problem calculating minimum amount of coins in change using python

Question

I have a homework assignment in which we have to write a program that outputs the change to be given by a vending machine using the lowest number of coins. E.g. £3.67 can be dispensed as 1x£2 + 1x£1 + 1x50p + 1x10p + 1x5p + 1x2p.

However, my program is outputting the wrong numbers. I know there will probably be rounding issues, but I think the current issue is to do with my method of coding this.

change=float(input("Input change"))

twocount=0
onecount=0
halfcount=0
pttwocount=0
ptonecount=0

while change!=0:
     if change-2>-1:
          change=change-2
          twocount+=1
     else:
          if change-1>-1:
               change=change-1
               onecount+=1
          else:
               if change-0.5>-1:
                    change=change-0.5
                    halfcount+=1
               else:
                    if change-0.2>-1:
                         change=change-0.2
                         pttwocount+=1
                    else:
                         if change-0.1>-1:
                              change=change-0.1
                              ptonecount+=1
                         else:
                              break

print(twocount,onecount,halfcount,pttwocount,ptonecount)

RESULTS:

Input: 2.3
Output: 11010
i.e. 3.2

Input: 3.2
Output:20010
i.e. 4.2

Input: 2
Output: 10001
i.e. 2.1

xXliolauXx · Accepted Answer · 2019-06-15 18:14:39Z

2

All your comparisons use >-1, so you give out change as long as you have more than -1 balance.

This would be correct if you were only dealing with integers, since there >-1 is equal to >=0.

For floating point numbers however, we have for example -0.5>-1, so we will give out change for negative balance (which we do not want).

So the correct way would be to replace all >-1 comparisons by >=0 (larger or equal to 0) comparisons.

answered Jun 15, 2019 at 18:14

xXliolauXx

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2 Comments

lee-tmy Over a year ago

Thanks, this seems to help a lot! However the answers are still incorrect - I'm guessing a rounding thing?

xXliolauXx Over a year ago

Possibly yes. A more consistent way of doing this would be to multiply your input by 100 and converting to int, in other words only deal with the balance in pence so you get around those nasty floating point issues.

Matthew Barlowe · Accepted Answer · 2019-06-15 18:07:44Z

The problem is how it calculates the change using your if/else statements. If you walk through the first example change-2>-1 will register true and then result will be .3 but on the next loop the if change - 1 > -1 you are expecting to be false but it's not it's actually -0.7. One of the best ways to do this would be with Python's floor // and mod % operators. You have to round some of the calculations because of the way Python handles floats

change=float(input("Input change"))

twocount=0
onecount=0
halfcount=0
pttwocount=0
ptonecount=0

twocount = int(change//2)
change = round(change%2,1)
if change//1 > 0:
    onecount = int(change//1)
    change = round(change%1,1)
if change//0.5 > 0:
    halfcount = int(change//0.5)
    change = round(change%0.5, 1)
if change//0.2 > 0:
    pttwocount = int(change//0.2)
    change = round(change%0.2, 1)
if change//0.1 > 0:
    ptonecount = int(change//0.1)
    change = round(change%0.1,1)


print(twocount,onecount,halfcount,pttwocount,ptonecount)

But given the inputs this produces

Input: 2.3
Output: 1 0 0 1 1


Input: 3.2
Output:1 1 0 1 0


Input: 2
Output: 1 0 0 0 0

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