implicit loop with either the B<-n> or B<-p> switch. It's just not the
mandatory default like it is in B<sed> and B<awk>.)
+=head2 Declarations
+
Perl is, for the most part, a free-form language. (The only
exception to this is format declarations, for obvious reasons.) Comments
are indicated by the "#" character, and extend to the end of the line. If
you attempt to use C</* */> C-style comments, it will be interpreted
either as division or pattern matching, depending on the context, and C++
-C<//> comments just look like a null regular expression, So don't do
+C<//> comments just look like a null regular expression, so don't do
that.
A declaration can be put anywhere a statement can, but has no effect on
the execution of the primary sequence of statements--declarations all
take effect at compile time. Typically all the declarations are put at
-the beginning or the end of the script.
+the beginning or the end of the script. However, if you're using
+lexically-scoped private variables created with my(), you'll have to make sure
+your format or subroutine definition is within the same block scope
+as the my if you expect to be able to access those private variables.
-As of Perl 5, declaring a subroutine allows a subroutine name to be used
-as if it were a list operator from that point forward in the program. You
-can declare a subroutine without defining it by saying just
+Declaring a subroutine allows a subroutine name to be used as if it were a
+list operator from that point forward in the program. You can declare a
+subroutine without defining it by saying C<sub name>, thus:
sub myname;
$me = myname $0 or die "can't get myname";
-Note that it functions as a list operator though, not a unary
-operator, so be careful to use C<or> instead of C<||> there.
+Note that it functions as a list operator, not as a unary operator; so
+be careful to use C<or> instead of C<||> in this case. However, if
+you were to declare the subroutine as C<sub myname ($)>, then
+C<myname> would functonion as a unary operator, so either C<or> or
+C<||> would work.
-Subroutines declarations can also be imported by a C<use> statement.
+Subroutines declarations can also be loaded up with the C<require> statement
+or both loaded and imported into your namespace with a C<use> statement.
+See L<perlmod> for details on this.
-Also as of Perl 5, a statement sequence may contain declarations of
-lexically scoped variables, but apart from declaring a variable name,
-the declaration acts like an ordinary statement, and is elaborated within
-the sequence of statements as if it were an ordinary statement.
+A statement sequence may contain declarations of lexically-scoped
+variables, but apart from declaring a variable name, the declaration acts
+like an ordinary statement, and is elaborated within the sequence of
+statements as if it were an ordinary statement. That means it actually
+has both compile-time and run-time effects.
=head2 Simple statements
side effects. Every simple statement must be terminated with a
semicolon, unless it is the final statement in a block, in which case
the semicolon is optional. (A semicolon is still encouraged there if the
-block takes up more than one line, since you may add another line.)
+block takes up more than one line, because you may eventually add another line.)
Note that there are some operators like C<eval {}> and C<do {}> that look
-like compound statements, but aren't (they're just TERMs in an expression),
-and thus need an explicit termination
-if used as the last item in a statement.
+like compound statements, but aren't (they're just TERMs in an expression),
+and thus need an explicit termination if used as the last item in a statement.
Any simple statement may optionally be followed by a I<SINGLE> modifier,
just before the terminating semicolon (or block ending). The possible
unless EXPR
while EXPR
until EXPR
+ foreach EXPR
The C<if> and C<unless> modifiers have the expected semantics,
-presuming you're a speaker of English. The C<while> and C<until>
-modifiers also have the usual "while loop" semantics (conditional
-evaluated first), except when applied to a do-BLOCK (or to the
-now-deprecated do-SUBROUTINE statement), in which case the block
-executes once before the conditional is evaluated. This is so that you
-can write loops like:
+presuming you're a speaker of English. The C<foreach> modifier is an
+iterator: For each value in EXPR, it aliases $_ to the value and
+executes the statement. The C<while> and C<until> modifiers have the
+usual "while loop" semantics (conditional evaluated first), except
+when applied to a do-BLOCK (or to the now-deprecated do-SUBROUTINE
+statement), in which case the block executes once before the
+conditional is evaluated. This is so that you can write loops like:
do {
- $_ = <STDIN>;
+ $line = <STDIN>;
...
- } until $_ eq ".\n";
+ } until $line eq ".\n";
See L<perlfunc/do>. Note also that the loop control
-statements described later will I<NOT> work in this construct, since
+statements described later will I<NOT> work in this construct, because
modifiers don't take loop labels. Sorry. You can always wrap
-another block around it to do that sort of thing.)
+another block around it to do that sort of thing.
