List index out of range when using negative values for find neighboring points

Question

A smaller piece of a program I'm writing finds points neighboring a point as if it were on a grid. Observe the cave drawing ...

The program works just fine with positive numbers -- output mockup:

Point: (5, 5) -
(4, 4), (5, 4), (6, 4), (4, 5), (6, 5) ...

and so on ... but let's see what happens if I try a negative value:

Point: (-3, 2) -
ACTUAL OUTPUT ↓ Below is debug
['-3', '2']
-4
1
-4
3
-3
2
-2
1
-2
3

And the error:

Traceback (most recent call last):
  File "C:\Users\jojaw\Documents\All\Stuff\Junk\Python\Continent generation-0.2fork.py", line 193, in <module>
    res = pf.neighbors_point((command[0], command[1]), (0, 0, 50, 50))
  File "C:\Users\jojaw\Documents\All\Stuff\Junk\Python\Continent generation-0.2fork.py", line 51, in neighbors_point
    for j in neighbors[i]:
IndexError: list index out of range

Code:

def neighbors_point(self, pt, bds): #pt is a two-element tuple, bds is a four-element tuple
    neighbors = []
    for x in range(-1, 2): # range excludes last number, actual range is [-1, 0, 1]
        for y in range(-1, 2): # range excludes last number, actual range is [-1, 0, 1]
            neighbors.append((pt[0]+x, pt[1]+y))
    for i in range(0, len(neighbors)): #debug code
        for j in neighbors[i]:
            print(j)
        #end debug code
        condition = neighbors[i][0] < bds[0] or neighbors[i][0] > bds[2] or neighbors[i][1] < bds[1] or neighbors[i][1] > bds[3]
        if condition:
            neighbors.pop(i)
    neighbors.pop(4)
    return neighbors

So, what's my screwup?

The problem is in the line where you pop array elements inside of the loop that iterates over your array. The loop starts with a range 0 to 9, but when the `condition` evaluates to true, you remove an element from the array, thus reducing the array size. — Dakshraj Sharma, Jul 02 '21 at 18:32
@DakshrajSharma I think I understand, what should I do instead? `for i in neighbors`? — , Jul 02 '21 at 18:46
@JoJodaMan Please read: https://stackoverflow.com/questions/1207406/how-to-remove-items-from-a-list-while-iterating — VPfB, Jul 02 '21 at 18:52

score 0 · Accepted Answer · answered Jul 02 '21 at 18:34

0

That's because you are removing elements using neighbors.pop(i) if condition else continue. You might remove the following lines:

for j in neighbors[i]:
    print(j)

Or you can use try and except in order to catch the error:

try:
    for j in neighbors[i]:
        print(j)

except IndexError as error:
    print(error)

Finally, you might use two separate lists, one for iteration and one for popping values out.

answered Jul 02 '21 at 18:34

Samir Ahmane

830
7
22

1

Used a list-iter loop eg `for i in list` -- to avoid out-of \-range issues -- and a counter to keep track of index of element I wanted to remove. Your solution helped. I'll check it! – Jul 02 '21 at 20:25

List index out of range when using negative values for find neighboring points

1 Answers1