summaryrefslogtreecommitdiff
path: root/Master/tlpkg/tlperl/lib/pods/perldata.pod
diff options
context:
space:
mode:
Diffstat (limited to 'Master/tlpkg/tlperl/lib/pods/perldata.pod')
-rw-r--r--Master/tlpkg/tlperl/lib/pods/perldata.pod887
1 files changed, 0 insertions, 887 deletions
diff --git a/Master/tlpkg/tlperl/lib/pods/perldata.pod b/Master/tlpkg/tlperl/lib/pods/perldata.pod
deleted file mode 100644
index 876382d29fe..00000000000
--- a/Master/tlpkg/tlperl/lib/pods/perldata.pod
+++ /dev/null
@@ -1,887 +0,0 @@
-=head1 NAME
-
-perldata - Perl data types
-
-=head1 DESCRIPTION
-
-=head2 Variable names
-X<variable, name> X<variable name> X<data type> X<type>
-
-Perl has three built-in data types: scalars, arrays of scalars, and
-associative arrays of scalars, known as "hashes". A scalar is a
-single string (of any size, limited only by the available memory),
-number, or a reference to something (which will be discussed
-in L<perlref>). Normal arrays are ordered lists of scalars indexed
-by number, starting with 0. 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 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 the slightly
-archaic C<'>); 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 that produces a reference
-to the value at runtime. This is described in more detail below
-and in L<perlref>.
-X<identifier>
-
-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 C<$> (see L<perlop> and L<perlre>).
-In addition, several special variables that provide windows into
-the inner working of Perl have names containing punctuation characters
-and control characters. These are documented in L<perlvar>.
-X<variable, built-in>
-
-Scalar values are always named with '$', 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.
-X<scalar>
-
- $days # the simple scalar value "days"
- $days[28] # the 29th element of array @days
- $days{'Feb'} # the 'Feb' value from hash %days
- $#days # the last index of array @days
-
-Entire arrays (and slices of arrays and hashes) are denoted by '@',
-which works much as the word "these" or "those" does in English,
-in that it indicates multiple values are expected.
-X<array>
-
- @days # ($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 '%':
-X<hash>
-
- %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 $foo
-and @foo are two different variables. It also means that C<$foo[1]>
-is a part of @foo, not a part of $foo. This may seem a bit weird,
-but that's okay, because it is weird.
-X<namespace>
-
-Because variable references always start with '$', '@', or '%', the
-"reserved" words aren't in fact reserved with respect to variable
-names. They I<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 "foo" are all different names. Names that start with a
-letter or underscore may also contain digits and underscores.
-X<identifier, case sensitivity>
-X<case>
-
-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 L<perlref>.
-
-Names that start with a digit may contain only more digits. Names
-that do not start with a letter, underscore, digit or a caret (i.e.
-a control character) 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.)
-
-=head2 Context
-X<context> X<scalar context> X<list 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 right-hand side in list context.
-
-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
-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 L<perlfunc/wantarray>
-for how you would dynamically discern your function's calling
-context.
-
-=head2 Scalar values
-X<scalar> X<number> X<string> X<reference>
-
-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 FALSE in the Boolean sense
-if it is undefined, the null string or the number 0 (or its
-string equivalent, "0"), and TRUE if it is anything else. The
-Boolean context is just a special kind of scalar context where no
-conversion to a string or a number is ever performed.
-X<boolean> X<bool> X<true> X<false> X<truth>
-
-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 C<"">.
-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 L<perlref>. 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.
-X<defined> X<undefined> X<undef> X<null> X<string, null>
-
-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 noises if warnings are on). That's
-because strings that aren't numbers count as 0, just as they do in B<awk>:
-
- if ($str == 0 && $str ne "0") {
- warn "That doesn't look like a number";
- }
-
-That method may be best because otherwise you won't treat IEEE
-notations like C<NaN> or C<Infinity> properly. At other times, you
-might prefer to determine whether string data can be used numerically
-by calling the POSIX::strtod() function or by inspecting your string
-with a regular expression (as documented in L<perlre>).
-
- warn "has nondigits" if /\D/;
- warn "not a natural number" unless /^\d+$/; # 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+))?$/;
-
-The length of an array is a scalar value. You may find the length
-of array @days by evaluating C<$#days>, as in B<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 C<$#days> actually changes the length of the array.
-Shortening an array this way destroys intervening values. Lengthening
-an array that was previously shortened does not recover values
-that were in those elements. (It used to do so in Perl 4, but we
-had to break this to make sure destructors were called when expected.)
-X<$#> X<array, length>
-
-You can also gain some minuscule 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:
-X<array, length>
-
- 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 C<"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. If a tied hash
-is evaluated in scalar context, the C<SCALAR> method is called (with a
-fallback to C<FIRSTKEY>).
