Python Float Lost with Big Numbers

Question

I am working with some large numbers and have run into a problem with lost floating values.

When multiplying large numbers, the float portion seems to go missing, lost, becomes zero. When using the same code with smaller numbers, this does not happen.

Trivial example:

import math
f_ProNum(31 * 61) ## Correct: DEBUG float: 314.833333
f_ProNum(5915587277 * 3367900313) ## Incorrect: DEBUG float: 3320518040297852928.000000

def f_ProNum(v_Num):
    """Syntax: (int); Returns: (float)"""
    print("DEBUG float: %f") %((v_Num - 2.0)/6.0) # DEBUG
    v_PDNum = (v_Num - 2.0)/6.0
    return v_PDNum

As seen in the second call, the float seems to get lost, or set to zero.

Why is it doing this, and how may it be addressed, fixed?

Some numbers, the float, right of the decimal point, as shown returned in the first call with smaller numbers. In the second call the lost float is ".83333333". — Raw_Input, Dec 09 '14 at 23:42
have you tried using the decimal module, related http://stackoverflow.com/questions/2663612/nicely-representing-a-floating-point-number-in-python — Padraic Cunningham, Dec 09 '14 at 23:47
@Padraic Cunningham, Erik Haugen answered with that. I tried it, and it worked. I selected his answer. Thanks for the feedback. Cheers. — Raw_Input, Dec 09 '14 at 23:55

score 4 · Accepted Answer · answered Dec 09 '14 at 23:44

Larger float values lose precision. To simplify a bit, floats have a fixed number of "digits" that are used, called the mantissa or significand, so large enough numbers are essentially going to use all their digits on the part to the left of the decimal place. For more see: https://en.wikipedia.org/wiki/Floating_point#Internal_representation

To solve your problem, you might try using the decimal module, which allows you to configure the precision you need: https://docs.python.org/2/library/decimal.html

Did the trick. Thanks so much. I did research this until the wee hours last night, but no joy. — Raw_Input, Dec 09 '14 at 23:54

Python Float Lost with Big Numbers

1 Answers1