X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperldata.pod;h=58c11234b4213f5e5bd066c6e2a173b89484efdd;hb=f648820cb158526d3c3e16f712206316f2112b7e;hp=c1144715d8f3ba67338238836d3be7452780ef39;hpb=55497cffdd24c959994f9a8ddd56db8ce85e1c5b;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perldata.pod b/pod/perldata.pod index c114471..58c1123 100644 --- a/pod/perldata.pod +++ b/pod/perldata.pod @@ -19,18 +19,18 @@ I, that is, a string beginning with a letter or underscore, and containing letters, underscores, and digits. In some cases, it may be a chain of identifiers, separated by C<::> (or by C<'>, but that's deprecated); all but the last are interpreted as names of -packages, in order to locate the namespace in which to look +packages, to locate the namespace in which to look up the final identifier (see L for details). It's possible to substitute for a simple identifier an expression -which produces a reference to the value at runtime; this is +that produces a reference to the value at runtime; this is described in more detail below, and in L. There are also special variables whose names don't follow these rules, so that they don't accidentally collide with one of your -normal variables. Strings which match parenthesized parts of a +normal variables. Strings that match parenthesized parts of a regular expression are saved under names containing only digits after the C<$> (see L and L). In addition, several special -variables which provide windows into the inner working of Perl have names +variables that provide windows into the inner working of Perl have names containing punctuation characters (see L). Scalar values are always named with '$', even when referring to a scalar @@ -65,14 +65,14 @@ 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 $foo. This may seem a bit weird, but that's okay, because it is weird. -Since variable and array references always start with '$', '@', or '%', +Because variable and array references always start with '$', '@', or '%', the "reserved" words aren't in fact reserved with respect to variable names. (They ARE reserved with respect to labels and filehandles, however, which don't have an initial special character. You can't have a filehandle named "log", for instance. Hint: you could say C rather than C. Using uppercase filehandles also improves readability and protects you from conflict -with future reserved words.) Case I significant--"FOO", "Foo" and +with future reserved words.) Case I significant--"FOO", "Foo", and "foo" are all different names. Names that start with a letter or underscore may also contain digits and underscores. @@ -80,9 +80,9 @@ It is possible to replace such an alphanumeric name with an expression that returns a reference to an object of that type. For a description of this, see L. -Names that start with a digit may only contain more digits. Names -which do not start with a letter, underscore, or digit are limited to -one character, e.g. C<$%> or C<$$>. (Most of these one character names +Names that start with a digit may contain only more digits. Names +that do not start with a letter, underscore, or digit are limited to +one character, e.g., C<$%> or C<$$>. (Most of these one character names have a predefined significance to Perl. For instance, C<$$> is the current process id.) @@ -135,7 +135,7 @@ Scalar variables may contain various kinds of singular data, such as numbers, strings, and references. In general, conversion from one form to another is transparent. (A scalar may not contain multiple values, but may contain a reference to an array or hash containing multiple values.) -Because of the automatic conversion of scalars, operations and functions +Because of the automatic conversion of scalars, operations, and functions that return scalars don't need to care (and, in fact, can't care) whether the context is looking for a string or a number. @@ -145,12 +145,12 @@ type "filehandle", or anything else. Perl is a contextually polymorphic language whose scalars can be strings, numbers, or references (which includes objects). While strings and numbers are considered pretty much the same thing for nearly all purposes, references are strongly-typed -uncastable pointers with built-in reference-counting and destructor +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. +Boolean context is just a special kind of scalar context. There are actually two varieties of null scalars: defined and undefined. Undefined null scalars are returned when there is no real @@ -160,54 +160,57 @@ array. An undefined null scalar may become defined the first time you use it as if it were defined, but prior to that you can use the defined() operator to determine whether the value is defined or not. -To find out whether a given string is a valid non-zero number, it's usually +To find out whether a given string is a valid nonzero number, it's usually enough to test it against both numeric 0 and also lexical "0" (although this will cause B<-w> noises). That's because strings that aren't numbers count as 0, just as they do in B: if ($str == 0 && $str ne "0") { warn "That doesn't look like a number"; - } + } That's usually preferable because otherwise you won't treat IEEE notations like C or C properly. At other times you might prefer to -use a regular expression to check whether data is numeric. See L -for details on regular expressions. +use the POSIX::strtod function or a regular expression to check whether +data is numeric. See L for details on regular expressions. warn "has nondigits" if /\D/; - warn "not a whole number" unless /^\d+$/; - warn "not an integer" unless /^[+-]?\d+$/ - warn "not a decimal number" unless /^[+-]?\d+\.?\d*$/ - warn "not a C float" + 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<$#days>, as in B. (Actually, it's not -the length of the array, it's the subscript of the last element, since +the length of the array, it's the subscript of the last element, because there is (ordinarily) a 0th element.) Assigning to C<$#days> changes the length of the array. Shortening an array by this method destroys intervening values. Lengthening an array that was previously shortened I recovers the values that were in those elements. (It used to in Perl 4, but we had to break this to make sure destructors were -called when expected.) You can also gain some measure of efficiency by -preextending an array that is going to get big. (You can also extend +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; + $#whatever = -1; If you evaluate a named array in a 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.) The following is always true: +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 just assume that +So in general you can assume that scalar(@whatever) == $#whatever + 1; @@ -216,17 +219,22 @@ left to doubt: $element_count = scalar(@whatever); -If you