-X<hash, scalar context> X<hash, bucket> X<bucket>
-
-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
-
-=head2 Scalar value constructors
-X<scalar, literal> X<scalar, constant>
-
-Numeric literals are specified in any of the following floating point or
-integer formats:
-
- 12345
- 12345.67
- .23E-10 # a very small number
- 3.14_15_92 # a very important number
- 4_294_967_296 # underscore for legibility
- 0xff # hex
- 0xdead_beef # more hex
- 0377 # octal (only numbers, begins with 0)
- 0b011011 # binary
-
-You are allowed to use underscores (underbars) in numeric literals
-between digits for legibility (but not multiple underscores in a row:
-C<23__500> is not legal; C<23_500> is).
-You could, for example, group binary
-digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
-or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
-X<number, literal>
-
-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
-characters such as newline, tab, etc., as well as some more exotic
-forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
-X<string, literal>
-
-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 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
-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 $Z<>100."
-X<interpolation>
-
- $Price = '$100'; # not interpolated
- print "The price is $Price.\n"; # interpolated
-
-There is no double interpolation in Perl, so the C<$100> is left as is.
-
-By default floating point numbers substituted inside strings use the
-dot (".") as the decimal separator. If C<use locale> is in effect,
-and POSIX::setlocale() has been called, the character used for the
-decimal separator is affected by the LC_NUMERIC locale.
-See L<perllocale> and L<POSIX>.
-
-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:
-X<interpolation>
-
- $who = "Larry";
- print PASSWD "${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 $whospeak, a
-C<$who::0>, and a C<$who's> variable. The last two would be the
-$0 and the $s variables in the (presumably) non-existent package
-C<who>.
-
-In fact, an identifier within such curlies is forced to be a string,
-as is any simple identifier within a hash subscript. Neither need
-quoting. Our earlier example, C<$days{'Feb'}> can be written as
-C<$days{Feb}> and the quotes will be assumed automatically. But
-anything more complicated in the subscript will be interpreted as an
-expression. This means for example that C<$version{2.0}++> is
-equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
-
-=head3 Version Strings
-X<version string> X<vstring> X<v-string>
-
-A literal of the form C<v1.20.300.4000> is parsed as a string composed
-of characters with the specified ordinals. This form, known as
-v-strings, 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 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. Note that using the v-strings for IPv4
-addresses is not portable unless you also use the
-inet_aton()/inet_ntoa() routines of the Socket package.
-
-Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
-are not v-strings before the C<< => >> operator (which is usually used
-to separate a hash key from a hash value); instead they are interpreted
-as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
-Perl 5.8.0, but that caused more confusion and breakage than good.
-Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
-be v-strings always.
-
-=head3 Special Literals
-X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
-X<end> X<data> X<^D> X<^Z>
-
-The special literals __FILE__, __LINE__, and __PACKAGE__
-represent the current filename, line number, and package name at that
-point in your program. __SUB__ gives a reference to the current
-subroutine. 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. (But the empty C<package;> is no longer supported, as of version
-5.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
-is only available in 5.16 or higher, and only with a C<use v5.16> or
-C<use feature "current_sub"> declaration.
-X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
-X<line> X<file> X<package>
-
-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__ 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
-line after __DATA__. The program should C<close DATA> when it is done
-reading from it. (Leaving it open leaks filehandles if the module is
-reloaded for any reason, so it's a safer practice to close it.) For
-compatibility with older scripts written before __DATA__ was
-introduced, __END__ behaves like __DATA__ in the top level 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
-__DATA__ (or __END__) token has not yet been seen.
-
-=head3 Barewords
-X<bareword>
-
-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 C<use warnings> pragma or the B<-w> switch,
-Perl will warn you about any such words. Perl limits barewords (like
-identifiers) to about 250 characters. Future versions of Perl are likely
-to eliminate these arbitrary limitations.
-
-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 C<no strict 'subs'>.
-
-=head3 Array Interpolation
-X<array, interpolation> X<interpolation, array> X<$">
-
-Arrays and slices are interpolated into double-quoted strings
-by joining the elements with the delimiter specified in the C<$">
-variable (C<$LIST_SEPARATOR> if "use English;" is specified),
-space by default. The following are equivalent:
-
- $temp = join($", @ARGV);
- system "echo $temp";
-
- system "echo @ARGV";
-
-Within search patterns (which also undergo double-quotish substitution)
-there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
-C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
-expression) or as C</${foo[bar]}/> (where C<[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 C<[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.
-
-If you're looking for the information on how to use here-documents,
-which used to be here, that's been moved to
-L<perlop/Quote and Quote-like Operators>.
-
-=head2 List value constructors
-X<list>
-
-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 I<NOT> interpolate, see L<perlref>.
