list index out of range python

Question

I have written a simple python program

l=[1,2,3,0,0,1] 

for i in range(0,len(l)): 

if l[i]==0: 

l.pop(i)

This gives me error 'list index out of range' on line if l[i]==0:

After debugging I could figure out that i is getting incremented and the list is getting reduced.

However, I have loop termination condition i < len(l). Then why I am getting such error?

Answer 1

This error is because you are reducing the length of your list l as you iterate over it, so as you approach the end of your indices in the range statement, some of those indices are no longer valid.

I think you want to do is:

l = [x for x in l if x != 0]

This will return a copy of l without any of the elements that were zero. You could even shorten that last part to just if x, since non-zero numbers evaluate to True.

You can write the fresh code as follows:

i = 0 

while i < len(l): 

  if l[i] == 0: 

     l.pop(i) 

  else: 

     i += 1

To know more about this you can have a look at the following video tutorial:-

Answer 2

Here, the expression len(l) is evaluated only one time, at the moment the range() builtin is evaluated. The range object constructed at that time does not change; it can't possibly know anything about the object l.