=head2 Compound statements
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
- LABEL foreach VAR (ARRAY) BLOCK
+ LABEL foreach VAR (LIST) BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
# a bit exotic, that last one
-The C<if> statement is straightforward. Since BLOCKs are always
+The C<if> statement is straightforward. Because BLOCKs are always
bounded by curly brackets, there is never any ambiguity about which
C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
the sense of the test is reversed.
The C<while> statement executes the block as long as the expression is
true (does not evaluate to the null string or 0 or "0"). The LABEL is
-optional, and if present, consists of an identifier followed by a
-colon. The LABEL identifies the loop for the loop control statements
-C<next>, C<last>, and C<redo> (see below). If there is a C<continue>
-BLOCK, it is always executed just before the conditional is about to be
-evaluated again, just like the third part of a C<for> loop in C.
-Thus it can be used to increment a loop variable, even when the loop
-has been continued via the C<next> statement (which is similar to the C
-C<continue> statement).
+optional, and if present, consists of an identifier followed by a colon.
+The LABEL identifies the loop for the loop control statements C<next>,
+C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
+refers to the innermost enclosing loop. This may include dynamically
+looking back your call-stack at run time to find the LABEL. Such
+desperate behavior triggers a warning if you use the B<-w> flag.
+
+If there is a C<continue> BLOCK, it is always executed just before the
+conditional is about to be evaluated again, just like the third part of a
+C<for> loop in C. Thus it can be used to increment a loop variable, even
+when the loop has been continued via the C<next> statement (which is
+similar to the C C<continue> statement).
+
+=head2 Loop Control
+
+The C<next> command is like the C<continue> statement in C; it starts
+the next iteration of the loop:
+
+ LINE: while (<STDIN>) {
+ next LINE if /^#/; # discard comments
+ ...
+ }
+
+The C<last> command is like the C<break> statement in C (as used in
+loops); it immediately exits the loop in question. The
+C<continue> block, if any, is not executed:
+
+ LINE: while (<STDIN>) {
+ last LINE if /^$/; # exit when done with header
+ ...
+ }
+
+The C<redo> command restarts the loop block without evaluating the
+conditional again. The C<continue> block, if any, is I<not> executed.
+This command is normally used by programs that want to lie to themselves
+about what was just input.
+
+For example, when processing a file like F</etc/termcap>.
+If your input lines might end in backslashes to indicate continuation, you
+want to skip ahead and get the next record.
+
+ while (<>) {
+ chomp;
+ if (s/\\$//) {
+ $_ .= <>;
+ redo unless eof();
+ }
+ # now process $_
+ }
+
+which is Perl short-hand for the more explicitly written version:
+
+ LINE: while (defined($line = <ARGV>)) {
+ chomp($line);
+ if ($line =~ s/\\$//) {
+ $line .= <ARGV>;
+ redo LINE unless eof(); # not eof(ARGV)!
+ }
+ # now process $line
+ }
+
+Or here's a simpleminded Pascal comment stripper (warning: assumes no
+{ or } in strings).
+
+ LINE: while (<STDIN>) {
+ while (s|({.*}.*){.*}|$1 |) {}
+ s|{.*}| |;
+ if (s|{.*| |) {
+ $front = $_;
+ while (<STDIN>) {
+ if (/}/) { # end of comment?
+ s|^|$front{|;
+ redo LINE;
+ }
+ }
+ }
+ print;
+ }
+
+Note that if there were a C<continue> block on the above code, it would get
+executed even on discarded lines.
If the word C<while> is replaced by the word C<until>, the sense of the
test is reversed, but the conditional is still tested before the first
iteration.
-In either the C<if> or the C<while> statement, you may replace "(EXPR)"
-with a BLOCK, and the conditional is true if the value of the last
-statement in that block is true. (This feature continues to work in Perl
-5 but is deprecated. Please change any occurrences of "if BLOCK" to
-"if (do BLOCK)".)
+The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
+available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
+
+=head2 For Loops
-The C-style C<for> loop works exactly like the corresponding C<while> loop:
+Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
+that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
-is the same as
+is the same as this:
$i = 1;
while ($i < 10) {
$i++;
}
-The foreach loop iterates over a normal list value and sets the
-variable VAR to be each element of the list in turn. The variable is
-implicitly local to the loop (unless declared previously with C<my>),
-and regains its former value upon exiting the loop. The C<foreach>
-keyword is actually a synonym for the C<for> keyword, so you can use
-C<foreach> for readability or C<for> for brevity. If VAR is omitted, $_
-is set to each value. If ARRAY is an actual array (as opposed to an
-expression returning a list value), you can modify each element of the
-array by modifying VAR inside the loop. Examples:
+(There is one minor difference: The first form implies a lexical scope
+for variables declared with C<my> in the initialization expression.)
