Float Multiplying Problem

Python Help
1

So, I wrote this code:

“num = 0.0
while True:
streq = input()
if streq == “p”:
num += 0.1
print(“Number is: {}”.format(num))”

here, every time I type p and press enter, num is supposed to be multiplied by 0.1 and printed on the screen, but after doing this a few times, it starts printing some weird numbers:

“p
Number is: 0.1
p
Number is: 0.2
p
Number is: 0.30000000000000004
p
Number is: 0.4
p
Number is: 0.5
p
Number is: 0.6
p
Number is: 0.7
p
Number is: 0.7999999999999999
p
Number is: 0.8999999999999999
p
Number is: 0.9999999999999999
p
Number is: 1.0999999999999999
p
Number is: 1.2
p
Number is: 1.3
p
Number is: 1.4000000000000001
p
Number is: 1.5000000000000002
p
Number is: 1.6000000000000003
p
Number is: 1.7000000000000004
p
Number is: 1.8000000000000005
p
Number is: 1.9000000000000006
p
Number is: 2.0000000000000004”

what am I doing wrong? please help.

2

Computers don’t do floating point with exact numbers: 15. Floating Point Arithmetic: Issues and Limitations — Python 3.10.2 documentation

Some of your possible alternatives:

  • Continue doing floating point, but put up with the imprecision and hide it by using round()
  • If you only need tenths, do everything 10 times bigger so it stays as integers, then divide the result by 10 for display
  • Use the Decimal module and have it add up (exact) tenths
3

This behaviour with floating point numbers occurs in all programming
languages, not just Python. It is an inescapable limitation of floating
point numbers.

https://docs.python.org/3/faq/design.html#why-am-i-getting-strange-results-with-simple-arithmetic-operations