Float comparison issues in Perl [duplicate]

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Possible Duplicate:

How do I fix this Perl code so that 1.1 + 2.2 == 3.3?

I'm working on a Perl script that compares strings representing gene models and prints out a summary of the comparison. If the gene models match perfectly, I print out a very terse summary, but if they are different, the summary is quite verbose.

The script looks at the value of a variable to determine whether it should do the terse or verbose summary--if the variable is equal to 1, it should print the terse summary; otherwis开发者_C百科e, it should print the verbose summary.

Since the value is numeric (a float), I've been using the == operator to do the comparison.

if($stats->{overall_simple_matching_coefficient} == 1)
{
  print "Gene structures match perfectly!\n";
}

This worked correctly for all of my tests and even for most of the new cases I am running now, but I found a weird case where the value was equal to 1 but the above comparison failed. I have not been able to figure out why the comparison failed, and stranger yet, when I changed the == operator to the eq operator, it seemed to work fine.

I thought the == was for numerical comparison and eq was for string comparison. Am I missing something here?

Update: If I print out the value right before the comparison...

printf("Test: '%f', '%d', '%s'\n", $stats->{overall_simple_matching_coefficient}, $stats->{overall_simple_matching_coefficient}, $stats->{overall_simple_matching_coefficient});

...I get this.

Test: '1.000000', '0', '1'

---

The first thing any computer language teacher should teach you about any computer language is that YOU CANNOT COMPARE FLOATS FOR EQUALITY. This is true of any language. Floating point arithmetic is not exact, and two floats that look like they're the same will be different in the insignificant digits somewhere where you can't see it. Instead, you can only compare that they are close to each other - like

if (abs(stats->{overall_simple_matching_coefficient)-1) < 0.0001)

---

What do you get if you print the value of $stats->{overall_simple_matching_coefficient} just before the comparison? If it's 1, try printf with a format of "%20.10f". I strongly suspect you have some rounding error (less then 1e-6) accumulated in the variable and it's not comparing equal numerically. However when converted to string, since the error is right of the 6th decimal place, and the default string format is to six places, it compares equal.