Iterating over arrays in Python 3

When you loop in an array like you did, your for variable(in this example i) is current element of your array.

For example if your ar is [1,5,10], the i value in each iteration is 1, 5, and 10. And because your array length is 3, the maximum index you can use is 2. so when i = 5 you get IndexError. You should change your code into something like this:

for i in ar:
    theSum = theSum + i

Or if you want to use indexes, you should create a range from 0 ro array length - 1.

for i in range(len(ar)):
    theSum = theSum + ar[i]

The for loop iterates over the elements of the array, not its indexes. Suppose you have a list ar = [2, 4, 6]:

When you iterate over it with for i in ar: the values of i will be 2, 4 and 6. So, when you try to access ar[i] for the first value, it might work (as the last position of the list is 2, a[2] equals 6), but not for the latter values, as a[4] does not exist.

If you intend to use indexes anyhow, try using for index, value in enumerate(ar):, then theSum = theSum + ar[index] should work just fine.