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diff --git a/Master/tlpkg/tlperl0/lib/pods/perldata.pod b/Master/tlpkg/tlperl0/lib/pods/perldata.pod new file mode 100755 index 00000000000..8f0feb7fed3 --- /dev/null +++ b/Master/tlpkg/tlperl0/lib/pods/perldata.pod @@ -0,0 +1,872 @@ +=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 + $#days # the last index of array @days + +Entire arrays (and slices of arrays and hashes) are denoted by '@', +which works much like the word "these" or "those" does in English, +in that it indicates multiple values are expected. +X<array> + + @days # ($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 righthand 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 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 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 - $[ + 1; + +Version 5 of Perl changed the semantics of C<$[>: 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 +X<$[> + + 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, a fatal error will result, since this +bucket usage information is currently not available for tied hashes. +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. 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> + +B<Note:> Version Strings (v-strings) have been deprecated. They will +be removed in some future release after Perl 5.8.1. The marginal +benefits of v-strings were greatly outweighed by the potential for +Surprise and Confusion. + +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. 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. +X<__FILE__> X<__LINE__> X<__PACKAGE__> 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 +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 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 Joining Delimiter +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 = $query->radio_group( + name => 'group_name', + values => ['eenie','meenie','minie'], + default => 'meenie', + linebreak => 'true', + labels => \%labels + ); + +Note that just because a hash is initialized in that order doesn't +mean that it comes out in that order. See L<perlfunc/sort> for examples +of how to arrange for an output ordering. + +=head2 Subscripts + +An array is subscripted by specifying a dollar sign (C<$>), 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"; + +=head2 Slices +X<slice> X<array, slice> X<hash, slice> + +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 + $dir = (getpwnam("daemon"))[7]; # likewise, but with list + +A slice accesses several elements of a list, an array, or a hash +simultaneously using a list of subscripts. It's more convenient +than writing out the individual elements as a list of separate +scalar values. + + ($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, $home; + } + +As noted earlier in this document, the scalar sense of list assignment +is the number of elements on the right-hand side of the assignment. +The null list contains no elements, so when the password file is +exhausted, the result is 0, not 2. + +If you're confused about why you use an '@' there on a hash slice +instead of a '%', think of it like this. The type of bracket (square +or curly) governs whether it's an array or a hash being looked at. +On the other hand, the leading symbol ('$' 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. |