[FIXED] Why do I get an IndexError (or TypeError) from "ar[i]" inside "for i in ar"?

Issue

I'm trying to sum the values of a list using a for loop. This is my code:

def sumAnArray(ar):
    theSum = 0
    for i in ar:
        theSum = theSum + ar[i]
    print(theSum)
    return theSum

I get the following error:

line 13, theSum = theSum + ar[i]
IndexError: list index out of range

I found that what I'm trying to do is apparently as simple as sum(ar). But I want to understand: Why do I get this IndexError, and how should I write the for loop instead? How does the loop actually work?

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_{For a technical overview of how Python implements for loops and the iterator protocol, see e.g. How does a Python for loop with iterable work?.}

Solution

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]

Answered By - Mr Alihoseiny