in reply to Re: Infinite loop but no reason why
in thread Infinite loop but no reason why

    sub fp_equal {
        my ($X, $Y, $POINTS) = @_;
        my ($tX, $tY);
        $tX = sprintf("%.${POINTS}g", $X);
        $tY = sprintf("%.${POINTS}g", $Y);
        return $tX eq $tY;
    }

    fp_equal($x, $y, 3);

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could be replaced with 

    sub fp_equal {
        return abs($_[0] - $_[1]) < $_[2];
    }

    fp_equal($x, $y, 0.001);

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Pros:

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Re^3: Infinite loop but no reason why
by jhourcle (Prior) on Jul 19, 2006 at 16:12 UTC

    I personally prefer relative precision, rather than absolute values, as you frequently don't know what the scale of the number is going to be for these sorts of calculations:

    sub fp_equal {
  my ($x,$y,$precision) = @_;
  $precision ||= 0.000001;

  return 1 if (! defined($x) and ! defined($y) );
  return 0 if (! defined($x) or  ! defined($y) );
  return 1 if ($x == $y);     # for 0 == 0
  ($x,$y) = ($y,$x) if (!$y); # for $y == 0
  return ( abs ( 1 - $x/$y ) <= $precision ) ? 1 : 0;
}

    [download]

    Update: I made an assumption based on the halving of the precision in the OP's problem that this was doing some sort of numerical analysis. ikegami is right in that sometimes you're looking at stepped measurements over a finite range, and so an absolute precision is necessary. (Most of my floating point comparisons are in scientific measurements, and I use relative precision so I don't have to have different definitions of precision for the units used and range of the measurements)

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[d/l]

      Relative presision removes the need to specify a precision (so it's easier to use), but it has limits.

      For example, most numbers in the real-time control system on which I work are have a range of 0 to 1 (inclusive). Some of the numbers with which I deal are measurements of valve positions. 0 means the valve is fully closed, 0.25 means it's one quarter open, and 1 means it's fully open. If we were to compare two valve positions, we'd consider 4/1000th open and 5/1000th open equal. Because relative precision has no idea these measurements are tiny compared to the range, it would consider them unequal.

      In my experience (in the real-time control systems world), relative presision should only be avoided whenever possible. Absolute precision should be used instead.

      Update:

      Oddly enough, we rarely use any kind of tolerance in our system except during automated testing. Equality checks are rarely if ever needed. Inequalities, on the other hand, are used throughout. We use deadbands to avoid jitter. 

      if ($x < 0.5) {
   # All is ok
   $ok = 1;
   ... normal operation mode ...
}
elsif ($x > 0.7) {
   # Things are definitely bad.
   $ok = 0;
   ... exceptional operation mode ...
}
elsif ($ok) {
   # In deadband. Was last ok.
   ... normal operation mode ...
}
else {
   # In deadband. Was last bad.
   ... exceptional operation mode ...
}

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[reply]
[d/l]

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