X<subroutine, declaration>
sub myname;
- $me = myname $0 or die "can't get myname";
+ $me = myname $0 or die "can't get myname";
A bare declaration like that declares the function to be a list operator,
not a unary operator, so you have to be careful to use parentheses (or
This is so that you can write loops like:
do {
- $line = <STDIN>;
- ...
+ $line = <STDIN>;
+ ...
} until !defined($line) || $line eq ".\n"
See L<perlfunc/do>. Note also that the loop control statements described
later will I<NOT> work in this construct, because modifiers don't take
loop labels. Sorry. You can always put another block inside of it
-(for C<next>) or around it (for C<last>) to do that sort of thing.
-For C<next>, just double the braces:
+(for C<next>/C<redo>) or around it (for C<last>) to do that sort of thing.
X<next> X<last> X<redo>
+For C<next> or C<redo>, just double the braces:
+
do {{
- next if $x == $y;
- # do something here
+ next if $x == $y;
+ # do something here
}} until $x++ > $z;
-For C<last>, you have to be more elaborate:
+For C<last>, you have to be more elaborate and put braces around it:
X<last>
- LOOP: {
- do {
- last if $x = $y**2;
- # do something here
- } while $x++ <= $z;
+ {
+ do {
+ last if $x == $y**2;
+ # do something here
+ } while $x++ <= $z;
+ }
+
+If you need both C<next> and C<last>, you have to do both and also use a
+loop label:
+
+ LOOP: {
+ do {{
+ next if $x == $y;
+ last LOOP if $x == $y**2;
+ # do something here
+ }} until $x++ > $z;
}
B<NOTE:> The behaviour of a C<my>, C<state>, or
Under the current implementation, the C<foreach> loop can be
anywhere within the C<when> modifier's dynamic scope, but must be
-within the C<given> block's lexical scope. This restricted may
-be relaxed in a future release. See L<"Switch Statements"> below.
+within the C<given> block's lexical scope. This restriction may
+be relaxed in a future release. See L</"Switch Statements"> below.
=head2 Compound Statements
X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
PHASE BLOCK
-The experimental C<given> statement is I<not automatically enabled>; see
+The experimental C<given> statement is I<not automatically enabled>; see
L</"Switch Statements"> below for how to do so, and the attendant caveats.
Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs,
die "Can't open $FOO: $!" unless open(FOO);
open(FOO) || die "Can't open $FOO: $!";
open(FOO) ? () : die "Can't open $FOO: $!";
- # a bit exotic, that last one
+ # a bit exotic, that last one
The C<if> statement is straightforward. Because BLOCKs are always
bounded by curly brackets, there is never any ambiguity about which
The C<next> command starts the next iteration of the loop:
LINE: while (<STDIN>) {
- next LINE if /^#/; # discard comments
- ...
+ next LINE if /^#/; # discard comments
+ ...
}
The C<last> command immediately exits the loop in question. The
C<continue> block, if any, is not executed:
LINE: while (<STDIN>) {
- last LINE if /^$/; # exit when done with header
- ...
+ last LINE if /^$/; # exit when done with header
+ ...
}
The C<redo> command restarts the loop block without evaluating the
want to skip ahead and get the next record.
while (<>) {
- chomp;
- if (s/\\$//) {
- $_ .= <>;
- redo unless eof();
- }
- # now process $_
+ chomp;
+ if (s/\\$//) {
+ $_ .= <>;
+ redo unless eof();
+ }
+ # now process $_
}
which is Perl shorthand for the more explicitly written version:
LINE: while (defined($line = <ARGV>)) {
- chomp($line);
- if ($line =~ s/\\$//) {
- $line .= <ARGV>;
- redo LINE unless eof(); # not eof(ARGV)!
- }
- # now process $line
+ chomp($line);
+ if ($line =~ s/\\$//) {
+ redo LINE unless eof(); # not eof(ARGV)!
+ }
+ # now process $line
}
Note that if there were a C<continue> block on the above code, it would
# inspired by :1,$g/fred/s//WILMA/
while (<>) {
- m?(fred)? && s//WILMA $1 WILMA/;
- m?(barney)? && s//BETTY $1 BETTY/;
- m?(homer)? && s//MARGE $1 MARGE/;
+ m?(fred)? && s//WILMA $1 WILMA/;
+ m?(barney)? && s//BETTY $1 BETTY/;
+ m?(homer)? && s//MARGE $1 MARGE/;
} continue {
- print "$ARGV $.: $_";
- close ARGV if eof; # reset $.
- reset if eof; # reset ?pat?
+ print "$ARGV $.: $_";
+ close ARGV if eof; # reset $.
+ reset if eof; # reset ?pat?
