Common 2D Array Patterns

A few ways of walking through a grid come up again and again. These examples use a square grid of size n:

var grid := [
	[1, 2, 3],
	[4, 5, 6],
	[7, 8, 9]
]
var n := grid.size()

Diagonals. On the main diagonal, the row index equals the column index: grid[i][i]. On the anti-diagonal, from the top-right corner to the bottom-left one, the column is n - 1 - i. One loop is enough:

for i in range(n):
	print(grid[i][i], " ", grid[i][n - 1 - i])

1 3
5 5
9 7

Rows, columns and borders. Fix one index and loop over the other. The top row is grid[0] and the bottom row is grid[n - 1]. A column keeps c fixed while the row changes:

var left_total := 0
for r in range(n):
	left_total += grid[r][0] # left column: 1 + 4 + 7
print(left_total) # 12

Neighbors. In grid games you often look at the cells around a position: up, down, left and right. Store the four steps as an array of [row, column] pairs, and skip the steps that leave the grid:

var r := 0
var c := 1
for step in [[-1, 0], [1, 0], [0, -1], [0, 1]]:
	var nr: int = r + step[0]
	var nc: int = c + step[1]
	if nr >= 0 and nr < n and nc >= 0 and nc < n:
		print(grid[nr][nc])

The cell grid[0][1] is on the top border, so it has 3 neighbors: 5, 1 and 3.