indexed by number, starting with 0. (Negative subscripts count from
the end.) Hash arrays are indexed by string.
+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
+I<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 C<::> (or by C<'>, 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 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
+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
+regular expression are saved under names containing only digits after
+the C<$> (see L<perlop> and L<perlre>). In addition, several special
+variables which provide windows into the inner working of Perl have names
+containing punctuation characters (see L<perlvar>).
+
Scalar values are always named with '$', even when referring to a scalar
that is part of an array. It works like the English word "the". Thus
we have:
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 '$', '@', or '%',
the "reserved" words aren't in fact reserved with respect to variable
names. (They ARE reserved with respect to labels and filehandles,
however, which don't have an initial special character. You can't have
a filehandle named "log", for instance. Hint: you could say
C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase
filehandles also improves readability and protects you from conflict
-with future reserved words.) Case I<IS> significant--"FOO", "Foo" and
+with future reserved words.) Case I<IS> significant--"FOO", "Foo", and
"foo" are all different names. Names that start with a letter or
underscore may also contain digits and underscores.
that returns a reference to an object of that type. For a description
of this, see L<perlref>.
-Names that start with a digit may only contain more digits. Names
+Names that start with a digit may contain only more digits. Names
which do not start with a letter, underscore, or digit are limited to
-one character, e.g. C<$%> or C<$$>. (Most of these one character names
+one character, e.g., C<$%> or C<$$>. (Most of these one character names
have a predefined significance to Perl. For instance, C<$$> is the
current process id.)
int( <STDIN> )
-the integer operation provides a scalar context for the <STDIN>
+the integer operation provides a scalar context for the E<lt>STDINE<gt>
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
+then the sort operation provides a list context for E<lt>STDINE<gt>, 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
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.
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 the pretty
-much same thing for nearly all purposes, but references are strongly-typed
+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.
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 B<-w> noises). That's because strings that aren't
-numbers count as 0, just as the do in I<awk>:
+numbers count as 0, just as they do in B<awk>:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
The length of an array is a scalar value. You may find the length of
array @days by evaluating C<$#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 make to make sure destructors were
+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.)
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 $[ is deprecated.)
-So in general you can just assume that
+Version 5 of Perl changed the semantics of C<$[>: 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 assume that
scalar(@whatever) == $#whatever + 1;
-Some programmer choose to use an explcit conversion so nothing's
+Some programmers choose to use an explicit conversion so nothing's
left to doubt:
$element_count = scalar(@whatever);
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
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 "C<\'>" and "C<\\>"). The usual Unix backslash rules apply for making
-characters such as newline, tab, etc., as well as some more exotic
-forms. See L<perlop/qq> for a list.
+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 "C<\'>" and "C<\\>").
+The usual Unix 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.
-You can also embed newlines directly in your strings, i.e. they can end
+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,
-identifiers beginning with $ or @, followed by an optional bracketed
+names beginning with $ or @, followed by an optional bracketed
expression as a subscript.) The following code segment prints out "The
-price is $100."
+price is $Z<>100."
$Price = '$100'; # not interpreted
print "The price is $Price.\n"; # interpreted
-As in some shells, you can put curly brackets around the identifier to
+As in some shells, you can put curly brackets around the name to
delimit it from following alphanumerics. In fact, an identifier
within such curlies is forced to be a string, as is any single
identifier within a hash subscript. Our earlier example,
Note that a
single-quoted string must be separated from a preceding word by a
-space, since single quote is a valid (though deprecated) character in
-an identifier (see L<perlmod/Packages>).
+space, because single quote is a valid (though deprecated) character in
+a variable name (see L<perlmod/Packages>).
Two special literals are __LINE__ and __FILE__, which represent the
current line number and filename at that point in your program. They
-may only be used as separate tokens; they will not be interpolated into
+may be used only as separate tokens; they will not be interpolated into
strings. In addition, the token __END__ may be used to indicate the
logical end of the script before the actual end of file. Any following
text is ignored, but may be read via the DATA filehandle. (The DATA
-filehandle may read data only from the main script, but not from any
+filehandle may read data from only the main script, but not from any
required file or evaluated string.) The two control characters ^D and
^Z are synonyms for __END__ (or __DATA__ in a module; see L<SelfLoader> for
details on __DATA__).
Array variables are interpolated into double-quoted strings by joining all
the elements of the array with the delimiter specified in the C<$">
-variable ($LIST_SEPARATOR in English), space by default. The following
+variable (C<$LIST_SEPARATOR> in English), space by default. The following
are equivalent:
$temp = join($",@ARGV);
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 C<E<lt>E<lt>> 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 the identifier. (If you put a space it
-will be treated as a null identifier, which is valid, and matches the
-first blank line--see the Merry Christmas example below.) The terminating
-string must appear by itself (unquoted and with no surrounding
-whitespace) on the terminating line.
-
- print <<EOF; # same as above
+A line-oriented form of quoting is based on the shell "here-doc"
+syntax. Following a C<E<lt>E<lt>> 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
+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
I said bar.
bar
- myfunc(<<"THIS", 23, <<'THAT'');
+ myfunc(<<"THIS", 23, <<'THAT');
Here's a line
or two.
THIS
$time = (stat($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];
$x = (($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:
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 quotes its left-hand operand, which makes
-it nice for initializing hashes:
+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,
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
-=head2 Typeglobs and FileHandles
+=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
pass arrays and hashes by reference into a function, but now that
-we have real references, this is seldom needed.
+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).
-One place where you still use typeglobs (or references thereto)
-is for passing or storing filehandles. If you want to save away
-a filehandle, do it this way:
+If you need to use a typeglob to save away a filehandle, do it this way:
$fh = *STDOUT;
$fh = \*STDOUT;
-This is also the way to create a local filehandle. For example:
+This is also a way to create a local filehandle. For example:
sub newopen {
my $path = shift;
}
$fh = newopen('/etc/passwd');
-See L<perlref>, L<perlsub>, and L<perlmod/"Symbols Tables"> for more
-discussion on typeglobs. See L<perlfunc/open> for other ways of
-generating filehandles.
+Another way to create local filehandles is with IO::Handle and its ilk,
+see the bottom of L<perlfunc/open()>.
+
+See L<perlref>, L<perlsub>, and L<perlmod/"Symbol Tables"> for more
+discussion on typeglobs.
projects / p5sagit/p5-mst-13.2.git / blobdiff