CODEWITHSUNDEEP
pythonmatrixindexing
ggordon
ggordon

Reputation: 259

Python - Matrix IndexError exception not working

I am trying to write a function which enables a move 1 unit to the left (in the x-axis) in a grid. It should only work if there is a valid coordinate to the left, and if not (i.e. the start point is on the edge of the grid), the function should return None. Here is my attempt:

def left_move(point):
    try: 
        LEFT = grid[point[1]][point[0]-1]
    except IndexError: 
        return None
    if LEFT == '.':
        change = [-1,0]
        for idx, amount in enumerate(change):
            point[idx]+=amount
        return point
    return None

Say the starting point is [0,3] and therefore on the edge of the grid and I only run the top part of the code:

def left_move(point):
    try: 
        LEFT = grid[point[1]][point[0]-1]
    except IndexError: 
        return None

It returns None as expected. But if the whole block is executed it returns [-1,3] which is outside of the grid and shouldn't be allowed by the try-except . Why is this? And how can it be corrected?

Upvotes: 1

Views: 94

Answers (2)

dheiberg
dheiberg

Reputation: 1914

Here, you need to place the rest of the code inside the try statement. Otherwise, it will try, fail, and then run the rest of the code. 

def left_move(point):
    if point[0]-1 != -1: 
        LEFT = grid[point[1]][point[0]-1]
        if LEFT == '.':
            change = [-1,0]
            for idx, amount in enumerate(change):
                point[idx]+=amount
            return point
        return None
    else: 
        return None

Upvotes: 2

Jokab
Jokab

Reputation: 2936

This is because Python's interpretation of -1 in array indexing is the last element of the array. -1 is a legal way of referencing array elements and thus will not trigger an IndexError exception.

>>> a = [0,1,2]
>>> print a[-1]
2

You would need to manually check if the value you are given resolves to -1 and then handle that accordingly.

Upvotes: 2

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