in reply to Re: Re: Perl Idioms Explained: && and || "Short Circuit" operators
in thread Perl Idioms Explained - && and || "Short Circuit" operators

C switch statements create an internal jump table that is more or less like a hash so that they can skip testing cases that are are going to fail, this makes them O(1). For example in the following switch statement if 'test' equals 'c' the code jumps right to case 'c', it doesn't test to see if 'test' equals 'a' or 'b': 
switch(test){
  case 'a':
   # do something
  case 'b':
   # do something else
  case 'c':
   # do something else
}

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This of course means that the possible cases (but not the test value) are static and must be known at compile time which decreases flexibilty but increases speed.
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Re: Re: Re: Re: Perl Idioms Explained: && and || "Short Circuit" operators
by demerphq (Chancellor) on Oct 22, 2003 at 23:35 UTC

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


    ---
    demerphq

      First they ignore you, then they laugh at you, then they fight you, then you win.
      -- Gandhi

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[d/l]
      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

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