}
If the word C<while> is replaced by the word C<until>, the sense of the
they aren't loops. You can double the braces to make them such, though.
if (/pattern/) {{
- last if /fred/;
- next if /barney/; # same effect as "last",
- # but doesn't document as well
- # do something here
+ last if /fred/;
+ next if /barney/; # same effect as "last",
+ # but doesn't document as well
+ # do something here
}}
This is caused by the fact that a block by itself acts as a loop that
-executes once, see L<"Basic BLOCKs">.
+executes once, see L</"Basic BLOCKs">.
The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
that means that this:
for ($i = 1; $i < 10; $i++) {
- ...
+ ...
}
is the same as this:
$i = 1;
while ($i < 10) {
- ...
+ ...
} continue {
- $i++;
+ $i++;
}
There is one minor difference: if variables are declared with C<my>
C<while> loop) is empty, it is treated as true. That is, both
for (;;) {
- ...
+ ...
}
and
while () {
- ...
+ ...
}
are treated as infinite loops.
$on_a_tty = -t STDIN && -t STDOUT;
sub prompt { print "yes? " if $on_a_tty }
for ( prompt(); <STDIN>; prompt() ) {
- # do something
+ # do something
}
Using C<readline> (or the operator form, C<< <EXPR> >>) as the
for (@ary) { s/foo/bar/ }
for my $elem (@elements) {
- $elem *= 2;
+ $elem *= 2;
}
for $count (reverse(1..10), "BOOM") {
- print $count, "\n";
- sleep(1);
+ print $count, "\n";
+ sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
- print "Item: $item\n";
+ print "Item: $item\n";
}
use feature "refaliasing";
no warnings "experimental::refaliasing";
foreach \my %hash (@array_of_hash_references) {
- # do something which each %hash
+ # do something which each %hash
}
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
+ 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
OUTER: for my $wid (@ary1) {
INNER: for my $jet (@ary2) {
- next OUTER if $wid > $jet;
- $wid += $jet;
- }
- }
+ 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
The BLOCK construct can be used to emulate case structures.
SWITCH: {
- if (/^abc/) { $abc = 1; last SWITCH; }
- if (/^def/) { $def = 1; last SWITCH; }
- if (/^xyz/) { $xyz = 1; last SWITCH; }
- $nothing = 1;
+ if (/^abc/) { $abc = 1; last SWITCH; }
+ if (/^def/) { $def = 1; last SWITCH; }
+ if (/^xyz/) { $xyz = 1; last SWITCH; }
+ $nothing = 1;
}
You'll also find that C<foreach> loop used to create a topicalizer
SWITCH:
for ($var) {
- if (/^abc/) { $abc = 1; last SWITCH; }
- if (/^def/) { $def = 1; last SWITCH; }
- if (/^xyz/) { $xyz = 1; last SWITCH; }
- $nothing = 1;
+ if (/^abc/) { $abc = 1; last SWITCH; }
+ if (/^def/) { $def = 1; last SWITCH; }
+ if (/^xyz/) { $xyz = 1; last SWITCH; }
+ $nothing = 1;
}
Such constructs are quite frequently used, both because older versions of
use v5.10.1;
for ($var) {
- when (/^abc/) { $abc = 1 }
- when (/^def/) { $def = 1 }
- when (/^xyz/) { $xyz = 1 }
- default { $nothing = 1 }
+ when (/^abc/) { $abc = 1 }
+ when (/^def/) { $def = 1 }
+ when (/^xyz/) { $xyz = 1 }
+ default { $nothing = 1 }
}
The C<foreach> is the non-experimental way to set a topicalizer.
use v5.10.1;
given ($var) {
- when (/^abc/) { $abc = 1 }
- when (/^def/) { $def = 1 }
- when (/^xyz/) { $xyz = 1 }
- default { $nothing = 1 }
+ when (/^abc/) { $abc = 1 }
+ when (/^def/) { $def = 1 }
+ when (/^xyz/) { $xyz = 1 }
+ default { $nothing = 1 }
}
As of 5.14, that can also be written this way:
use v5.14;
for ($var) {
- $abc = 1 when /^abc/;
- $def = 1 when /^def/;
- $xyz = 1 when /^xyz/;
- default { $nothing = 1 }
+ $abc = 1 when /^abc/;
+ $def = 1 when /^def/;
+ $xyz = 1 when /^xyz/;
+ default { $nothing = 1 }
}
Or if you don't care to play it safe, like this:
use v5.14;
given ($var) {
- $abc = 1 when /^abc/;
- $def = 1 when /^def/;
- $xyz = 1 when /^xyz/;
- default { $nothing = 1 }
+ $abc = 1 when /^abc/;
+ $def = 1 when /^def/;
+ $xyz = 1 when /^xyz/;
+ default { $nothing = 1 }
}
The arguments to C<given> and C<when> are in scalar context,
C<given> is merely a lexically scoped copy of the original, not a
dynamically scoped alias to the original, as it would be if it were a
C<foreach> or under both the original and the current Perl 6 language
-specification. This bug was fixed in Perl
-5.18. If you really want a lexical C<$_>,
-specify that explicitly, but note that C<my $_>
-is now deprecated and will warn unless warnings
-have been disabled:
-
- given(my $_ = EXPR) { ... }
+specification. This bug was fixed in Perl 5.18 (and lexicalized C<$_> itself
+was removed in Perl 5.24).
