and containing letters, underscores, and digits. In some cases, it
may be a chain of identifiers, separated by C<::> (or by C<'>, but
that's deprecated); all but the last are interpreted as names of
-packages, in order to locate the namespace in which to look
+packages, to locate the namespace in which to look
up the final identifier (see L<perlmod/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
+that produces a reference to the value at runtime; this is
described in more detail below, and in L<perlref>.
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
+normal variables. Strings that match parenthesized parts of a
regular expression are saved under names containing only digits after
the C<$> (see L<perlop> and L<perlre>). In addition, several special
-variables which provide windows into the inner working of Perl have names
+variables that provide windows into the inner working of Perl have names
containing punctuation characters (see L<perlvar>).
Scalar values are always named with '$', even when referring to a scalar
means that C<$foo[1]> is a part of @foo, not a part of $foo. This may
seem a bit weird, but that's okay, because it is weird.
-Since variable and array references always start with '$', '@', or '%',
+Because variable and array references always start with '$', '@', or '%',
the "reserved" words aren't in fact reserved with respect to variable
names. (They ARE reserved with respect to labels and filehandles,
however, which don't have an initial special character. You can't have
a filehandle named "log", for instance. Hint: you could say
C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase
filehandles also improves readability and protects you from conflict
-with future reserved words.) Case I<IS> significant--"FOO", "Foo" and
+with future reserved words.) Case I<IS> significant--"FOO", "Foo", and
"foo" are all different names. Names that start with a letter or
underscore may also contain digits and underscores.
that returns a reference to an object of that type. For a description
of this, see L<perlref>.
-Names that start with a digit may only contain more digits. Names
-which do not start with a letter, underscore, or digit are limited to
-one character, e.g. C<$%> or C<$$>. (Most of these one character names
+Names that start with a digit may contain only more digits. Names
+that do not start with a letter, underscore, or digit are limited to
+one character, e.g., C<$%> or C<$$>. (Most of these one character names
have a predefined significance to Perl. For instance, C<$$> is the
current process id.)
numbers, strings, and references. In general, conversion from one form to
another is transparent. (A scalar may not contain multiple values, but
may contain a reference to an array or hash containing multiple values.)
-Because of the automatic conversion of scalars, operations and functions
+Because of the automatic conversion of scalars, operations, and functions
that return scalars don't need to care (and, in fact, can't care) whether
the context is looking for a string or a number.
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
+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.
+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
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
+To find out whether a given string is a valid nonzero number, it's usually
enough to test it against both numeric 0 and also lexical "0" (although
this will cause B<-w> noises). That's because strings that aren't
numbers count as 0, just as they do in B<awk>:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
- }
+ }
That's usually preferable because otherwise you won't treat IEEE notations
like C<NaN> or C<Infinity> properly. At other times you might prefer to
-use a regular expression to check whether data is numeric. See L<perlre>
-for details on regular expressions.
+use the POSIX::strtod function or a regular expression to check whether
+data is numeric. See L<perlre> for details on regular expressions.
warn "has nondigits" if /\D/;
- warn "not a whole number" unless /^\d+$/;
- warn "not an integer" unless /^[+-]?\d+$/
- warn "not a decimal number" unless /^[+-]?\d+\.?\d*$/
- warn "not a C float"
+ 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 C<$#days>, as in B<csh>. (Actually, it's not
-the length of the array, it's the subscript of the last element, since
+the length of the array, it's the subscript of the last element, because
there is (ordinarily) a 0th element.) Assigning to C<$#days> changes the
length of the array. Shortening an array by this method destroys
intervening values. Lengthening an array that was previously shortened
I<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
+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;
+ $#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:
+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 - $[ + 1;
Version 5 of Perl changed the semantics of C<$[>: files that don't set
the value of C<$[> no longer need to worry about whether another
file changed its value. (In other words, use of C<$[> is deprecated.)
-So in general you can just assume that
+So in general you can assume that
scalar(@whatever) == $#whatever + 1;
$element_count = scalar(@whatever);
-If you evaluate a hash in a scalar context, it returns a value which is
+If you evaluate a hash in a scalar context, it returns a value that 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 only useful to find out whether Perl's
+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.)
