Why Am I Getting an IndexError When Summing List Values in Python?

Understanding Array Indexing in Python

When attempting to sum the values of a list using a for loop, one may encounter an IndexError as a result of incorrectly using array indexing. The purpose of this article is to elucidate the underlying reason for such errors and provide guidance on how to rectify them.

The for loop in the provided code snippet is attempting to access elements of the array ar by using ar[i] as its expression. However, the loop variable i represents the current element of the list, not the index. Therefore, the statement ar[i] will attempt to access the element at that specific value, leading to an IndexError when the value of i exceeds the array bounds.

To resolve this issue, one should use the value of i as an index into the array. The modified loop below corrects this error:

<code class="python">for i in ar:
    theSum += i</code>

Alternatively, one can also utilize Python's built-in functions for more concise code:

<code class="python">theSum = sum(ar)</code>

To use array indexing explicitly, one can employ a range() function to generate the valid indices:

<code class="python">for i in range(len(ar)):
    theSum += ar[i]</code>

By adhering to these principles, one can avoid IndexingErrors and write reliable and efficient Python code.

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