+
+Besides the normal array index looping, C<for> can lend itself
+to many other interesting applications. Here's one that avoids the
+problem you get into if you explicitly test for end-of-file on
+an interactive file descriptor causing your program to appear to
+hang.
+
+ $on_a_tty = -t STDIN && -t STDOUT;
+ sub prompt { print "yes? " if $on_a_tty }
+ for ( prompt(); <STDIN>; prompt() ) {
+ # do something
+ }
+
+=head2 Foreach Loops
+
+The C<foreach> loop iterates over a normal list value and sets the
+variable VAR to be each element of the list in turn. If the variable
+is preceded with the keyword C<my>, then it is lexically scoped, and
+is therefore visible only within the loop. Otherwise, the variable is
+implicitly local to the loop and regains its former value upon exiting
+the loop. If the variable was previously declared with C<my>, it uses
+that variable instead of the global one, but it's still localized to
+the loop. (Note that a lexically scoped variable can cause problems
+if you have subroutine or format declarations within the loop which
+refer to it.)
+
+The C<foreach> keyword is actually a synonym for the C<for> keyword, so
+you can use C<foreach> for readability or C<for> for brevity. If VAR is
+omitted, $_ is set to each value. If any element of LIST is an lvalue,
+you can modify it by modifying VAR inside the loop. That's because
+the C<foreach> loop index variable is an implicit alias for each item
+in the list that you're looping over.
+
+If any part of LIST is an array, C<foreach> will get very confused if
+you add or remove elements within the loop body, for example with
+C<splice>. So don't do that.
+
+C<foreach> probably won't do what you expect if VAR is a tied or other
+special variable. Don't do that either.
+
+Examples:
- for (@ary) { s/foo/bar/; }
+ for (@ary) { s/foo/bar/ }
- foreach $elem (@elements) {
+ foreach my $elem (@elements) {
$elem *= 2;
}
- for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
- print $_, "\n"; sleep(1);
+ for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
+ print $count, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
- foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) {
+ foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
print "Item: $item\n";
}
-A BLOCK by itself (labeled or not) is semantically equivalent to a loop
-that executes once. Thus you can use any of the loop control
-statements in it to leave or restart the block. The C<continue> block
-is optional. This construct is particularly nice for doing case
+Here's how a C programmer might code up a particular algorithm in Perl:
+
+ for (my $i = 0; $i < @ary1; $i++) {
+ for (my $j = 0; $j < @ary2; $j++) {
+ if ($ary1[$i] > $ary2[$j]) {
+ last; # can't go to outer :-(
+ }
+ $ary1[$i] += $ary2[$j];
+ }
+ # this is where that last takes me
+ }
+
+Whereas here's how a Perl programmer more comfortable with the idiom might
+do it:
+
+ OUTER: foreach my $wid (@ary1) {
+ INNER: foreach my $jet (@ary2) {
+ next OUTER if $wid > $jet;
+ $wid += $jet;
+ }
+ }
+
+See how much easier this is? It's cleaner, safer, and faster. It's
+cleaner because it's less noisy. It's safer because if code gets added
+between the inner and outer loops later on, the new code won't be
+accidentally executed. The C<next> explicitly iterates the other loop
+rather than merely terminating the inner one. And it's faster because
+Perl executes a C<foreach> statement more rapidly than it would the
+equivalent C<for> loop.
+
+=head2 Basic BLOCKs and Switch Statements
+
+A BLOCK by itself (labeled or not) is semantically equivalent to a
+loop that executes once. Thus you can use any of the loop control
+statements in it to leave or restart the block. (Note that this is
+I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
+C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
+block is optional.
+
+The BLOCK construct is particularly nice for doing case
structures.
SWITCH: {
$nothing = 1;
}
-(That's actually not as strange as it looks one you realize that you can
+(That's actually not as strange as it looks once you realize that you can
use loop control "operators" within an expression, That's just the normal
C comma operator.)
or formatted so it stands out more as a "proper" switch statement:
SWITCH: {
- /^abc/ && do {
- $abc = 1;
- last SWITCH;
+ /^abc/ && do {
+ $abc = 1;
+ last SWITCH;
};
- /^def/ && do {
- $def = 1;
- last SWITCH;
+ /^def/ && do {
+ $def = 1;
+ last SWITCH;
};
- /^xyz/ && do {
- $xyz = 1;
- last SWITCH;
+ /^xyz/ && do {
+ $xyz = 1;
+ last SWITCH;
};
$nothing = 1;
}
else
{ $nothing = 1 }
+
+A common idiom for a switch statement is to use C<foreach>'s aliasing to make
+a temporary assignment to $_ for convenient matching:
+
+ SWITCH: for ($where) {
+ /In Card Names/ && do { push @flags, '-e'; last; };
+ /Anywhere/ && do { push @flags, '-h'; last; };
+ /In Rulings/ && do { last; };
+ die "unknown value for form variable where: `$where'";
+ }
+
+Another interesting approach to a switch statement is arrange
+for a C<do> block to return the proper value:
+
+ $amode = do {
+ if ($flag & O_RDONLY) { "r" }
+ elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
+ elsif ($flag & O_RDWR) {
+ if ($flag & O_CREAT) { "w+" }
+ else { ($flag & O_APPEND) ? "a+" : "r+" }
+ }
+ };
+
+=head2 Goto
+
+Although not for the faint of heart, Perl does support a C<goto> statement.
