Why is this if/else always true?

Question

Whatever color the input is it returns "green" :(

sub next_traffic_lights {
  my ($x) = @_;
  if ($x == "red") { 
    return "green";
  } 
  elsif ($x == "green") {
    return "yellow";
  } else {
    return "red";
  }
 }

If you had used `use strict; use warnings;`, Perl would have told you. — melpomene, Oct 21 '17 at 18:33

tadman · Answer 1 · 2017-10-21T18:26:36.267

2

In Perl == means numerical equivalence where eq is string equivalence. You just need to flip those == to eq to do the right comparisons.

Both "red" and "green" are equal to zero numerically, so they're considered identical here using numerical comparison.

When writing code like this you should steer towards a lookup table anyway, as this is basically a simple state machine:

my %next_light = (
  "red" => "green",
  "yellow" => "red",
  "green" => "yellow"
);

sub next_traffic_lights {
  return $next_light{$_[0]};
}

edited Oct 21 '17 at 18:26

answered Oct 21 '17 at 18:20

tadman

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Much appreciated, thanks :) – Echoding Oct 21 '17 at 18:48
1

Anecdote: In Germany things are much more complicated. The traffic lights go "red" → "red&yellow" → "green" → "yellow" → "red". – PerlDuck Oct 23 '17 at 18:43

Why is this if/else always true?

1 Answers1