python – Why do I get List index out of range when trying to add consecutive numbers in a list using for i in list?

python – Why do I get List index out of range when trying to add consecutive numbers in a list using for i in list?

1. In your for loop, youre iterating through the elements of a list a. But in the body of the loop, youre using those items to index that list, when you actually want indexes.
   Imagine if the list a would contain 5 items, a number 100 would be among them and the for loop would reach it. You will essentially attempt to retrieve the 100th element of the list a, which obviously is not there. This will give you an IndexError.

   We can fix this issue by iterating over a range of indexes instead:

   for i in range(len(a))
   

   and access the as items like that: a[i]. This wont give any errors.

2. In the loops body, youre indexing not only a[i], but also a[i+1]. This is also a place for a potential error. If your list contains 5 items and youre iterating over it like Ive shown in the point 1, youll get an IndexError. Why? Because range(5) is essentially 0 1 2 3 4, so when the loop reaches 4, you will attempt to get the a[5] item. Since indexing in Python starts with 0 and your list contains 5 items, the last item would have an index 4, so getting the a[5] would mean getting the sixth element which does not exist.

   To fix that, you should subtract 1 from len(a) in order to get a range sequence 0 1 2 3. Since youre using an index i+1, youll still get the last element, but this way you will avoid the error.

3. There are many different ways to accomplish what youre trying to do here. Some of them are quite elegant and more pythonic, like list comprehensions:

   b = [a[i] + a[i+1] for i in range(len(a) - 1)]
   

   This does the job in only one line.

Reduce the range of the for loop to range(len(a) - 1):

a = [0, 1, 2, 3]
b = []

for i in range(len(a) - 1):
    b.append(a[i] + a[i+1])

This can also be written as a list comprehension:

b = [a[i] + a[i+1] for i in range(len(a) - 1)]

python – Why do I get List index out of range when trying to add consecutive numbers in a list using for i in list?

When you call for i in a:, you are getting the actual elements, not the indexes. When we reach the last element, that is 3, b.append(a[i+1]-a[i]) looks for a[4], doesnt find one and then fails. Instead, try iterating over the indexes while stopping just short of the last one, like

for i in range(0, len(a)-1): Do something

Your current code wont work yet for the do something part though 😉