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. Most often, it consists of 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 ', but that's deprecated); all but the last are interpreted as names of
packages, to locate the namespace in which to look up the final identifier
(see Packages for details). It's possible to substitute for a simple identifier an
expression which produces a reference to the value at runtime; this is
described in more detail below, and in the perlref manpage.
There are also special variables whose names don't follow these rules, so
that they don't accidentally collide with one of your normal variables.
Strings which 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 which provide windows into the
inner working of Perl have names containing punctuation characters (see the perlvar manpage).
Scalar values are always named with '$', even when referring to a scalar that is part of an array. It works like the English word ``the''. Thus we have:
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$days{'Feb'} # the 'Feb' value from hash %days
$#days # the last index of array @days
but entire arrays or array slices are denoted by '@', which works much like the word ``these'' or ``those'':
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as @days[3..5]
@days{'a','c'} # same as ($days{'a'},$days{'c'})
and entire hashes are denoted by '%':
%days # (key1, val1, key2, val2 ...)
In addition, subroutines are named with an initial '&', though this is optional when it's otherwise unambiguous (just as ``do'' is often redundant in English). Symbol table entries can be named with an initial '*', but you don't really care about that yet.
Every variable type has its own namespace. You can, without fear of
conflict, use the same name for a scalar variable, an array, or a hash (or,
for that matter, a filehandle, a subroutine 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 $foo. This may seem a bit weird, but
that's okay, because it is weird.
Because variable and array references always start with '$', '@', 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 rather than open. 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 an object of that type. For a description of this, see the perlref manpage.
Names that start with a digit may contain only more digits. Names which 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.)
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 a scalar context for the < STDIN> 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 a list context for < STDIN>, 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 righthand side in a scalar context, while assignment to an array or array slice evaluates the righthand side in a list context. Assignment to a list also evaluates the righthand side in a list context.
User defined subroutines may choose to care whether they are being called in a scalar or list context, but most subroutines do not need to care, because scalars are automatically interpolated into lists. See wantarray.
Scalars aren't necessarily one thing or another. There's no place to declare a scalar variable to be of type ``string'', or of type ``number'', or type ``filehandle'', or anything else. Perl is a contextually polymorphic language whose scalars can be strings, numbers, or references (which includes objects). While strings and numbers are considered pretty much the same thing for nearly all purposes, references are strongly-typed uncastable pointers with built-in 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.
There are actually two varieties of null scalars: defined and undefined.
Undefined null scalars are returned when 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. An undefined
null scalar may become defined the first time you use it as if it were
defined, but prior to that you can use the defined operator to
determine whether the value is defined or not.
To find out whether a given string is a valid non-zero number, it's usually 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 && $str ne "0") {
warn "That doesn't look like a number";
}
That's usually preferable because otherwise you won't treat
IEEE notations like NaN or Infinity properly. At other times you might prefer to use a regular expression to
check whether data is numeric. See the perlre manpage
for details on regular expressions.
warn "has nondigits" if /\D/;
warn "not a whole number" unless /^\d+$/;
warn "not an integer" unless /^[+-]?\d+$/
warn "not a decimal number" unless /^[+-]?\d+\.?\d*$/
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. (Actually, it's not the length of the array, it's the subscript of the
last element, because there is (ordinarily) a 0th element.) Assigning to $#days changes the length of the array. Shortening an array by this method
destroys intervening values. Lengthening an array that was previously
shortened
NO LONGER recovers the values that were in those elements. (It used to in Perl 4, but
we had to break this to make sure destructors were called when expected.)
You can also gain some measure of efficiency by preextending 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 a named array in a 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.) 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 nothing's left to doubt:
$element_count = scalar(@whatever);
If you evaluate a hash in a scalar context, it returns a value which is
true if and only if the hash contains any key/value pairs. (If there are
any key/value pairs, 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 (compiled
in) 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.)
12345
12345.67
.23E-10
0xffff # hex
0377 # octal
4_294_967_296 # underline for legibility
String literals are usually delimited by either single or double quotes.
