Python counting through a number with >=

Question

I'm learning Python(2.7) at the moment, and an exercise says to write a program which counts how many coins you need to pay a specific sum. My solution is this:

sum = input("Bitte gebe einen Euro Betrag ein: ")
coins = []
euro = [20,10,5,2,1,0.5,0.2,0.1,0.05,0.02,0.01]

for i in euro:
    while sum >= i:
        sum -= i
        coins.append(i)

print coins

This is nearly working, but when I input e.g. 17,79 it gives me the coins for 17,78.

Bitte gebe einen Euro Betrag ein: 17.79
[10, 5, 2, 0.5, 0.2, 0.05, 0.02, 0.01]

Why? Has this something to do with round?

Answer 1

For currency calculations it's best to avoid float type if you can, because of accumulating rounding errors. You can do it in a way similar to this:

amount= input("Bitte gib einen Euro Betrag ein: ")
coins = []
cents = [2000, 1000, 500, 200, 100, 50, 20, 10, 5, 2, 1]
amount = int(float(amount) * 100)
for cent in cents:
    while amount >= cent:
        amount -= cent
        coins.append(cent)

print [coin / 100.0 for coin in coins]

I've also changed the variable name from sum to amount - sum will shadow the sum built-in function.

Result:

Bitte gebe einen Euro Betrag ein: 17.79
[10.0, 5.0, 2.0, 0.5, 0.2, 0.05, 0.02, 0.02]

Alternatively, you can implement this without inner while loop, like this:

for cent in cents:
    n = int(math.floor(amount / cent))
    amount -= n * cent
    coins += [cent] * n

It's possible to exit loop earlier (if not amount: break) and avoid unnecessary operations (if not n: continue), but I omitted these guards for readability.

Another possible alternative is to use the decimal data type.

Answer 2

It has. If after executing the code you check the 'sum' value, you get something like 0,009999999999999133, which IS smaller than the smallest coin.

My suggestion? Do everything in cents!

sum = int(input("Your text") * 100)
cents = [2000,1000,500,200,100,50,20,10,5,2,1]
coins = []

for i in euro:
    while True:
        if i <= sum:
            coins.append(i)
            sum -= i
        else:
            break

for i in range(len(coins)):
    coins[i] /= 100

print(coins)