Hello everyone,

A recent thread reminded me of a script I wrote years ago while learning about the flip-flop operator (aka the range operator in scalar context), and I thought I'd share it in case it helps someone else.

The script, which I've included below the output, runs through a sequence of true/false values, tests whether the right-hand-side and/or left-hand-side of the operator is evaluated (marked by an asterisk in the table), gets the return value of the operator, and outputs all that in the following handy table. I hope it illustrates the difference between the .. (two-dot) and ... (three-dot) versions of the operator: two dots will immediately evaluate the RHS if the LHS is true, three dots will wait until the next evaluation.

*** Demonstration of the Flip-Flop Operators ***

__A______B____X___   __A______B____X___
  0* ..  0  =          0* ... 0  =     
  0* ..  1  =          0* ... 1  =     
  0* ..  0  =          0* ... 0  =     
  1* ..  0* = 1        1* ... 0  = 1   
  0  ..  0* = 2        0  ... 0* = 2   
  0  ..  0* = 3        0  ... 0* = 3   
  0  ..  1* = 4E0      0  ... 1* = 4E0 
  0* ..  0  =          0* ... 0  =     
  0* ..  0  =          0* ... 0  =     
  1* ..  1* = 1E0      1* ... 1  = 1   
  0* ..  0  =          0  ... 0* = 2   
  0* ..  0  =          0  ... 0* = 3   
  1* ..  0* = 1        1  ... 0* = 4   
  0  ..  0* = 2        0  ... 0* = 5   
  0  ..  1* = 3E0      0  ... 1* = 6E0 
  0* ..  1  =          0* ... 1  =     
  1* ..  1* = 1E0      1* ... 1  = 1   
  1* ..  0* = 1        1  ... 0* = 2   
  0  ..  0* = 2        0  ... 0* = 3   
  0  ..  1* = 3E0      0  ... 1* = 4E0 
  0* ..  0  =          0* ... 0  =     
  1* ..  1* = 1E0      1* ... 1  = 1   
  1* ..  1* = 1E0      1  ... 1* = 2E0 
  1* ..  1* = 1E0      1* ... 1  = 1   
  0* ..  0  =          0  ... 0* = 2   
  0* ..  1  =          0  ... 1* = 3E0 
  0* ..  0  =          0* ... 0  =     

(* = Evaluated)
[download]

#!/usr/bin/env perl
use warnings;
use strict;

print "*** Demonstration of the Flip-Flop Operators ***\n\n";

my @tests = qw/
    0:0 0:1 0:0 1:0 0:0 0:0 0:1 0:0 0:0
    1:1 0:0 0:0 1:0 0:0 0:1 0:1 1:1 1:0
    0:0 0:1 0:0 1:1 1:1 1:1 0:0 0:1 0:0 /;

print join('   ', ("__A______B____X___") x 2), "\n";
sub fmt {
    my ($lv,$le,$op,$rv,$re,$v) = @_;
    return sprintf '  %s%s %-3s %s%s = %-3s ',
        $lv, $le?'*':' ', $op,
        $rv, $re?'*':' ', $v;
}

for (@tests) {
    my ($lv, $rv) = split /:/;
    
    my ($le, $re);
    my $lf = sub { $le=1; $lv };
    my $rf = sub { $re=1; $rv };
    
    ($le, $re) = (0,0);
    my $v1 = &$lf ..  &$rf;
    print fmt($lv,$le,'..' ,$rv,$re,$v1);
    
    print '   ';
    
    ($le, $re) = (0,0);
    my $v2 = &$lf ... &$rf;
    print fmt($lv,$le,'...',$rv,$re,$v2);
    
    print "\n";
}

print "\n(* = Evaluated)\n";
[download]

Regards,
-- Hauke D