+You can preallocate space for a hash by assigning to the keys() function.
+This rounds up the allocated bucked to the next power of two:
+
+ keys(%users) = 1000; # allocate 1024 buckets
+
=head2 Scalar value constructors
Numeric literals are specified in any of the customary floating point or
newline, tab, etc., as well as some more exotic forms. See
L<perlop/Quote and Quotelike Operators> for a list.
-You can also embed newlines directly in your strings, i.e. they can end
+Octal or hex representations in string literals (e.g. '0xffff') are not
+automatically converted to their integer representation. The hex() and
+oct() functions make these conversions for you. See L<perlfunc/hex> and
+L<perlfunc/oct> 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
Note that a
single-quoted string must be separated from a preceding word by a
-space, since single quote is a valid (though deprecated) character in
+space, because single quote is a valid (though deprecated) character in
a variable name (see L<perlmod/Packages>).
-Two special literals are __LINE__ and __FILE__, which represent the
-current line number and filename at that point in your program. They
-may only be used 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 only from 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 L<SelfLoader> for
-details on __DATA__).
+Three special literals are __FILE__, __LINE__, and __PACKAGE__, which
+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 C<package;> directive), __PACKAGE__ is the undefined value.
+
+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, but may be read via a DATA filehandle: main::DATA for __END__,
+or PACKNAME::DATA (where PACKNAME is the current package) for __DATA__.
+The two control characters ^D and ^Z are synonyms for __END__ (or
+__DATA__ in a module). See L<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.
A word that has no other interpretation in the grammar will
be treated as if it were a quoted string. These are known as
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
+end of the enclosing block. An inner block may countermand this
by saying C<no strict 'subs'>.
Array variables are interpolated into double-quoted strings by joining all
text, just as in regular quoting. An unquoted identifier works like
double quotes. There must be no space between the C<E<lt>E<lt>> 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
+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;
+ print <<EOF;
The price is $Price.
EOF
Here's a line
or two.
THIS
- and here another.
+ 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
+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
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:
+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 an
+You may have an optional comma before the closing parenthesis of a
list literal, so that you can say:
@foo = (
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
+individual element were a member of LIST. Thus arrays and hashes lose their
identity in a LIST--the list
- (@foo,@bar,&SomeSub)
+ (@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 when
-it's called in a list context.
+followed by all the elements returned by the subroutine named SomeSub
+called in a list context, followed by the key/value pairs of %glarch.
To make a list reference that does I<NOT> interpolate, see L<perlref>.
The null list is represented by (). Interpolating it in a list
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:
+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 PARENS
+ $time = stat($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];
+You may assign to C<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);
+
Lists may be assigned to if and only if each element of the list
is legal to assign to:
($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
-Array assignment in a scalar context returns the number of elements
+List 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 $x to f()'s return count
This is very handy when you want to do a list assignment in a Boolean
-context, since most list functions return a null list when finished,
+context, because most list functions return a null list when finished,
which when assigned produces a 0, which is interpreted as FALSE.
The final element may be an array or a hash:
($a, $b, @rest) = split;
- local($a, $b, %rest) = @_;
+ my($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 get
It is often more readable to use the C<=E<gt>> operator between key/value
pairs. The C<=E<gt>> 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.
+interpreted as a string--if it's a bareword that would be a legal identifier.
This makes it nice for initializing hashes:
%map = (
or for using call-by-named-parameter to complicated functions:
- $field = $query->radio_group(
+ $field = $query->radio_group(
name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
-=head2 Typeglobs
+=head2 Typeglobs and Filehandles
Perl uses an internal type called a I<typeglob> to hold an entire
symbol table entry. The type prefix of a typeglob is a C<*>, because
-it represents all types. This used to be the preferred way to
+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.
+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 L<perlmod/"Symbol Tables"> 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 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 L<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 can be used to create new symbol table entries,
+but *foo{THING} cannot.
+
+Another way to create anonymous filehandles is 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
+L<perlfunc/open()> for an example.
See L<perlref>, L<perlsub>, and L<perlmod/"Symbol Tables"> for more
-discussion on typeglobs.
+discussion on typeglobs and the *foo{THING} syntax.
https://perl5.git.perl.org / perl5.git / blobdiff