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diff --git a/contrib/perl5/pod/perldata.pod b/contrib/perl5/pod/perldata.pod deleted file mode 100644 index 315f716..0000000 --- a/contrib/perl5/pod/perldata.pod +++ /dev/null @@ -1,829 +0,0 @@ -=head1 NAME - -perldata - Perl data types - -=head1 DESCRIPTION - -=head2 Variable names - -Perl has three built-in data types: scalars, arrays of scalars, and -associative arrays of scalars, known as "hashes". Normal arrays -are ordered lists of scalars indexed by number, starting with 0 and with -negative subscripts counting from the end. 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>. - -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<$> (see L<perlop> and L<perlre>). -In addition, several special variables that provide windows into -the inner working of Perl have names containing punctuation characters -and control characters. These are documented in L<perlvar>. - -Scalar values are always named with '$', 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. - - $days # the simple scalar value "days" - $days[28] # the 29th element of array @days - $days{'Feb'} # the 'Feb' value from hash %days - $#days # the last index of array @days - -Entire arrays (and slices of arrays and hashes) are denoted by '@', -which works much like the word "these" or "those" does in English, -in that it indicates multiple values are expected. - - @days # ($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 '%': - - %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 $foo. This may seem a bit weird, -but that's okay, because it is weird. - -Because variable references always start with '$', '@', 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. - -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, or digit are limited to -one character, e.g., C<$%> or C<$$>. (Most of these one character names -have a predefined significance to Perl. For instance, C<$$> is the -current process id.) - -=head2 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 - -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. - -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. - -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 B<-w> noises). That's because strings -that aren't numbers count as 0, just as they do in B<awk>: - - if ($str == 0 && $str ne "0") { - warn "That doesn't look like a number"; - } - -That method may be best because otherwise you won't treat IEEE -notations like C<NaN> or C<Infinity> properly. At other times, you -might prefer to determine whether string data can be used numerically -by calling the POSIX::strtod() function or by inspecting your string -with a regular expression (as documented in L<perlre>). - - warn "has nondigits" if /\D/; - warn "not a natural number" unless /^\d+$/; # 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.) - -You can also gain some miniscule measure of efficiency by pre-extending -an array that is going to get big. You can also extend an array -by assigning to an element that is off the end of the array. You -can truncate an array down to nothing by assigning the null list -() to it. The following are equivalent: - - @whatever = (); - $#whatever = -1; - -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: - - 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 - - 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. - -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 - -Numeric literals are specified in any of the following floating point or -integer formats: - - 12345 - 12345.67 - .23E-10 # a very small number - 4_294_967_296 # underline for legibility - 0xff # hex - 0377 # octal - 0b011011 # binary - -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. - -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." - - $Price = '$100'; # not interpreted - print "The price is $Price.\n"; # interpreted - -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: - - $who = "Larry"; - print PASSWD "${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<$days{Feb}> and the quotes will be assumed automatically. But -anything more complicated in the subscript will be interpreted as -an expression. - -A literal of the form C<v1.20.300.4000> is parsed as a string composed -of characters with the specified ordinals. This provides an alternative, -more readable way to construct strings, rather than use the somewhat less -readable interpolation form C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful -for representing Unicode strings, and for comparing version "numbers" -using the string comparison operators, C<cmp>, C<gt>, C<lt> etc. -If there are two or more dots in the literal, the leading C<v> may be -omitted. - - print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}" - print v102.111.111; # prints "foo" - print 102.111.111; # same - -Such literals are accepted by both C<require> and C<use> for -doing a version check. The C<$^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 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 -__DATA__ (or __END__) token has not yet been seen. - -A word that has no other interpretation in the grammar will -be treated as if it were a quoted string. These are known as -"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. 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'>. - -Arrays and slices are interpolated into double-quoted strings -by joining the elements with the delimiter specified in the C<$"> -variable (C<$LIST_SEPARATOR> in English), 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. - -A line-oriented form of quoting is based on the shell "here-document" -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<< << >> and -the identifier, unless the identifier is quoted. (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 surrounding whitespace) on the terminating line. - - print <<EOF; - The price is $Price. - EOF - - print << "EOF"; # same as above - The price is $Price. - EOF - - print << `EOC`; # execute commands - echo hi there - echo lo there - EOC - - print <<"foo", <<"bar"; # you can stack them - I said foo. - foo - I said bar. - bar - - myfunc(<< "THIS", 23, <<'THAT'); - Here's a line - or two. - THIS - and here's another. - THAT - -Just don't forget that you have to put a semicolon on the end -to finish the statement, as Perl doesn't know you're not going to -try to do this: - - print <<ABC - 179231 - ABC - + 20; - -If you want your here-docs to be indented with the -rest of the code, you'll need to remove leading whitespace -from each line manually: - - ($quote = <<'FINIS') =~ s/^\s+//gm; - The Road goes ever on and on, - down from the door where it began. - FINIS - -If you use a here-doc within a delimited construct, such as in C<s///eg>, -the quoted material must come on the lines following the final delimiter. -So instead of - - s/this/<<E . 'that' - the other - E - . 'more '/eg; - -you have to write - - s/this/<<E . 'that' - . 'more '/eg; - the other - E - -If the terminating identifier is on the last line of the program, you -must be sure there is a newline after it; otherwise, Perl will give the -warning B<Can't find string terminator "END" anywhere before EOF...>. - -Additionally, the quoting rules for the identifier are not related to -Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported -in place of C<''> and C<"">, and the only interpolation is for backslashing -the quoting character: - - print << "abc\"def"; - testing... - abc"def - -Finally, quoted strings cannot span multiple lines. The general rule is -that the identifier must be a string literal. Stick with that, and you -should be safe. - -=head2 List value constructors - -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 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 $x to f()'s return count - -This is handy when you want to do a list assignment in a Boolean -context, because most list functions return a null list when finished, -which when assigned produces a 0, which is interpreted as FALSE. - -The final element may be an array or a hash: - - ($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 identifier. -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 Slices - -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[0], $folks[-1]); - -Since changing a slice changes the original array or hash that it's -slicing, a C<foreach> construct will alter some--or even all--of the -values of the array or hash. - - foreach (@array[ 4 .. 10 ]) { s/peter/paul/ } - - foreach (@hash{keys %hash}) { - s/^\s+//; # trim leading whitespace - s/\s+$//; # 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 - -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 - } - -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. |
