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PERLDATA(1)
NAME
perldata - Perl data types
DESCRIPTION
Variable names
Perl has three built-in data types: scalars, arrays of scalars, and
associative arrays of scalars, known as "hashes". Normal arrays are
ordered lists of scalars indexed by number, starting with 0 and with
negative subscripts counting from the end. 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 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
"$" (see the perlop manpage and the perlre manpage). In addition, several
special variables that provide windows into the inner working of Perl have
names containing punctuation characters and control characters. These are
documented in the perlvar manpage.
Scalar values are always named with '$', even when referring to a scalar
that is part of an array or a hash. The '$' symbol works semantically like
the English word "the" in that it indicates a single value is expected.
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$days{'Feb'} # the 'Feb' value from hash %days
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are denoted by '@', which
works much like the word "these" or "those" does in English, in that it
indicates multiple values are expected.
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as ($days[3],$days[4],$days[5])
@days{'a','c'} # same as ($days{'a'},$days{'c'})
Entire hashes are denoted by '%':
%days # (key1, val1, key2, val2 ...)
In addition, subroutines are named with an initial '&', though this is
optional when unambiguous, just as the word "do" is often redundant in
English. Symbol table entries can be named with an initial '*', but you
don't really care about that yet (if ever :-).
Every variable type has its own namespace, as do several non-variable
identifiers. This means that you can, without fear of conflict, use the
same name for a scalar variable, an array, or a hash--or, for that matter,
for a filehandle, a directory handle, a subroutine name, a format name, or
a label. This means that $foo and @foo are two different variables. It
also means that "$foo[1]" is a part of @foo, not a part of $foo. This may
seem a bit weird, but that's okay, because it is weird.
Because variable references always start with '$', '@', or '%', the
"reserved" words aren't in fact reserved with respect to variable names.
They are reserved with respect to labels and filehandles, however, which
don't have an initial special character. You can't have a filehandle named
"log", for instance. Hint: you could say "open(LOG,'logfile')" rather than
"open(log,'logfile')". Using uppercase filehandles also improves
readability and protects you from conflict with future reserved words.
Case is significant--"FOO", "Foo", and "foo" are all different names.
Names that start with a letter or underscore may also contain digits and
underscores.
It is possible to replace such an alphanumeric name with an expression that
returns a reference to the appropriate type. For a description of this,
see the perlref manpage.
Names that start with a digit may contain only more digits. Names that do
not start with a letter, underscore, or digit are limited to one character,
e.g., "$%" or "$$". (Most of these one character names have a predefined
significance to Perl. For instance, "$$" is the current process id.)
Context
The interpretation of operations and values in Perl sometimes depends on
the requirements of the context around the operation or value. There are
two major contexts: list and scalar. Certain operations return list values
in contexts wanting a list, and scalar values otherwise. If this is true
of an operation it will be mentioned in the documentation for that
operation. In other words, Perl overloads certain operations based on
whether the expected return value is singular or plural. Some words in
English work this way, like "fish" and "sheep".
In a reciprocal fashion, an operation provides either a scalar or a list
context to each of its arguments. For example, if you say
int( <STDIN> )
the integer operation provides scalar context for the <> operator, which
responds by reading one line from STDIN and passing it back to the integer
operation, which will then find the integer value of that line and return
that. If, on the other hand, you say
sort( <STDIN> )
then the sort operation provides list context for <>, which will proceed to
read every line available up to the end of file, and pass that list of
lines back to the sort routine, which will then sort those lines and return
them as a list to whatever the context of the sort was.
Assignment is a little bit special in that it uses its left argument to
determine the context for the right argument. Assignment to a scalar
evaluates the right-hand side in scalar context, while assignment to an
array or hash evaluates the righthand side in list context. Assignment to
a list (or slice, which is just a list anyway) also evaluates the righthand
side in list context.
When you use the "use warnings" pragma or Perl's -w command-line option,
you may see warnings about useless uses of constants or functions in "void
context". Void context just means the value has been discarded, such as a
statement containing only ""fred";" or "getpwuid(0);". It still counts as
scalar context for functions that care whether or not they're being called
in list context.
User-defined subroutines may choose to care whether they are being called
in a void, scalar, or list context. Most subroutines do not need to
bother, though. That's because both scalars and lists are automatically
interpolated into lists. See 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 values.
Scalars aren't necessarily one thing or another. There's no place to
declare a scalar variable to be of type "string", type "number", type
"reference", or anything else. Because of the automatic conversion of
scalars, operations that return scalars don't need to care (and in fact,
cannot care) whether their caller is looking for a string, a number, or a
reference. Perl is a contextually polymorphic language whose scalars can
be strings, numbers, or references (which includes objects). Although
strings and numbers are considered pretty much the same thing for nearly
all purposes, references are strongly-typed, uncastable pointers with
builtin reference-counting and destructor invocation.
