Python index error in a 2d array

Question

So i am making a game involving a 2d array, and i ask the user to input a grid height and width, here is the code that makes the grid

Grid=[['O'for x in range(gridS[0])]for y in range(gridS[1])]

I later change gridS[0] and 1 to gridX and Y

then i try to add players into the grid, again I ask the user how many players (stored in variable opponents) are playing, then add that many players

            if opponents>0:
            Grid[gridX-1][gridY-2]='P'
            if opponents>1:
                Grid[0][(gridY-2)]='P'
                if opponents>3:
                    Grid[gridX-1][0]='P'

however, when testing, it comes up with this error:

>>> How many bots do you want to play against?
>>> 10
>>> Please enter a number between 0 and 3
>>> 2
2

    Grid[gridX-1][gridY-2]='P'
IndexError: list assignment index out of range

It is a pedantic point, and loosely, it could be referred to as an array, but it is better to be precise. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jul 12, 2017 at 18:35
What do gridX and gridY hold before that line? Print them there and you will see your error — Ofer Sadan
– Ofer Sadan, Commented Jul 12, 2017 at 18:39

juanpa.arrivillaga · Accepted Answer · 2017-07-12 18:41:41Z

The problem is your nested comprehension constructs a nested array with the dimensions "flipped". Consider:

>>> dims = (4, 2)
>>> GRID = [['0' for x in range(dims[0])] for y in range(dims[1])]
>>> GRID
[['0', '0', '0', '0'], ['0', '0', '0', '0']]

That would typically be called a 2X4 "matrix". And although that is a red flag, it is a notational issue. The fundamental problem is you are indexing into the first axis using gridX, gridY = dims. So:

>>> x, y = dims
>>> x
4
>>> len(GRID)
2
>>> GRID[x]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

To make everything work out, use the following comprehension:

 Grid=[['O'for y in range(gridS[1])]for x in range(gridS[0])]

In other words, the inner-comprehension creates the second axis, the outer creates the first. But you were thinking about it in the opposite way.

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