Why Do I Get an IndexError When Accessing Elements in a List with \'ar[i]\' Inside a \'for i in ar\' Loop?

Understanding IndexError in "ar[i]" within "for i in ar"

When attempting to sum the elements of a list using a for loop, many encounter an IndexError when trying to access elements using "ar[i]". This occurs due to a misunderstanding of the iterator's purpose within the loop.

The Role of the Iterator Variable

The for loop's iterator variable (in this case, "i") represents the current element of the list. For instance, given a list "ar = [1, 5, 10]", "i" will sequentially contain the values 1, 5, and 10. The error arises when "i" attempts to access the 5th element, which is outside the valid index range of 0-2 for the given list.

Alternative Loop Construction

To address this issue, the loop should be written without indexing the list:

for i in ar:
    theSum = theSum + i

This code correctly iterates over the list elements, adding each to "theSum".

Using Range to Index

Alternatively, if indexing is required, a range() can be employed:

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

This approach ensures that "i" takes on only valid index values for "ar", preventing the IndexError.

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