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; otherwise, 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