Math.ceil(x) using while loop

Question

Last time i was learning math module and I can not understand how math.ceil() works. I was trying to use it with while loop, but I do not know how exacly it works. Here is my little code:

import math

x = 20.4
y = 20.4

x = math.ceil(x)

while y != x:
    y += 0.1
    print(y)

It just can not stop calculating, why is that? It even prints numbers like 20152.599999987233 or higher.

Answer 1

Python has issues with comparing two floating point numbers, due to problems with precision/rounding. Python actually evaluates y to the closest approximation of 21.0 when you expect it to be 21. Due to this, they're not equal and the condition of y != x never evaluates to False, causing the infinite loop you're experiencing.

You can use the math.isclose function instead of using equality to fix the issue:

import math

x = math.ceil(20.4)
y = 20.4

while not math.isclose(x,y):
    y += 0.1
    print(y)

Now, your loop should terminate when y is "close to" 21 - this was my output:

20.5
20.6
20.700000000000003
20.800000000000004
20.900000000000006
21.000000000000007