Why does increasing a variable by a list element give me an IndexError?

Question

I am new to Python. I am trying to make a slot machine game and I keep getting this error:

totalScore += scores[score]
IndexError: list index out of range

Here is the code:

for dollars in range(money):
if money < 5:
    print("You don't have enough money to play anymore!")
    break
score = pyslot()
scores.append(score)
money -= 5

totalScore = 0
winnings = 0

for score in scores:
    totalScore += scores[score]
    if scores[score] < 20:
        winnings -= 5
    elif scores[score] < 50:
        winnings += 5
    elif scores[score] >= 50:
        winnings += 10

I'm not sure why Python is giving me this error. I don't know much about it but I thought that the "for score in scores" would keep this from happening.

Answer 1

for score in scores loops over the elements in scores . If scores is a list containing [50, 100, 200] you will get 50 in score in the first iteration; scores[50] is clearly out of bounds, and hence you get an IndexError .

If you actually want to loop over the positions in the array, try

for idx in range(len(scores)):
    totalScore += scores[idx]

but this is unnecessarily complex and slow here.

(There are situations where it makes a difference to your algorithm where in the sequence you are in each iteration. For example, if you wanted to compare the current scores[idx] to the adjacent scores in the list, that's somewhat hard to do if you loop directly over the items, but easy with scores[idx-1] and scores[idx+1] - obviously, you have to take care to avoid an IndexError at the first and last elements, but let's skip that complication here.)