Python float round error 117.285 round to 117.28 not 117.29

Question

I am using python 2.7, and the code I have is:

a = 10.5 * 22.34 / 2.0
print \"%.2f\" % a

and the result I expect is 117.29, but it shows 11

Answer 1

If you want a simple solution and don't care about performance, you could use a function like this to convert to integer, round, and convert back to float:

def round_exact(number, decimal_places=0):
    """Round the number to the given number of decimal places by converting to 
    and from integers to avoid floating point error."""
    factor = 10**(decimal_places + 1)
    rounded_int = int(number * factor)
    if rounded_int % 10 >= 5:
        # Round up
        return (int(rounded_int//10) + 1) / float(factor//10)
    # Round down
    return int(rounded_int//10) / float(factor//10)