How to resolve Python: IndexError: list index out of range?

Question

I am trying to count the occurrence of '-1' in the list 'args'. '-1' occurs at many locations, so when ever it occurs more than once consecutively, I wish to count that.

I am getting an "list index out of range" error, however it is wrong. I am trying to access the 16th element, the length of 'args' is 19. On line 5 and 6 I am individually printing the index and the element of the list, these lines are executing without error.

Why am I getting the error? Moreover, print statement in line 10 is not printing, what is the reason?

args=[-3, -1, -1, -1, -1, -2, -1, -1, -2, -1, -1, -1, -1, -3, -1, -2, -1, -1, -1]
i=0
while  i<= len(args)-1:
    count=0
    print(i)
    print(args[i])
    while args[i]==-1:
        count+=1
        i+=1 
    print("count="+str(count)+"-"+str(i))
    i+=1


$python main.py
0
-3
count=0-0
1
-1
count=4-5
6
-1
count=2-8
9
-1
count=4-13
14
-1
count=1-15
16
-1
Traceback (most recent call last):
  File "main.py", line 8, in <module>
    while args[i]==-1:
IndexError: list index out of range

the error does not occur in the outer loop but in the inner one. your last element is -1, therefore it gets executed once more and causes the error. because, it does not need to be -1 there to cause the error, the error occurs before the check — muzzletov, Jul 25 '21 at 12:42
I highly recommend not to use `while` instead of a `if` clause (which I assume you tried to). I also recommend Python‘s [for (each) loop](https://www.geeksforgeeks.org/iterate-over-a-list-in-python/). — Michael Dorner, Jul 25 '21 at 12:46

score 0 · Accepted Answer · answered Jul 25 '21 at 12:42

The main problem here happens when you do while args[i] == -1.

The reason that is problematic is because if your last value will be -1, you will increment the index and then you'll try to access args in an index that does not exist.

There is a much faster solution for your general problem (counting consecutive values), as answered by @Karin here:

from itertools import groupby
list1 = [-1, -1, 1, 1, 1, -1, 1]
count_dups = [sum(1 for _ in group) for _, group in groupby(list1)]
print(count_dups)

score 0 · Answer 2 · answered Jul 25 '21 at 13:00

The IndexError is because of your Inner while loop. You are incrementing i without having a check if it exceeds the list length and trying to access it.

There's another way to solve this.

You could keep track of previously accessed element , check if it's -1 and check if previous and current element are same and only then increase the counter count

args=[-3, -1, -1, -1, -1, -2, -1, -1, -2, -1, -1, -1, -1, -3, -1, -2, -1, -1, -1]

prev = args[0]
count = 0 
i = 1
while  i < len(args):
    if args[i] == -1 and args[i] == prev:
        count += 1
    else:
        if count > 1:
            print(count)
        count = 1
    prev = args[i]
    if i == len(args) - 1 and count > 1:
        print(count)
    i+=1

This will print the count of -1s that occur consecutively in the list.

How to resolve Python: IndexError: list index out of range?

2 Answers2