Index error saying that the index is out of range

Question

I'm trying to return the sum of the numbers in the array, returning 0 for an empty array. Except the number 13 it doesn't count and the number immediately after 13.

def sum13(nums):
  p = 0
  for i in range(len(nums)):
    if nums[i] == 13 and nums[i + 1]:
      continue
    p += nums[i]
  return p

When I run it, it gives me an index error list index our of range, what am I doing wrong?

I suspect `nums[i+1]` is indexing out of range when `i` reaches the highest value in the range. — lurker, Apr 17 '19 at 02:53
Yes, if `i` is at the end of the range already, `i+1` would be beyond it. — kindall, Apr 17 '19 at 02:55
Yeah if the last element of the list is 13, then your code would try to access the last + 1 element. I also don't think the code does what you think it does. — T Tse, Apr 17 '19 at 02:55

score 2 · Answer 1 · answered Apr 17 '19 at 03:05

2

Here is what you want:

def sum13(nums):
    p = 0
    skip_next = False
    for i in range(len(nums)):
        if nums[i] == 13:
           skip_next = True
           continue
        if skip_next:
            skip_next = False
            continue
        p += nums[i]
    return p

answered Apr 17 '19 at 03:05

emilaz

1,722
1
15
31

You could make this significantly shorter if use user `iter` and `next` such as [Skip multiple iter...](https://stackoverflow.com/a/22296065/8150685) – Error - Syntactical Remorse Apr 17 '19 at 04:12

score 1 · Answer 2 · answered Apr 17 '19 at 02:56

1

range(len(nums))'s size is the same size of nums.
Hence, doing nums[i+1] doesn't exist.

answered Apr 17 '19 at 02:56

ggrelet

1,071
7
22

score 0 · Answer 3 · answered Apr 17 '19 at 03:08

if nums[i]==13 and nums[i+1] won't work, since i+1 will be bigger than the length of the list, also nums[i]+1 just checks if the number exists, it doesn't skip it.
You want to skip a number if it is 13, or the number before it is 13, as follows

def sum13(nums):

    p = 0
    for i in range(len(nums)):
        if nums[i] == 13 or nums[i-1] == 13:
            continue
        else:
            p += nums[i]
    return p

print(sum13([1,2,13,14,5,6,7]))
#21
print(sum13([13,14]))
#0
print(sum13([13]))
#0
print(sum13([]))
#0

score 0 · Accepted Answer · answered Apr 17 '19 at 03:21

The problem is with nums[i+1], it is out of the range of your list. Here is another solution using while.

def sum13(nums):
    p=0; i=0
    while i < len(nums):
        if nums[i]==13:
            i = i + 2 #exclude the position after 13
            continue
        else:
            p+=nums[i]
            i=i+1
    return p

Index error saying that the index is out of range

4 Answers4