diff options
Diffstat (limited to 'pod/perlop.pod')
| -rw-r--r-- | pod/perlop.pod | 47 |
1 files changed, 34 insertions, 13 deletions
diff --git a/pod/perlop.pod b/pod/perlop.pod index 4781b7fbbe..69e4fcb0d9 100644 --- a/pod/perlop.pod +++ b/pod/perlop.pod @@ -599,7 +599,7 @@ a transliteration, the first ten of these sequences may be used. \Q quote regexp metacharacters till \E If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> -and <\U> is taken from the current locale. See L<perllocale>. +and C<\U> is taken from the current locale. See L<perllocale>. Patterns are subject to an additional level of interpretation as a regular expression. This is done as a second pass, after variables are @@ -897,7 +897,7 @@ text is not evaluated as a command. If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing quotes, e.g., C<s(foo)(bar)> or C<sE<lt>fooE<gt>/bar/>. A C</e> will cause the -replacement portion to be interpreter as a full-fledged Perl expression +replacement portion to be interpreted as a full-fledged Perl expression and eval()ed right then and there. It is, however, syntax checked at compile-time. @@ -1031,7 +1031,7 @@ an eval(): =head2 I/O Operators There are several I/O operators you should know about. -A string is enclosed by backticks (grave accents) first undergoes +A string enclosed by backticks (grave accents) first undergoes variable substitution just like a double quoted string. It is then interpreted as a command, and the output of that command is the value of the pseudo-literal, like in a shell. In a scalar context, a single @@ -1054,17 +1054,35 @@ Ordinarily you must assign that value to a variable, but there is one situation where an automatic assignment happens. I<If and ONLY if> the input symbol is the only thing inside the conditional of a C<while> or C<for(;;)> loop, the value is automatically assigned to the variable -C<$_>. The assigned value is then tested to see if it is defined. -(This may seem like an odd thing to you, but you'll use the construct -in almost every Perl script you write.) Anyway, the following lines -are equivalent to each other: +C<$_>. In these loop constructs, the assigned value (whether assignment +is automatic or explcit) is then tested to see if it is defined. +The defined test avoids problems where line has a string value +that would be treated as false by perl e.g. "" or "0" with no trailing +newline. (This may seem like an odd thing to you, but you'll use the +construct in almost every Perl script you write.) Anyway, the following +lines are equivalent to each other: while (defined($_ = <STDIN>)) { print; } + while ($_ = <STDIN>) { print; } while (<STDIN>) { print; } for (;<STDIN>;) { print; } print while defined($_ = <STDIN>); + print while ($_ = <STDIN>); print while <STDIN>; +and this also behaves similarly, but avoids the use of $_ : + + while (my $line = <STDIN>) { print $line } + +If you really mean such values to terminate the loop they should be +tested for explcitly: + + while (($_ = <STDIN>) ne '0') { ... } + while (<STDIN>) { last unless $_; ... } + +In other boolean contexts C<E<lt>I<filehandle>E<gt>> without explcit C<defined> +test or comparison will solicit a warning if C<-w> is in effect. + The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in packages, where they would be interpreted as local identifiers rather @@ -1124,9 +1142,9 @@ Getopts modules or put a loop on the front like this: ... # code for each line } -The E<lt>E<gt> symbol will return FALSE only once. If you call it again after -this it will assume you are processing another @ARGV list, and if you -haven't set @ARGV, will input from STDIN. +The E<lt>E<gt> symbol will return C<undef> for end-of-file only once. +If you call it again after this it will assume you are processing another +@ARGV list, and if you haven't set @ARGV, will input from STDIN. If the string inside the angle brackets is a reference to a scalar variable (e.g., E<lt>$fooE<gt>), then that variable contains the name of the @@ -1174,9 +1192,12 @@ A glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In a list context this isn't important, because you automatically get them all anyway. In a scalar context, however, the operator returns the next value -each time it is called, or a FALSE value if you've just run out. Again, -FALSE is returned only once. So if you're expecting a single value from -a glob, it is much better to say +each time it is called, or a C<undef> value if you've just run out. As +for filehandles an automatic C<defined> is generated when the glob +occurs in the test part of a C<while> or C<for> - because legal glob returns +(e.g. a file called F<0>) would otherwise terminate the loop. +Again, C<undef> is returned only once. So if you're expecting a single value +from a glob, it is much better to say ($file) = <blurch*>; |