evaluate a hash in a scalar context, it returns a value which is +If you evaluate a hash in a scalar context, it returns a value that is true if and only if the hash contains any key/value pairs. (If there are any key/value pairs, the value returned is a string consisting of the number of used buckets and the number of allocated buckets, separated -by a slash. This is pretty much only useful to find out whether Perl's +by a slash. This is pretty much useful only to find out whether Perl's (compiled in) hashing algorithm is performing poorly on your data set. For example, you stick 10,000 things in a hash, but evaluating %HASH in scalar context reveals "1/16", which means only one out of sixteen buckets 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 bucked to the next power of two: + + keys(%users) = 1000; # allocate 1024 buckets + =head2 Scalar value constructors Numeric literals are specified in any of the customary floating point or @@ -247,7 +255,12 @@ The usual Unix backslash rules apply for making characters such as newline, tab, etc., as well as some more exotic forms. See L for a list. -You can also embed newlines directly in your strings, i.e. they can end +Octal or hex representations in string literals (e.g. '0xffff') are not +automatically converted to their integer representation. The hex() and +oct() functions make these conversions for you. See L and +L 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 @@ -276,19 +289,25 @@ in the subscript will be interpreted as an expression. Note that a single-quoted string must be separated from a preceding word by a -space, since single quote is a valid (though deprecated) character in +space, because single quote is a valid (though deprecated) character in a variable name (see L). -Two special literals are __LINE__ and __FILE__, which represent the -current line number and filename at that point in your program. They -may only be used as separate tokens; they will not be interpolated into -strings. In addition, the token __END__ may be used to indicate the -logical end of the script before the actual end of file. Any following -text is ignored, but may be read via the DATA filehandle. (The DATA -filehandle may read data only from the main script, but not from any -required file or evaluated string.) The two control characters ^D and -^Z are synonyms for __END__ (or __DATA__ in a module; see L for -details on __DATA__). +Three special literals are __FILE__, __LINE__, and __PACKAGE__, which +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 directive), __PACKAGE__ is the undefined value. + +The tokens __END__ and __DATA__ may be used to indicate the logical end +of the script before the actual end of file. Any following text is +ignored, but may be read via a DATA filehandle: main::DATA for __END__, +or PACKNAME::DATA (where PACKNAME is the current package) for __DATA__. +The two control characters ^D and ^Z are synonyms for __END__ (or +__DATA__ in a module). See L 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 @@ -302,7 +321,7 @@ say 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 +end of the enclosing block. An inner block may countermand this by saying C. Array variables are interpolated into double-quoted strings by joining all @@ -334,11 +353,11 @@ 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 CE> and the identifier. (If you put a space it will be treated as a null -identifier, which is valid, and matches the first blank line.) The +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 < interpolate, see L. The null list is represented by (). Interpolating it in a list @@ -426,13 +445,13 @@ interpolating an array with no elements is the same as if no array had been interpolated at that point. A list value may also be subscripted like a normal array. You must -put the list in parentheses to avoid ambiguity. Examples: +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 PARENS + $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES # Find a hex digit. $hexdigit = ('a','b','c','d','e','f')[$digit-10]; @@ -440,6 +459,11 @@ put the list in parentheses to avoid ambiguity. Examples: # A "reverse comma operator". return (pop(@foo),pop(@foo))[0]; +You may assign to C 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); + Lists may be assigned to if and only if each element of the list is legal to assign to: @@ -454,13 +478,13 @@ produced by the expression on the right side of the assignment: $x = (($foo,$bar) = f()); # set $x to f()'s return count This is very handy when you want to do a list assignment in a Boolean -context, since most list functions return a null list when finished, +context, because most list functions return a null list when finished, which when assigned produces a 0, which is interpreted as FALSE. The final element may be an array or a hash: ($a, $b, @rest) = split; - local($a, $b, %rest) = @_; + my($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 get @@ -482,7 +506,7 @@ key/value pairs. That's why it's good to use references sometimes. It is often more readable to use the C<=E> operator between key/value pairs. The C<=E> 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 which would be a legal identifier. +interpreted as a string--if it's a bareword that would be a legal identifier. This makes it nice for initializing hashes: %map = ( @@ -501,7 +525,7 @@ or for initializing hash references to be used as records: or for using call-by-named-parameter to complicated functions: - $field = $query->radio_group( + $field = $query->radio_group( name => 'group_name', values => ['eenie','meenie','minie'], default => 'meenie', @@ -513,16 +537,67 @@ Note that just because a hash is initialized in that order doesn't mean that it comes out in that order. See L for examples of how to arrange for an output ordering. -=head2 Typeglobs +=head2 Typeglobs and Filehandles Perl uses an internal type called a I 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 +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. It also used to be the -preferred way to pass filehandles into a function, but now -that we have the *foo{THING} notation it isn't often needed for that, -either. +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 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 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 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 *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 +*HANDLE{IO} only works if HANDLE has already been used as a handle. +In other words, *FH can be used to create new symbol table entries, +but *foo{THING} cannot. + +Another way to create anonymous filehandles is 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 for an example. See L, L, and L for more -discussion on typeglobs. +discussion on typeglobs and the *foo{THING} syntax.