diff options
Diffstat (limited to 'pod/perldata.pod')
| -rw-r--r-- | pod/perldata.pod | 56 |
1 files changed, 39 insertions, 17 deletions
diff --git a/pod/perldata.pod b/pod/perldata.pod index c1144715d8..407a25204f 100644 --- a/pod/perldata.pod +++ b/pod/perldata.pod @@ -19,7 +19,7 @@ I<identifier>, that is, a string beginning with a letter or underscore, and containing letters, underscores, and digits. In some cases, it may be a chain of identifiers, separated by C<::> (or by C<'>, but that's deprecated); all but the last are interpreted as names of -packages, 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<perlmod/Packages> for details). It's possible to substitute for a simple identifier an expression which produces a reference to the value at runtime; this is @@ -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 '$', '@', or '%', the "reserved" words aren't in fact reserved with respect to variable names. (They ARE reserved with respect to labels and filehandles, however, which don't have an initial special character. You can't have a filehandle named "log", for instance. Hint: you could say C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase filehandles also improves readability and protects you from conflict -with future reserved words.) Case I<IS> significant--"FOO", "Foo" and +with future reserved words.) Case I<IS> significant--"FOO", "Foo", and "foo" are all different names. Names that start with a letter or underscore may also contain digits and underscores. @@ -80,9 +80,9 @@ It is possible to replace such an alphanumeric name with an expression that returns a reference to an object of that type. For a description of this, see L<perlref>. -Names that start with a digit may only contain more digits. Names +Names that start with a digit may contain only more digits. Names which do not start with a letter, underscore, or digit are limited to -one character, e.g. C<$%> or C<$$>. (Most of these one character names +one character, e.g., C<$%> or C<$$>. (Most of these one character names have a predefined significance to Perl. For instance, C<$$> is the current process id.) @@ -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. @@ -183,7 +183,7 @@ for details on regular expressions. The length of an array is a scalar value. You may find the length of array @days by evaluating C<$#days>, as in B<csh>. (Actually, it's not -the length of the array, it's the subscript of the last element, since +the length of the array, it's the subscript of the last element, because there is (ordinarily) a 0th element.) Assigning to C<$#days> changes the length of the array. Shortening an array by this method destroys intervening values. Lengthening an array that was previously shortened @@ -207,7 +207,7 @@ last value, like the C comma operator.) The following is always true: 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; @@ -220,7 +220,7 @@ If you evaluate a hash in a scalar context, it returns a value which 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 @@ -247,7 +247,7 @@ The usual Unix backslash rules apply for making characters such as newline, tab, etc., as well as some more exotic forms. See L<perlop/Quote and Quotelike Operators> for a list. -You can also embed newlines directly in your strings, i.e. they can end +You can also embed newlines directly in your strings, i.e., they can end on a different line than they begin. This is nice, but if you forget your trailing quote, the error will not be reported until Perl finds another line containing the quote character, which may be much further @@ -276,16 +276,16 @@ 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<perlmod/Packages>). Two special literals are __LINE__ and __FILE__, which represent the current line number and filename at that point in your program. They -may only be used as separate tokens; they will not be interpolated into +may be used only as separate tokens; they will not be interpolated into strings. In addition, the token __END__ may be used to indicate the logical end of the script before the actual end of file. Any following text is ignored, but may be read via the DATA filehandle. (The DATA -filehandle may read data only from the main script, but not from any +filehandle may read data from only the main script, but not from any required file or evaluated string.) The two control characters ^D and ^Z are synonyms for __END__ (or __DATA__ in a module; see L<SelfLoader> for details on __DATA__). @@ -432,7 +432,7 @@ put the list in parentheses to avoid ambiguity. Examples: $time = (stat($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]; @@ -454,7 +454,7 @@ 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: @@ -513,7 +513,7 @@ 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 Typeglobs +=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 @@ -522,7 +522,29 @@ 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. +either. It is still needed to pass new filehandles into functions +(*HANDLE{IO} only works if HANDLE has already been used). + +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; + +This is also a way to create a local filehandle. For example: + + sub newopen { + my $path = shift; + local *FH; # not my! + open (FH, $path) || return undef; + return \*FH; + } + $fh = newopen('/etc/passwd'); + +Another way to create local filehandles is with IO::Handle and its ilk, +see the bottom of L<perlfunc/open()>. See L<perlref>, L<perlsub>, and L<perlmod/"Symbol Tables"> for more discussion on typeglobs. |