my loop will run into negative numbers even when I am an if statement telling not to

Question

this is a simple program where I am getting input as change owed in $, and I have to return the total amount of coins. My program runs correctly until it comes down to counting pennies. Can someone see, Why does it give negative output, even when I told the program with an if statement not to run after it comes to 0.

import sys
import cs50


val = cs50.get_float(("Change owed: "))

count = 0
newVal = val

while newVal > .25:
        newVal = round(newVal - 0.25, 2)
        print(newVal)
        count +=1
        if newVal == 0.25:
          newVal = round(newVal - 0.25, 2)
          print(newVal)
          count +=1
        elif newVal < 0.25:
          newVal = round(newVal - 0.10, 2)
          print(newVal)
          count +=1
          if newVal < 0.10:
            newVal = round(newVal - 0.05, 2)
            print(newVal)
            count +=1
            if newVal < 0.05:
              newVal = round(newVal - 0.01, 2)
              print(newVal)
              count +=1
              if newVal < 0:
                break
              
              
print(count)

1

Maybe the "<=" operator could help? – Apr 20 '21 at 02:35

score 1 · Accepted Answer · answered Apr 20 '21 at 02:51

Personally, I have no idea what your code is doing. Here's a version using division and mod. It's much more straightforward.

import sys
import cs50


val = cs50.get_float(("Change owed: "))

count = 0
newVal = int(val * 100)

if newVal >= 25:
    count += int(newVal / 25)
    newVal = newVal % 25

if newVal >= 10:
    count += int(newVal / 10)
    newVal = newVal % 10

if newVal >= 5:
    count += int(newVal / 5)
    newVal = newVal % 5

if newVal >= 1:
    count += int(newVal / 1)

print(count)

YousefZ01 · Answer 2 · 2021-04-20T02:41:50.850

0

The issue you are having is that it's a floating point error. i.e.

>>> 0.03 - 0.01
0.019999999999999997

Perhaps it would be better if you made you multiplied the given value by 100, and so you'd be dealing with integer values instead of decimals.

On a different note:

you should probably divide instead of looping and subtracting, i.e.

coin_types = (25, 10, 5, 1)

def solution(value: int):
    coins = 0

    for coin in coin_types:
        coins += value // coin
        value = value % coin

        if value == 0:
            return coins

val = cs50.get_float(("Change owed: "))
print(solution(int(val*100)))

edited Apr 20 '21 at 02:41

answered Apr 20 '21 at 02:38

YousefZ01

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3
9

1

Try and give it numbers such as .66 and .64 and so on. Doesnt work. – Buddy Bob Apr 20 '21 at 02:40
then you are looking at a floating point error. – YousefZ01 Apr 20 '21 at 02:42
1

Can you explain? Im just saying to test your own program. before posting it. – Buddy Bob Apr 20 '21 at 02:45
1

Floating point errors are caused because of how decimal values (floats) are stored in memory. This is a well known and documented error, that I suggest you look at online. there are examples [here](https://stackoverflow.com/questions/2100490/floating-point-inaccuracy-examples) – YousefZ01 Apr 20 '21 at 02:47
oh apologies. Can edit your question to show what an example output would look like – YousefZ01 Apr 20 '21 at 02:51

my loop will run into negative numbers even when I am an if statement telling not to

2 Answers2