+A loop's LABEL is not actually a valid target for a C<goto>;
+it's just the name of the loop. There are three forms: goto-LABEL,
+goto-EXPR, and goto-&NAME.
+
+The goto-LABEL form finds the statement labeled with LABEL and resumes
+execution there. It may not be used to go into any construct that
+requires initialization, such as a subroutine or a foreach loop. It
+also can't be used to go into a construct that is optimized away. It
+can be used to go almost anywhere else within the dynamic scope,
+including out of subroutines, but it's usually better to use some other
+construct such as last or die. The author of Perl has never felt the
+need to use this form of goto (in Perl, that is--C is another matter).
+
+The goto-EXPR form expects a label name, whose scope will be resolved
+dynamically. This allows for computed gotos per FORTRAN, but isn't
+necessarily recommended if you're optimizing for maintainability:
+
+ goto ("FOO", "BAR", "GLARCH")[$i];
+
+The goto-&NAME form is highly magical, and substitutes a call to the
+named subroutine for the currently running subroutine. This is used by
+AUTOLOAD() subroutines that wish to load another subroutine and then
+pretend that the other subroutine had been called in the first place
+(except that any modifications to @_ in the current subroutine are
+propagated to the other subroutine.) After the C<goto>, not even caller()
+will be able to tell that this routine was called first.
+
+In almost all cases like this, it's usually a far, far better idea to use the
+structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
+resorting to a C<goto>. For certain applications, the catch and throw pair of
+C<eval{}> and die() for exception processing can also be a prudent approach.
+
+=head2 PODs: Embedded Documentation
+
+Perl has a mechanism for intermixing documentation with source code.
+While it's expecting the beginning of a new statement, if the compiler
+encounters a line that begins with an equal sign and a word, like this
+
+ =head1 Here There Be Pods!
+
+Then that text and all remaining text up through and including a line
+beginning with C<=cut> will be ignored. The format of the intervening
+text is described in L<perlpod>.
+
+This allows you to intermix your source code
+and your documentation text freely, as in
+
+ =item snazzle($)
+
+ The snazzle() function will behave in the most spectacular
+ form that you can possibly imagine, not even excepting
+ cybernetic pyrotechnics.
+
+ =cut back to the compiler, nuff of this pod stuff!
+
+ sub snazzle($) {
+ my $thingie = shift;
+ .........
+ }
+
+Note that pod translators should look at only paragraphs beginning
+with a pod directive (it makes parsing easier), whereas the compiler
+actually knows to look for pod escapes even in the middle of a
+paragraph. This means that the following secret stuff will be
+ignored by both the compiler and the translators.
+
+ $a=3;
+ =secret stuff
+ warn "Neither POD nor CODE!?"
+ =cut back
+ print "got $a\n";
+
+You probably shouldn't rely upon the warn() being podded out forever.
+Not all pod translators are well-behaved in this regard, and perhaps
+the compiler will become pickier.
+
+One may also use pod directives to quickly comment out a section
+of code.
+
+=head2 Plain Old Comments (Not!)
+
+Much like the C preprocessor, perl can process line directives. Using
+this, one can control perl's idea of filenames and line numbers in
+error or warning messages (especially for strings that are processed
+with eval()). The syntax for this mechanism is the same as for most
+C preprocessors: it matches the regular expression
+C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
+number for the next line, and C<$2> being the optional filename
+(specified within quotes).
+
+Here are some examples that you should be able to type into your command
+shell:
+
+ % perl
+ # line 200 "bzzzt"
+ # the `#' on the previous line must be the first char on line
+ die 'foo';
+ __END__
+ foo at bzzzt line 201.
+
+ % perl
+ # line 200 "bzzzt"
+ eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
+ __END__
+ foo at - line 2001.
+
+ % perl
+ eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
+ __END__
+ foo at foo bar line 200.
+
+ % perl
+ # line 345 "goop"
+ eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
+ print $@;
+ __END__
+ foo at goop line 345.
+
+=cut