Perl has three built-in data types: scalars, arrays of scalars, and
associative arrays of scalars, known as "hashes". Normal arrays
-are ordered lists indexed by number, starting with 0 and with
+are ordered lists of scalars indexed by number, starting with 0 and with
negative subscripts counting from the end. Hashes are unordered
-collections of values indexed by their associated string key.
+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
int( <STDIN> )
-the integer operation provides scalar context for the E<lt><gt>
+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 E<lt><gt>, which
+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
context. Assignment to a list (or slice, which is just a list
anyway) also evaluates the righthand side in list context.
-When you use Perl's B<-w> command-line option, you may see warnings
+When you use the C<use warnings> pragma or Perl's B<-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 C<"fred";> or C<getpwuid(0);>. It still
reference-counting and destructor invocation.
A scalar value is interpreted as TRUE in the Boolean sense if it is not
-the empty string or the number 0 (or its string equivalent, "0"). The
+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.
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 empty list
+can truncate an array down to nothing by assigning the null list
() to it. The following are equivalent:
@whatever = ();
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 "C<\'>" and
-"C<\\>"). The usual C-style backslash rules apply for making
+substitution; single-quoted strings are not (except for C<\'> and
+C<\\>). The usual C-style backslash rules apply for making
characters such as newline, tab, etc., as well as some more exotic
-forms. See L<perlop/"Quote and Quotelike Operators"> for a list.
+forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
Hexadecimal, octal, or binary, representations in string literals
(e.g. '0xff') are not automatically converted to their integer
anything more complicated in the subscript will be interpreted as
an expression.
+A literal of the form C<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 C<"\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, C<cmp>, C<gt>, C<lt> etc.
+If there are two or more dots in the literal, the leading C<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 C<require> and C<use> for
+doing a version check. The C<$^V> special variable also contains the
+running Perl interpreter's version in this form. See L<perlvar/$^V>.
+
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 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__ in the main program, __DATA__ in a separate
-module. See L<SelfLoader> for more description of __DATA__, and
+(due to an empty C<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 C<PACKNAME::DATA>,
+where C<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
+C<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
+C<require> or C<do>) and leaves the remaining contents of the
+file accessible via C<main::DATA>.
+
+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
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 B<-w> switch, Perl will warn you about any
+words, and if you use the C<use warnings> pragma or the B<-w> switch,
+Perl will warn you about any
such words. Some people may wish to outlaw barewords entirely. If you
say
braces as above.
A line-oriented form of quoting is based on the shell "here-document"
-syntax. Following a C<E<lt>E<lt>> you specify a string to terminate
+syntax. Following a C<< << >> 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 C<E<lt>E<lt>> and
+double quotes. There must be no space between the C<< << >> and
the identifier. (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
called in list context, followed by the key/value pairs of %glarch.
To make a list reference that does I<NOT> interpolate, see L<perlref>.
-The empty list is represented by (). Interpolating it in a list
+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.
$x = (($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 empty 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:
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 C<=E<gt>> operator between key/value
-pairs. The C<=E<gt>> operator is mostly just a more visually distinctive
+It is often more readable to use the C<< => >> operator between key/value
+pairs. The C<< => >> 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:
=head2 Slices
-A common way 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.
+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
$dir = (getpwnam("daemon"))[7]; # likewise, but with list
A slice accesses several elements of a list, an array, or a hash
-simultaneously using a list of subscripts. It's a more convenient
-that writing out the individual elements as a list of separate
+simultaneously using a list of subscripts. It's more convenient
+than writing out the individual elements as a list of separate
scalar values.
($him, $her) = @folks[0,-1]; # array slice
($folks[0], $folks[-1]) = ($folks[0], $folks[-1]);
Since changing a slice changes the original array or hash that it's
-slicing, a C<foreach> construct will alter through some--or even
-all--of the values of the array or hash.
+slicing, a C<foreach> construct will alter some--or even all--of the
+values of the array or hash.
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
s/(\w+)/\u\L$1/g; # "titlecase" words
}
-You couldn't just loop through C<values %hash> to do this because
-that function produces a new list which is a copy of the values,
-so changing them doesn't change the original.
+A slice of an empty list is still an empty list. Thus:
+
+ @a = ()[1,0]; # @a has no elements
+ @b = (@a)[0,1]; # @b has no elements
+ @c = (0,1)[2,3]; # @c has no elements
+
+But:
+
+ @a = (1)[1,0]; # @a has two elements
+ @b = (1,undef)[1,0,2]; # @b has three elements
-As a special rule, if a slice would produce a list consisting entirely
-of undefined values, the empty list is produced instead. This makes
-it easy to write loops that terminate when an empty list is returned:
+This makes it easy to write loops that terminate when a null list
+is returned:
while ( ($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 empty list contains no elements, so when the password file is
+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