there is ordinarily a 0th element. Assigning
to "$#days" actually changes the length of the array.
Shortening an array this way destroys intervening values.
Lengthening an array that was previously shortened does
not recover values that were in those elements. (It used
to do so in Perl 4, but we had to break this to make sure
destructors were called when expected.)
You can also gain some miniscule 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 fol
lowing are equivalent:
@whatever = ();
$#whatever = -1;
If you evaluate an array in scalar context, it returns the
length of the array. (Note that this is not true of
lists, which return the last value, like the C comma
operator, nor of built-in functions, which return whatever
they feel like returning.) The following is always true:
scalar(@whatever) == $#whatever - $[ + 1;
Version 5 of Perl changed the semantics of "$[": files
that don't set the value of "$[" no longer need to worry
about whether another file changed its value. (In other
words, use of "$[" is deprecated.) So in general you can
assume that
scalar(@whatever) == $#whatever + 1;
Some programmers choose to use an explicit conversion so
as to leave nothing to doubt:
$element_count = scalar(@whatever);
If you evaluate a hash in scalar context, it returns false
if the hash is empty. If there are any key/value pairs,
it returns true; more precisely, the value returned is a
string consisting of the number of used buckets and the
number of allocated buckets, separated by a slash. This
is pretty much useful only to find out whether Perl's
internal hashing algorithm is performing poorly on your
data set. For example, you stick 10,000 things in a hash,
but evaluating %HASH in scalar context reveals ""1/16"",
which means only one out of sixteen buckets has been
touched, and presumably contains all 10,000 of your items.
This isn't supposed to happen.
You can preallocate space for a hash by assigning to the
keys() function. This rounds up the allocated buckets to
the next power of two:
keys(%users) = 1000; # allocate 1024 buckets
Scalar value constructors
Numeric literals are specified in any of the following
floating point or integer formats:
12345
12345.67
.23E-10 # a very small number
4_294_967_296 # underline for legibility
0xff # hex
0377 # octal
0b011011 # binary
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 new
line, tab, etc., as well as some more exotic forms. See
the Quote and Quote-like Operators entry in the perlop
manpage 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() func
tions make these conversions for you. See the hex entry
in the perlfunc manpage and the oct entry in the perlfunc
manpage 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 sub
script.) The following code segment prints out "The price
is $100."
$Price = '$100'; # not interpreted
print "The price is $Price.\n"; # interpreted
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 interpolat
ing 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 $whos
peak, a "$who::0", and a "$who's" variable. The last two
would be the $0 and the $s variables in the (presumably)
non-existent package "who".
In fact, an identifier within such curlies is forced to be
a string, as is any simple identifier within a hash sub
script. Neither need quoting. Our earlier example,
"$days{'Feb'}" can be written as "$days{Feb}" and the
quotes will be assumed automatically. But anything more
complicated in the subscript will be interpreted as an
expression.
A literal of the form "v1.20.300.4000" is parsed as a
string composed of characters with the specified ordinals.
This provides an alternative, more readable way to con
struct strings, rather than use the somewhat less readable
interpolation form ""\x{1}\x{14}\x{12c}\x{fa0}"". This is
useful for representing Unicode strings, and for comparing
version "numbers" using the string comparison operators,
"cmp", "gt", "lt" etc. If there are two or more dots in
the literal, the leading "v" may be omitted.
print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
print v102.111.111; # prints "foo"
print 102.111.111; # same
Such literals are accepted by both "require" and "use" for
doing a version check. The "$^V" special variable also
contains the running Perl interpreter's version in this
form. See the section on "$^V" in the perlvar manpage.
The special literals __FILE__, __LINE__, and __PACKAGE__
represent the current filename, line number, and package
name at that point in your program. They may be used only
as separate tokens; they will not be interpolated into
strings. If there is no current package (due to an empty
"package;" directive), __PACKAGE__ is the undefined value.
The two control characters ^D and ^Z, and the tokens
__END__ and __DATA__ may be used to indicate the logical
end of the script before the actual end of file. Any fol
lowing 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 the SelfLoader manpage 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 compila
tion), at which point the corresponding __DATA__ (or
__END__) token has not yet been seen.
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 sub
routine 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'".
Arrays and slices are interpolated into double-quoted
strings by joining the elements with the delimiter speci
fied in the "$"" variable ("$LIST_SEPARATOR" in English),
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.
