Python flatten rows of 2D list

Question

Currently working on a program that requires the flattening of columns, rows, and diagonals of a 2D list. I have written the following code that flattens the columns and diagonals but haven't been able to flatten the rows. I am unsure of what I am executing incorrectly.

cols = []
rows = []
max_col = len(grid[0])
max_row = len(grid)

for y in range(max_row):
   rows.append(grid[y][x])
   for x in range(max_col):
       cols.append(grid[y][x])
print(cols)
print(rows)

firstDiagonal = [grid[i][i] for i in range(len(grid))]
secondDiagonal = [grid[i][len(grid)-1-i] for i in range(len(grid))]

The output resembles: ['G', 'A', 'O', 'C', 'T']['T', 'E', 'B', 'R', 'S'] The row output should be longer mimicking the column output. ['A', 'S', 'T', 'R', 'V', 'Y', 'V', 'B', 'B', 'G']

Answer 1

To flatten along the opposite dimension, you need to swap the direction of your loop. If you flatten columns by doing

cols = []
for y in range(max_row):
    for x in range(max_col):
        cols.append(grid[y][x])

then you flatten rows by doing

rows = []
for x in range(max_col):
    for y in range(max_row):
        rows.append(grid[y][x])

You can greatly simplify the computation of cols using the extend method, since the inner loop iterates over the entire row:

cols = []
for y in range(max_row):
    cols.extend(grid[y])

Or better yet:

cols = []
for row in grid:
    cols.extend(row)

Such a simplification would not work for rows, but you can use the transpose idiom using zip:

rows = []
for col in zip(*grid):
    rows.extend(col)

Finally, each of the expressions can be written as a one-liner using any of the following nested comprehensions:

cols = [grid[y][x] for y in range(max_row) for x in range(max_col)]
cols = [item for row in grid for item in row]
rows = [grid[y][x] for x in range(max_col) for y in range(max_row)]
rows = [item for col in zip(*grid) for item in col]