They work much like shell quotes: double-quoted string literals are subject
to backslash and variable substitution; single-quoted strings are not
(except for ``\''' and ``\\''). The usual Unix backslash rules apply for making characters such as
newline, tab, etc., as well as some more exotic forms. See
Quote and Quotelike Operators for a list.
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 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 put curly brackets around the name to delimit it from following alphanumerics. In fact, an identifier within such curlies is forced to be a string, as is any single identifier within a hash subscript. 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.
Note that a single-quoted string must be separated from a preceding word by a space, because single quote is a valid (though deprecated) character in a variable name (see Packages).
Two special literals are __LINE__ and __FILE__, which represent the current line number and filename at that point in your program. They may be used only as separate tokens; they will not be interpolated into strings. In addition, the token __END__ may be used to indicate the logical end of the script before the actual end of file. Any following text is ignored, but may be read via the DATA filehandle. (The DATA filehandle may read data from only the main script, but not from any required file or evaluated string.) The two control characters ^D and ^Z are synonyms for __END__ (or __DATA__ in a module; see the SelfLoader manpage for details on __DATA__).
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 -w switch, Perl will warn you about any such words. Some people may wish to outlaw barewords entirely. If you say
use strict 'subs';
then any bareword that would
NOT be interpreted as a subroutine call produces a
compile-time error instead. The restriction lasts to the end of the
enclosing block. An inner block may countermand this by saying no strict 'subs'.
Array variables are interpolated into double-quoted strings by joining all
the elements of the array with the delimiter specified in the $"
variable ($LIST_SEPARATOR in English), space by default. The following are equivalent:
$temp = join($",@ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish substitution)
there is a bad ambiguity: Is /$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
brackets as above.
A line-oriented form of quoting is based on the shell
``here-doc'' 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. (If you put a space it will be treated as a null
identifier, which is valid, and matches the first blank 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 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;
(LIST)
In a context not requiring a list value, the value of the list literal is 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 variable foo. Note that the value of
an actual array in a scalar context is the length of the array; the
following assigns to $foo the value 3:
@foo = ('cc', '-E', $bar);
$foo = @foo; # $foo gets 3
You may have an optional comma before the closing parenthesis of an list literal, so that you can say:
@foo = (
1,
2,
3,
);
LISTs do automatic interpolation of sublists. That is, when a LIST is evaluated, each element of the list is evaluated in a list context, and the resulting list value is interpolated into LIST just as if each individual element were a member of LIST. Thus arrays lose their identity in a LIST--the list
(@foo,@bar,&SomeSub)
contains all the elements of @foo followed by all the elements
of @bar, followed by all the elements returned by the subroutine named
SomeSub when it's called in a list context. 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.
A list value may also be subscripted like a normal array. You must put the list in parentheses to avoid ambiguity. Examples:
# 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 if and only if each element of the list is legal to assign to:
($a, $b, $c) = (1, 2, 3);
($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
Array assignment in a 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 $x to f()'s return count
This is very handy when you want to do a list assignment in a Boolean context, because most list functions return a null list when finished, which when assigned produces a 0, which is interpreted as FALSE.
The final element may be an array or a hash:
($a, $b, @rest) = split;
local($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 get
a null value. This may be useful in a local or
my.
A hash literal contains pairs of values 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 usually 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 which 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 sort for examples of how to arrange for an output ordering.
*, 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. It also used to be the preferred way to pass filehandles into a function, but now that we have the *foo{THING} notation it isn't often needed for that, either. It is still needed to pass new filehandles into functions
(*HANDLE{IO} only works if
HANDLE has already been used).
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;
This is also a way to create a local filehandle. For example:
sub newopen {
my $path = shift;
local *FH; # not my!
open (FH, $path) || return undef;
return *FH;
}
$fh = newopen('/etc/passwd');
Another way to create local filehandles is with IO::Handle and its ilk, see the bottom of open().
See the perlref manpage, the perlsub manpage, and Symbol Tables for more discussion on typeglobs.