Comparing Floating-Point Numbers

Problem

Floating-point arithmetic isn't precise. You want to compare two floating-point numbers and know if they're equal when carried out to a certain number of decimal places. Most of the time, this is the way you should compare floating-point numbers for equality.

Solution

Use sprintf to format the numbers to a certain number of decimal places, then compare the resulting strings:

# equal(NUM1, NUM2, ACCURACY) : returns true if NUM1 and NUM2 are # equal to ACCURACY number of decimal places sub equal {
 my ($A, $B, $dp) = @_;
return sprintf("%.${dp}g", $A) eq sprintf("%.${dp}g", $B);
}

Alternatively, store the numbers as integers by assuming the decimal place.

Discussion

You need the equal routine because most computers' floating-point representations aren't accurate. See the Introduction for a discussion of this issue.

If you have a fixed number of decimal places, as with currency, you can sidestep the problem by storing your values as integers. Storing $3.50 as 350 instead of 3.5 removes the need for floating-point values. Reintroduce the decimal point on output:

$wage = 536; # $5.36/hour $week = 40 * $wage; # $214.40 printf("One week's wage is: \$%.2f\n", $week/100); One week's wage is: $214.40

It rarely makes sense to compare to more than 15 decimal places.