Python computation Error

Question

I found something really strange happening in python with that code:

num = 99999999999999999999999999999

for i in range(2, num):
    if num % i == 0:
        j = int(num / i)
        print(num, '=', i, '*', j)
        break
else:
    print(num, 'is prime')

Python is giving me

99999999999999999999999999999 = 3 * 33333333333333333409747959808

as output, what is obviouly wrong. And as bigger num is getting, the wronger the output gets.

Can someone tell me, what's going on here?

score 1 · Accepted Answer · answered Dec 20 '15 at 09:45

I expect you're using Python 3. In Python 3, when dividing integers with the / operator, it converts them to float and produces a float result. Floats have limited precision, so you're seeing roundoff errors.

To get integer division, without roundoff errors, you can use the // operator, which gives true integer division. Just change the assignment to j to:

j = num // i

Python computation Error

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