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]
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?
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?.
When looping over a list, the for variable (in this example i) represents the current element of the list.
For example, given ar = [1, 5, 10], i will have the successive values 1, 5 and 10 each time through the loop. Since the length of the list is 3, the maximum permitted index is 2. Thus, the second time through the loop, when i == 5, an IndexError is raised.
The code should be like this instead:
for i in ar:
theSum = theSum + i
To be able to index into the list, use a range instead of iterating over the list directly:
for i in range(len(ar)):
theSum = theSum + ar[i]
This way, i naturally takes on all the valid index values for ar.