/* */ C-style comments, it will be interpreted either as division or pattern matching, depending on the context, and
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. 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.
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 sub name, thus:
sub myname;
$me = myname $0 or die "can't get myname";
Note that it functions as a list operator, not as a unary operator; so be
careful to use or instead of || in this case. However, if you were to declare the subroutine as sub myname ($), then
myname would functonion as a unary operator, so either or or
|| would work.
Subroutines declarations can also be loaded up with the require statement or both loaded and imported into your namespace with a use statement. See the perlmod manpage for details on this.
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.
Any simple statement may optionally be followed by a SINGLE modifier, just before the terminating semicolon (or block ending). The possible modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
The if and unless modifiers have the expected semantics, presuming you're a speaker of
English. The while and 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:
do {
$line = <STDIN>;
...
} until $line eq ".\n";
See do. Note also that the loop control statements described later will 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.
But generally, a block is delimited by curly brackets, also known as braces. We will call this syntactic construct a BLOCK.
The following compound statements may be used to control flow:
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (LIST) BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs, not statements. This means that the curly brackets are required--no dangling statements allowed. If you want to write conditionals without curly brackets there are several other ways to do it. The following all do the same thing:
if (!open(FOO)) { die "Can't open $FOO: $!"; }
die "Can't open $FOO: $!" unless open(FOO);
open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
# a bit exotic, that last one
The if statement is straightforward. Because BLOCKs are always bounded by curly
brackets, there is never any ambiguity about which
if an else goes with. If you use unless in place of if, the sense of the test is reversed.
The while statement executes the block as long as the expression is true (does not evaluate to the null string or 0 or ``0''). The
LABEL is optional, and if present, consists of an identifier followed by a colon. The
LABEL identifies the loop for the loop control statements
next,
last, and redo. If the
LABEL is omitted, the loop control statement refers to the innermost enclosing loop. This may include dynamically looking back your call-stack at run time to find the
LABEL. Such desperate behavior triggers a warning if you use the
-w flag.
If there is a continue
BLOCK, it is always executed just before the
conditional is about to be evaluated again, just like the third part of a
for loop in
C. Thus it can be used to increment a loop variable,
even when the loop has been continued via the next statement (which is similar to the
C continue statement).
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
The last command is like the break statement in
C (as used in loops); it immediately exits the loop in
question. The
continue block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
...
}
The redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is 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 /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 continue block on the above code, it would get executed even on discarded lines.
If the word while is replaced by the word until, the sense of the test is reversed, but the conditional is still tested
before the first iteration.
The form while/if BLOCK BLOCK, available in Perl 4, is no longer available. Replace any occurrence of if BLOCK by if (do BLOCK).
for loop works exactly like the corresponding while loop; that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as this:
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
(There is one minor difference: The first form implies a lexical scope for variables declared with my in the initialization expression.)
Besides the normal array index looping, 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
}
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 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 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 with
you have subroutine or format declarations.)
The foreach keyword is actually a synonym for the for keyword, so you can use foreach for readability or for for brevity. If
VAR is omitted, $_ is set to each value. If
LIST 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. That's because the
foreach
loop index variable is an implicit alias for each item in the list that
you're looping over.
Examples:
for (@ary) { s/foo/bar/ }
foreach my $elem (@elements) {
$elem *= 2;
}
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})) {
print "Item: $item\n";
}
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 next explicitly iterates the other loop rather than merely terminating the inner
one. And it's faster because Perl executes a foreach statement more rapidly than it would the equivalent for loop.
The BLOCK construct is particularly nice for doing case structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
There is no official switch statement in Perl, because there are already several ways to write the equivalent. In addition to the above, you could write
SWITCH: {
$abc = 1, last SWITCH if /^abc/;
$def = 1, last SWITCH if /^def/;
$xyz = 1, last SWITCH if /^xyz/;
$nothing = 1;
}
(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
SWITCH: {
/^abc/ && do { $abc = 1; last SWITCH; };
/^def/ && do { $def = 1; last SWITCH; };
/^xyz/ && do { $xyz = 1; last SWITCH; };
$nothing = 1;
}
or formatted so it stands out more as a ``proper'' switch statement:
SWITCH: {
/^abc/ && do {
$abc = 1;
last SWITCH;
};
/^def/ && do {
$def = 1;
last SWITCH;
};
/^xyz/ && do {
$xyz = 1;
last SWITCH;
};
$nothing = 1;
}
or
SWITCH: {
/^abc/ and $abc = 1, last SWITCH;
/^def/ and $def = 1, last SWITCH;
/^xyz/ and $xyz = 1, last SWITCH;
$nothing = 1;
}
or even, horrors,
if (/^abc/)
{ $abc = 1 }
elsif (/^def/)
{ $def = 1 }
elsif (/^xyz/)
{ $xyz = 1 }
else
{ $nothing = 1 }
A common idiom for a switch statement is to use 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 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 { ($flag & O_APPEND) ? "a+" : "r+" }
}
};
The goto-LABEL form finds the statement labeled with LABEL and resumes execution there. It may not be used to go into any construct that requires initialization, such as a subroutine or a foreach loop. It also can't be used to go into a construct that is optimized away. It can be used to go almost anywhere else within the dynamic scope, including out of subroutines, but it's usually better to use some other construct such as last or die. The author of Perl has never felt the need to use this form of goto (in Perl, that is--C is another matter).
The goto-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computed gotos per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$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 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 next, last, or redo instead of resorting to a goto. For certain applications, the catch and throw pair of
eval{} and die() for exception processing can also be a prudent
approach.
=head1 Here There Be Pods!
Then that text and all remaining text up through and including a line
beginning with =cut will be ignored. The format of the intervening text is described in the perlpod manpage.
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.
eval()). The syntax for this mechanism is the same as for most
C preprocessors: it matches the regular expression
/^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/ with $1 being the line number for the next line, and $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.