If statement in for loop, index out of range with one additional condition?-CodePudding

I'm trying to create an if statement in a for loop to look at an element in a list and compare it to the next element with enumerate().

arr = ["NORTH", "SOUTH", "SOUTH", "EAST", "WEST", "NORTH", "WEST"]
liste = []
for idx,i in enumerate(arr):
    if (i == 'NORTH' and arr[idx 1] == 'SOUTH') or (i == 'SOUTH' and arr[idx 1] == 'NORTH') or (i == 'EAST' and arr[idx 1] == 'WEST') or (i == 'WEST' and arr[idx 1] == 'EAST'):
        liste.append(idx)
        liste.append(idx 1)
print(liste)

expected

[0, 1, 3, 4]

got

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Input In [44], in <cell line: 2>()
      1 liste = []
      2 for idx,i in enumerate(arr):
----> 3     if (i == 'NORTH' and arr[idx 1] == 'SOUTH') or (i == 'SOUTH' and arr[idx 1] == 'NORTH') or (i == 'EAST' and arr[idx 1] == 'WEST') or (i == 'WEST' and arr[idx 1] == 'EAST'):
      4         liste.append(idx)
      5         liste.append(idx 1)

IndexError: list index out of range

but if the original if is (without the last "or")

for idx,i in enumerate(arr):
    if (i == 'NORTH' and arr[idx 1] == 'SOUTH') or (i == 'SOUTH' and arr[idx 1] == 'NORTH') or (i == 'EAST' and arr[idx 1] == 'WEST'):

it goes through fine and gives the expected outcome (this case has no reversed west/east anyway, but I of course want it to work for random lists).

What's up with that? It's a codewars problem and I can come up with a workaround myself, so I don't want the solution to the whole problem, I'm just trying to understand why it's behaving this way. EDIT: I just realized it's because the last element in the list is actually "WEST" so then it's checking idx 1 which for the last element is not in the list. In that case I would be interested in how to avoid that!

CodePudding user response：

When you reach your last element, idx 1 is out of bounds for the array. You would want to keep that in mind in your logic. One way to resolve is to enumerate over the length-1 of the array so it is never trying to access an index out of bounds.

For example:

arr = ["NORTH", "SOUTH", "SOUTH", "EAST", "WEST", "NORTH", "WEST"]
liste = []
for idx,i in enumerate(arr[:len(arr)-1]):
    if (i == 'NORTH' and arr[idx 1] == 'SOUTH') or (i == 'SOUTH' and arr[idx 1] == 'NORTH') or (i == 'EAST' and arr[idx 1] == 'WEST') or (i == 'WEST' and arr[idx 1] == 'EAST'):
        liste.append(idx)
        liste.append(idx 1)
print(liste)

outputs [0,1,3,4]

CodePudding user response：

When your loop arrives at the last element, it checks for 'WEST' and then tries to check the next element, but there is none. This gives the index error. A quick fix may be to append an element like 'END', for which none of the checks succeeds. Thus, no element past the end gets checked.

CodePudding user response：

The behavior you're describing is due to short-circuit evaluation. This condition:

i == 'WEST' and arr[idx 1] == 'EAST'

will check if i is equal to "WEST". Only when this check passes will it compare if arr[idx 1] is equal to "EAST" -- at which point an IndexError occurs. The other three conditions in your if statement behave analogously, which is why they don't cause an IndexError to occur.

You should create a slice so that you don't check past the last element:

from itertools import islice
for idx, i in enumerate(islice(arr, len(arr) - 1)):
    ...