A line-oriented form of quoting is based on the shell
"here-document" syntax. Following a "<<" you specify a
string to terminate the quoted material, and all lines
following the current line down to the terminating string
are the value of the item. The terminating string may be
either an identifier (a word), or some quoted text. If
quoted, the type of quotes you use determines the treat
ment of the text, just as in regular quoting. An unquoted
identifier works like double quotes. There must be no
space between the "<<" and the identifier, unless the
identifier is quoted. (If you put a space it will be
treated as a null identifier, which is valid, and matches
the first empty line.) The terminating string must appear
by itself (unquoted and with no surrounding whitespace) on
the terminating line.
print <<EOF;
The price is $Price.
EOF
print << "EOF"; # same as above
The price is $Price.
EOF
print << `EOC`; # execute commands
echo hi there
echo lo there
EOC
print <<"foo", <<"bar"; # you can stack them
I said foo.
foo
I said bar.
bar
myfunc(<< "THIS", 23, <<'THAT');
Here's a line
or two.
THIS
and here's another.
THAT
Just don't forget that you have to put a semicolon on the
end to finish the statement, as Perl doesn't know you're
not going to try to do this:
print <<ABC
179231
ABC
+ 20;
If you want your here-docs to be indented with the rest of
the code, you'll need to remove leading whitespace from
each line manually:
($quote = <<'FINIS') =~ s/^\s+//gm;
The Road goes ever on and on,
down from the door where it began.
FINIS
If you use a here-doc within a delimited construct, such
as in "s///eg", the quoted material must come on the lines
following the final delimiter. So instead of
s/this/<<E . 'that'
the other
E
. 'more '/eg;
you have to write
s/this/<<E . 'that'
. 'more '/eg;
the other
E
If the terminating identifier is on the last line of the
program, you must be sure there is a newline after it;
otherwise, Perl will give the warning Can't find string
terminator "END" anywhere before EOF....
Additionally, the quoting rules for the identifier are not
related to Perl's quoting rules -- "q()", "qq()", and the
like are not supported in place of "''" and """", and the
only interpolation is for backslashing the quoting charac
ter:
print << "abc\"def";
testing...
abc"def
Finally, quoted strings cannot span multiple lines. The
general rule is that the identifier must be a string lit
eral. Stick with that, and you should be safe.
List value constructors
List values are denoted by separating individual values by
commas (and enclosing the list in parentheses where prece
dence 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 paren
thesis 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 ele
ments of @bar, followed by all the elements returned by
the subroutine named SomeSub called in list context, fol
lowed by the key/value pairs of %glarch. To make a list
reference that does NOT interpolate, see the perlref man
page.
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 open
ing and closing parentheses are optional (except necessary
for precedence) and lists may end with an optional comma
to mean that multiple commas within lists are legal syn
tax. 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 $x to f()'s return count
This is handy when you want to do a list assignment in a
Boolean context, because most list functions return a null
list when finished, which when assigned produces a 0,
which is interpreted as FALSE.
The final element may be an array or a hash:
($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 inter
changeable, 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 (includ
ing 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 identi
fier. 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 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 the
sort entry in the perlfunc manpage for examples of how to
arrange for an output ordering.
Slices
A common way to access an array or a hash is one scalar
element at a time. You can also subscript a list to get a
single element from it.
$whoami = $ENV{"USER"}; # one element from the hash
$parent = $ISA[0]; # one element from the array
$dir = (getpwnam("daemon"))[7]; # likewise, but with list
A slice accesses several elements of a list, an array, or
a hash simultaneously using a list of subscripts. It's
more convenient than writing out the individual elements
as a list of separate scalar values.
($him, $her) = @folks[0,-1]; # array slice
@them = @folks[0 .. 3]; # array slice
($who, $home) = @ENV{"USER", "HOME"}; # hash slice
($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also
assign to an array or hash slice.
@days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
= (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
($colors{'red'}, $colors{'blue'}, $colors{'green'})
= (0xff0000, 0x0000ff, 0x00ff00);
($folks[0], $folks[-1]) = ($folks[0], $folks[-1]);
Since changing a slice changes the original array or hash
that it's slicing, a "foreach" construct will alter
some--or even all--of the values of the array or hash.
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
foreach (@hash{keys %hash}) {
s/^\s+//; # trim leading whitespace
s/\s+$//; # 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 refer
ence into a function, but now that we have real refer
ences, 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 the
Symbol Tables entry in the perlmod manpage for more exam
ples 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 the perlsub manpage 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 "*HAN
DLE{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 uninitial
ized 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
}
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 the open() entry in the perlfunc manpage for an exam
ple.
SEE ALSO
See the perlvar manpage for a description of Perl's built-
in variables and a discussion of legal variable names.
See the perlref manpage, the perlsub manpage, and the Sym
bol Tables entry in the perlmod manpage for more discus
sion on typeglobs and the "*foo{THING}" syntax.
2001-04-07 perl v5.6.1 PERLDATA(1)
