Re: Re: Re: Re: Perl Idioms Explained: && and || "Short Circuit" operators

[demerphq]

Im still confused how this works. :-) How does it map discontiguous values into a jump table without doing a lot of work?

[fletcher_the_dog]

Think of the c compiler doing something like this:

sub CompileSwitch{
  my %hash;
  my @commands;
  my $command = 0;
  my $mydefault;
  while (@_) {
    my $case = shift;
    my $code = shift;
    if ($case eq "default") {
      $mydefault = $code;
      last;
    }
    $hash{$case} = $command++;
    push @commands,$code;

  }
  return sub{
    my $case = shift;
    my $i = $hash{$case};
    if (defined $i) {
      # fall through , a C switch doesn't test following cases it
      # just goes to the end or till it hits a break
      for (;$i<@commands;$i++) {
    # execute code
    &{$commands[$i]}
      }
    }
    elsif (defined $mydefault) {
      &$mydefault;
    }
  }
}

my $switch = CompileSwitch(
               'a'=>sub { print "a"},
               'b'=>sub { print "b"},
               'c'=>sub { print "c"; last},
               'x'=>sub { print "x"},
               'y'=>sub { print "y"},
               'z'=>sub { print "z"},
               'default'=>sub { print "unknown case"}
);

foreach (qw(a b c d e f x y z)) {
  print "testing $_\n";
  $switch->($_);
  print "\n";
}

__OUTPUT__
testing a
abc
testing b
bc
testing c
c
testing d
unknown case
testing e
unknown case
testing f
unknown case
testing x
xyz
testing y
yz
testing z
z