Python Forum

Why does the "insert.(index, element)" function work backwards in this for loop when used in the context "myList.insert(1, myList[v])"? Instead of adding 0, 1, and 2 to the list to the "myList[1]" location during iteration it will instead add 1 to myList[0], myList[1], and myList[2] during iteration.

Used outside of the loop "myList.insert(4, 8)" it will behave as expected adding 8 to "myList[4]" location.

myList = [1,2,3]
print("This is my initial list: ", myList)
for v in range(len(myList)):
    print("This is v: ", v)
    myList.insert(1, myList[v])
    print("This is my list during iteration: ", myList)
print(myList)

myList.insert(4, 8)
print(myList)

I couldn't upload an image of my output, but here is a copy/paste view of my output.

Output:
This is my initial list:  [1, 2, 3]
This is v:  0
This is my list during iteration:  [1, 1, 2, 3]
This is v:  1
This is my list during iteration:  [1, 1, 1, 2, 3]
This is v:  2
This is my list during iteration:  [1, 1, 1, 1, 2, 3]
[1, 1, 1, 1, 2, 3]
[1, 1, 1, 1, 8, 2, 3]

What do you mean by "work backward"? The action is a bit confusing because you're modifying a list at the same time as you're reading it for information.

Start with [1, 2, 3].
First time through the loop, you tell it copy element 0 (which is a 1) to spot 1. -> [1, 1, 2, 3]
Next time through the loop, you tell it copy element 1 (which is now a 1) to spot 1 -> [1, 1, 1, 2, 3]
Next time through the loop, you tell it copy element 2 (which is now a 1) to spot 1 -> [1, 1, 1, 1, 2, 3]

It doesn't add 1, 2, 3 because myList has changed by the time the element is looked for.