-
-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 when necessary for
-precedence) and lists may end with an optional comma to mean that
-multiple commas within lists are legal syntax. The list C<1,,3> is a
-concatenation of two lists, C<1,> and C<3>, the first of which ends
-with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
-similarly for C<1,,,3> is C<(1,),(,),3> is C<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'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
-
-An exception to this is that you may assign to C<undef> in a list.
-This is useful for throwing away some of the return values of a
-function:
-
- ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
-
-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.
-
-It's also the source of a useful idiom for executing a function or
-performing an operation in list context and then counting the number of
-return values, by assigning to an empty list and then using that
-assignment in scalar context. For example, this code:
-
- $count = () = $string =~ /\d+/g;
-
-will place into $count the number of digit groups found in $string.
-This happens because the pattern match is in list context (since it
-is being assigned to the empty list), and will therefore return a list
-of all matching parts of the string. The list assignment in scalar
-context will translate that into the number of elements (here, the
-number of times the pattern matched) and assign that to $count. Note
-that simply using
-
- $count = $string =~ /\d+/g;
-
-would not have worked, since a pattern match in scalar context will
-only return true or false, rather than a count of matches.
-
-The final element of a list assignment may be an array or a hash:
-
- ($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 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 simple
-identifier. C<< => >> doesn't quote compound identifiers, that contain
-double colons. 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 = $query->radio_group(
- name => 'group_name',
- values => ['eenie','meenie','minie'],
- default => 'meenie',
- linebreak => 'true',
- labels => \%labels
- );
-
-Note that just because a hash is initialized in that order doesn't
-mean that it comes out in that order. See L<perlfunc/sort> for examples
-of how to arrange for an output ordering.
-
-=head2 Subscripts
-
-An array can be accessed one scalar at a
-time by specifying a dollar sign (C<$>), then the
-name of the array (without the leading C<@>), then the subscript inside
-square brackets. For example:
-
- @myarray = (5, 50, 500, 5000);
- print "The Third Element is", $myarray[2], "\n";
-
-The array indices start with 0. A negative subscript retrieves its
-value from the end. In our example, C<$myarray[-1]> would have been
-5000, and C<$myarray[-2]> would have been 500.
-
-Hash subscripts are similar, only instead of square brackets curly brackets
-are used. For example:
-
- %scientists =
- (
- "Newton" => "Isaac",
- "Einstein" => "Albert",
- "Darwin" => "Charles",
- "Feynman" => "Richard",
- );
-
- print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
-
-You can also subscript a list to get a single element from it:
-
- $dir = (getpwnam("daemon"))[7];
-
-=head2 Multi-dimensional array emulation
-
-Multidimensional arrays may be emulated by subscripting a hash with a
-list. The elements of the list are joined with the subscript separator
-(see L<perlvar/$;>).
-
- $foo{$a,$b,$c}
-
-is equivalent to
-
- $foo{join($;, $a, $b, $c)}
-
-The default subscript separator is "\034", the same as SUBSEP in B<awk>.
-
-=head2 Slices
-X<slice> X<array, slice> X<hash, slice>
-
-A slice accesses several elements of a list, an array, or a hash
-simultaneously using a list of subscripts. It's more convenient
-than writing out the individual elements as a list of separate
-scalar values.
-
- ($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[-1], $folks[0]);
-
-Since changing a slice changes the original array or hash that it's
-slicing, a C<foreach> construct will alter some--or even all--of the
-values of the array or hash.
-
- foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
-
- foreach (@hash{qw[key1 key2]}) {
- s/^\s+//; # trim leading whitespace
- s/\s+$//; # 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.
-
-Slices in scalar context return the last item of the slice.
-
- @a = qw/first second third/;
- %h = (first => 'A', second => 'B');
- $t = @a[0, 1]; # $t is now 'second'
- $u = @h{'first', 'second'}; # $u is now 'B'
-
-If you're confused about why you use an '@' there on a hash slice
-instead of a '%', think of it like this. The type of bracket (square
-or curly) governs whether it's an array or a hash being looked at.
-On the other hand, the leading symbol ('$' or '@') on the array or
-hash indicates whether you are getting back a singular value (a
-scalar) or a plural one (a list).
-
-=head2 Typeglobs and Filehandles
-X<typeglob> X<filehandle> X<*>
-
-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.
-
-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 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 C<*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
-C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
-In other words, C<*FH> must be used to create new symbol table entries;
-C<*foo{THING}> cannot. When in doubt, use C<*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 C<open(my $fh, ...)> and C<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
- }
-
-Note that if an initialized scalar variable is used instead the
-result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
-to C<open( *{'zzz'}, ...)>.
-C<use strict 'refs'> forbids such practice.
-
-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 L<perlfunc/open> for an
-example.
-
-=head1 SEE ALSO
-
-See L<perlvar> for a description of Perl's built-in variables and
-a discussion of legal variable names. See L<perlref>, L<perlsub>,
-and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
-the C<*foo{THING}> syntax.