Diagonal Indexing on Grids

Constant i+j or i-j labels diagonals for sweeping tricks.

Try this in Python

grid = [[1, 2], [3, 4]]
for i, row in enumerate(grid):
    for j, v in enumerate(row):
        print(v, "diag", i + j)

Key takeaways

• Map diagonals to dict of lists for grouping.
• Mind off-by-one in boundaries.
• Anti-diagonals flip column index.