My code is doing something I really can't understand. I'm performing some linear interpolation, and test the result against the number 75. I can get two different results depending on where the &adjust_hue subroutine is called. Here's the code 
use strict;

use constant HUE_RANGE => 255;

my @HUES = (
    0,         25,     50,     75,     120,     168,     195,     240,
+     268,     315);
#    0.0        0.1        0.2        0.3        0.4        0.5       
+ 0.6        0.7        0.8        0.9        1.0

&adjust_hue(0.3);

my @hues = (0 .. 4);
$_ *= 0.1 for @hues;

for (@hues) {
    &adjust_hue($_);
}

sub adjust_hue {
    my ($scale) = @_;

    my @hues = @HUES;
    push @hues, HUE_RANGE unless $hues[-1] == HUE_RANGE;
#    warn 'hues ', Dumper \@hues;
#    warn "n hues ", (scalar @hues), "\n";
    my $n_unique_hues = -1 + scalar @hues;
    my $scaled_index = $scale*$n_unique_hues;
    my $index = int $scaled_index;
    my $adjusted_hue = $hues[$index] + ($scaled_index - $index)*($hues
+[$index + 1] - $hues[$index]);
#    $adjusted_hue = eval $adjusted_hue; # This shouldn't be necessary
+. Somehow this sub was producing strings instead of numbers. Later, t
+his strings would fail comparisons.
    if ($adjusted_hue == 75) {
        warn "  adjusted_hue ($adjusted_hue) equals 75\n";
    } else {
        warn "  adjusted_hue ($adjusted_hue) does not equal 75\n";
    }
    warn "scale $scale, scaled_index $scaled_index, index $index, adju
+sted_hue $adjusted_hue\n\n";
    $adjusted_hue;
}

[download]
The output is the following for me: 
  adjusted_hue (75) equals 75
scale 0.3, scaled_index 3, index 3, adjusted_hue 75

  adjusted_hue (0) does not equal 75
scale 0, scaled_index 0, index 0, adjusted_hue 0

  adjusted_hue (25) does not equal 75
scale 0.1, scaled_index 1, index 1, adjusted_hue 25

  adjusted_hue (50) does not equal 75
scale 0.2, scaled_index 2, index 2, adjusted_hue 50

  adjusted_hue (75) does not equal 75
scale 0.3, scaled_index 3, index 3, adjusted_hue 75

  adjusted_hue (120) does not equal 75
scale 0.4, scaled_index 4, index 4, adjusted_hue 120

[download]
I really don't understand why the comparison to 75 is true in one case with the parameter 0.3 and false also with the parameter 0.3 Thanks, John

In reply to Why is this comparison failing? by johnrcomeau

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