A scalar value is interpreted as TRUE in the Boolean sense if it is not the
null string or the number 0 (or its string equivalent, "0"). The Boolean
context is just a special kind of scalar context where no conversion to a
string or a number is ever performed.
There are actually two varieties of null strings (sometimes referred to as
"empty" strings), a defined one and an undefined one. The defined version
is just a string of length zero, such as """". The undefined version is
the value that indicates that there is no real value for something, such as
when there was an error, or at end of file, or when you refer to an
uninitialized variable or element of an array or hash. Although in early
versions of Perl, an undefined scalar could become defined when first used
in a place expecting a defined value, this no longer happens except for
rare cases of autovivification as explained in 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 following are
equivalent:
@whatever = ();
$#whatever = -1;
If you evaluate an array in scalar context, it returns the length of the
array. (Note that this is not true of lists, which return the last value,
like the C comma operator, nor of built-in functions, which return whatever
they feel like returning.) The following is always true:
scalar(@whatever) == $#whatever - $[ + 1;
Version 5 of Perl changed the semantics of "$[": files that don't set the
value of "$[" no longer need to worry about whether another file changed
its value. (In other words, use of "$[" is deprecated.) So in general you
can assume that
scalar(@whatever) == $#whatever + 1;
Some programmers choose to use an explicit conversion so as to leave
nothing to doubt:
$element_count = scalar(@whatever);
If you evaluate a hash in scalar context, it returns false if the hash is
empty. If there are any key/value pairs, it returns true; more precisely,
the value returned is a string consisting of the number of used buckets and
the number of allocated buckets, separated by a slash. This is pretty much
useful only to find out whether Perl's internal hashing algorithm is
performing poorly on your data set. For example, you stick 10,000 things
in a hash, but evaluating %HASH in scalar context reveals ""1/16"", which
means only one out of sixteen buckets has been touched, and presumably
contains all 10,000 of your items. This isn't supposed to happen.
You can preallocate space for a hash by assigning to the keys() function.
This rounds up the allocated buckets to the next power of two:
keys(%users) = 1000; # allocate 1024 buckets
Scalar value constructors
Numeric literals are specified in any of the following floating point or
integer formats:
12345
12345.67
.23E-10 # a very small number
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 newline, 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() functions 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
subscript.) 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 interpolating a variable into a string to separate the
variable name from a following double-colon or an apostrophe, since these
would be otherwise treated as a package separator:
$who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
print "We use ${who}speak when ${who}'s here.\n";
Without the braces, Perl would have looked for a $whospeak, a "$who::0",
and a "$who's" variable. The last two would be the $0 and the $s variables
in the (presumably) non-existent package "who".
In fact, an identifier within such curlies is forced to be a string, as is
any simple identifier within a hash subscript. Neither need quoting. Our
earlier example, "$days{'Feb'}" can be written as "$days{Feb}" and the
quotes will be assumed automatically. But anything more complicated in the
subscript will be interpreted as an expression.
A literal of the form "v1.20.300.4000" is parsed as a string composed of
characters with the specified ordinals. This provides an alternative, more
readable way to construct strings, rather than use the somewhat less
readable interpolation form ""\x{1}\x{14}\x{12c}\x{fa0}"". This is useful
for representing Unicode strings, and for comparing version "numbers" using
the string comparison operators, "cmp", "gt", "lt" etc. If there are two
or more dots in the literal, the leading "v" may be omitted.
print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
print v102.111.111; # prints "foo"
print 102.111.111; # same
Such literals are accepted by both "require" and "use" for doing a version
check. The "$^V" special variable also contains the running Perl
interpreter's version in this form. See 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 following text is ignored.
Text after __DATA__ but may be read via the filehandle "PACKNAME::DATA",
where "PACKNAME" is the package that was current when the __DATA__ token
was encountered. The filehandle is left open pointing to the contents
after __DATA__. It is the program's responsibility to "close DATA" when it
is done reading from it. For compatibility with older scripts written
before __DATA__ was introduced, __END__ behaves like __DATA__ in the
toplevel script (but not in files loaded with "require" or "do") and leaves
the remaining contents of the file accessible via "main::DATA".
See 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
compilation), 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 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'".
Arrays and slices are interpolated into double-quoted strings by joining
the elements with the delimiter specified 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 treatment 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 character:
print << "abc\"def";
testing...
abc"def
Finally, quoted strings cannot span multiple lines. The general rule is
that the identifier must be a string literal. 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 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 the perlref manpage.
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 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 $x to f()'s return count
This is handy when you want to do a list assignment in a Boolean context,
because most list functions return a null list when finished, which when
assigned produces a 0, which is interpreted as FALSE.
The final element may be an array or a hash:
($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 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 identifier. 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 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
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 the Symbol Tables entry in the perlmod manpage 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 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
"*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
}
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 example.
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 Symbol Tables entry in the perlmod manpage for more
discussion on typeglobs and the "*foo{THING}" syntax.
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