I am trying to carry out a simple comparison of two numbers in Perl, but I have encountered some interesting results. If someone could point me to a foolish mistake somewhere I would be more than grateful!

I understand that eq carries out a string comparison, whereas == carries out a numerical comparison, however when I try to compare numbers after a simple arithmetic calculation == returns false when I would expect true, while eq returns true correctly.

$num1     = 523.20;
$num2     = 23.2;
$eq        = 500;
print ("$num1,    $num2,    ($eq,    ". ($num1 - $num2) ."),    ". ($n
+um1 - $num2 eq $eq) .",    ". (($num1 - $num2) == $eq). ".\n");

[download]

prints

523.2,    23.2,    (500,    500),    1,    .

I have come to the conclusion that this is because of floating point arithmetic (correct me if I'm wrong), as printing $num1 - $num2 yeilds a value of 1.13686837721616e-13.

My confusion comes because if I alter the above script from 23.2 to 0.2, the result for == is true (correctly), and I can't understand where the difference lies that should cause == to deviate in such a manner.

Of course the ultimate question is: is there something I'm doing (or not doing) which can account for this? Is there a better way of doing this simple comparison which will yield consistent results?

In reply to eq vs == by spikeheap

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