If your code still needs to run on older versions,
stick to C<foreach> for your topicalizer and
sub unimplemented { ... }
eval { unimplemented() };
if ($@ =~ /^Unimplemented at /) {
- say "I found an ellipsis!";
+ say "I found an ellipsis!";
}
You can only use the elliptical statement to stand in for a
...;
eval { ... };
sub somemeth {
- my $self = shift;
- ...;
+ my $self = shift;
+ ...;
}
$x = do {
- my $n;
- ...;
- say "Hurrah!";
- $n;
+ my $n;
+ ...;
+ say "Hurrah!";
+ $n;
};
The elliptical statement cannot stand in for an expression that
=cut back to the compiler, nuff of this pod stuff!
sub snazzle($) {
- my $thingie = shift;
- .........
+ my $thingie = shift;
+ .........
}
Note that pod translators should look at only paragraphs beginning
use feature ":5.10";
given ($foo) {
- when (undef) {
- say '$foo is undefined';
- }
- when ("foo") {
- say '$foo is the string "foo"';
- }
- when ([1,3,5,7,9]) {
- say '$foo is an odd digit';
- continue; # Fall through
- }
- when ($_ < 100) {
- say '$foo is numerically less than 100';
- }
- when (\&complicated_check) {
- say 'a complicated check for $foo is true';
- }
- default {
- die q(I don't know what to do with $foo);
- }
+ when (undef) {
+ say '$foo is undefined';
+ }
+ when ("foo") {
+ say '$foo is the string "foo"';
+ }
+ when ([1,3,5,7,9]) {
+ say '$foo is an odd digit';
+ continue; # Fall through
+ }
+ when ($_ < 100) {
+ say '$foo is numerically less than 100';
+ }
+ when (\&complicated_check) {
+ say 'a complicated check for $foo is true';
+ }
+ default {
+ die q(I don't know what to do with $foo);
+ }
}
Before Perl 5.18, C<given(EXPR)> assigned the value of I<EXPR> to
merely a lexically scoped I<B<copy>> (!) of C<$_>, not a dynamically
scoped alias the way C<foreach> does. That made it similar to
- do { my $_ = EXPR; ... }
+ do { my $_ = EXPR; ... }
except that the block was automatically broken out of by a successful
C<when> or an explicit C<break>. Because it was only a copy, and because
A smart match that uses an explicit C<~~> operator, such as C<EXPR ~~ EXPR>.
-B<NOTE:> You will often have to use C<$c ~~ $_> because the default case
+B<NOTE:> You will often have to use C<$c ~~ $_> because the default case
uses C<$_ ~~ $c> , which is frequentlythe opposite of what you want.
=item Z<>4.
case to the next:
given($foo) {
- when (/x/) { say '$foo contains an x'; continue }
- when (/y/) { say '$foo contains a y' }
- default { say '$foo does not contain a y' }
+ when (/x/) { say '$foo contains an x'; continue }
+ when (/y/) { say '$foo contains a y' }
+ default { say '$foo does not contain a y' }
}
=head3 Return value
evaluate to an empty list.
my $price = do {
- given ($item) {
- when (["pear", "apple"]) { 1 }
- break when "vote"; # My vote cannot be bought
- 1e10 when /Mona Lisa/;
- "unknown";
- }
+ given ($item) {
+ when (["pear", "apple"]) { 1 }
+ break when "vote"; # My vote cannot be bought
+ 1e10 when /Mona Lisa/;
+ "unknown";
+ }
};
Currently, C<given> blocks can't always
use v5.10.1;
my $count = 0;
for (@array) {
- when ("foo") { ++$count }
+ when ("foo") { ++$count }
}
print "\@array contains $count copies of 'foo'\n";
use v5.14;
my $count = 0;
for (@array) {
- ++$count when "foo";
+ ++$count when "foo";
}
print "\@array contains $count copies of 'foo'\n";
because Perl 5 would parse the expression
given $foo {
- ...
+ ...
}
as though the argument to C<given> were an element of the hash
https://perl5.git.perl.org / perl5.git / blobdiff