Python loop not providing expected output

Question

Here is a python while loop code I am using:

i = 0.0
while i < 9.0:
    y = 13.0 - i
    if (y -i) == 6.0:
        print '[+] Solution found!'
        print 'x = ', i
        print 'y = ', y
        print 'z =', 8.0 - i
    i += 0.1

The problem is that 'if' block never gets executed even when condition is satisfied. On the other hand, if I put increment of 0.5 i.e. i += 0.5 then the code works as it should. Any idea what is causing the issue?

are you using python 2 ? I'd suggest switching for python3, for which the syntax is different concerning print statements. Plus, in your example, i is not defined — Sileo, Apr 19 '20 at 17:08
Type printing the values of `i` outside the `if` then see: [Floating Point Arithmetic](https://docs.python.org/3/tutorial/floatingpoint.html) — Mark, Apr 19 '20 at 17:11
Oh yeah I forgot about floating point math in Python. Use a library such as decimal for operations. — unkn0wn.dev, Apr 19 '20 at 17:19
Due to limited, floating point precision ``y - i - i`` will be ``5.9999999999999964``, not ``6.0``. — MisterMiyagi, Apr 19 '20 at 17:19
@MisterMiyagi when printing the values of `i`, `y` & `y - i`, `y - i` is correctly displayed as 6.0 — dek0der, Apr 19 '20 at 17:23

score 0 · Accepted Answer · answered Apr 19 '20 at 17:23

0

Try this Code :

i = 0.0
while i < 9.0:
    y = 13.0 - i
    if round((y -i),1) == 6.0:
        print('[+] Solution found!')
        print('x = ', round(i,1))
        print('y = ', round(y,1))
        print('z =', round(8.0 - i,1))
    i += 0.1

answered Apr 19 '20 at 17:23

Saragada Ramesh

191
2
5

score 0 · Answer 2 · answered Apr 19 '20 at 17:23

This is an issue of floating point precision. It's a really old problem. A way to get around this is to use Decimal and limit the precision to 2 digits after the comma. A possible way would like this (switched to python3 to test on my machine):

from decimal import *
getcontext().prec = 2
i = Decimal(0.0)
while i < Decimal(9.0):
    y = Decimal(13.0) - i
    if (y -i) == 6.0:
        print('[+] Solution found!')
        print('x = ', i)
        print('y = ', y)
        print('z =', Decimal(8.0) - i)
    i += Decimal(0.1)

score 0 · Answer 3 · answered Apr 19 '20 at 17:24

0

It is caused by floating point precision. As MisterMiyagi commented, check Is floating point math broken ?. I would suggest using integers here.

answered Apr 19 '20 at 17:24

Sileo

307
2
18

Python loop not providing expected output

3 Answers3