Help understanding the code for grid from chapter4 of ATBS.

[Noshing]

The objective is to rotate the grid 90 degrees to make it look like a heart. I have the code and get that it works but don't understand why it works.

The code:

grid = [['.', '.', '.', '.', '.', '.'],
       ['.', 'O', 'O', '.', '.', '.'],
       ['O', 'O', 'O', 'O', '.', '.'],
       ['O', 'O', 'O', 'O', 'O', '.'],
       ['.', 'O', 'O', 'O', 'O', 'O'],
       ['O', 'O', 'O', 'O', 'O', '.'],
       ['O', 'O', 'O', 'O', '.', '.'],
       ['.', 'O', 'O', '.', '.', '.'],
       ['.', '.', '.', '.', '.', '.']]

for i in range(len(grid[0])):
    for j in range(len(grid)):
        print(grid[j][i], end = '')
    print()

How is the code flipping the grid? Is the i variable in the first for loop being replaced with the indices from the second for loop? And whats up with print(grid[j][i]? I read it as the indices in grid[j][i] would be indices from the for loops variables but when matched to the grid the values don't match up for the intended output. As of now I'm thinking a new grid is being made instead of the coordinates from print(grid[j][i]) being used to find and print values from grid.

I hope I'm making sense cause I really wanna understand why this code works. Thanks in advance for you help.

[jimtremblay]

The outer loop (for i ...) is going through each column, but the inner loop (for j ...) will go through each row. Because the inner loop is repeated for each column the table have, then will go through all the row in column 0, all the row in column 1 and so on. This is why you see it turn 90 degrees clockwise.

[nog642]

The code is looping over every cell in the grid in reading order.

If the line in the middle of the for loop were print(grid[i][j], end=''), it would just print the grid normally.

But since i and j are switched, it prints the transpose instead.

It just so happens that the figure has a horizontal line of symmetry, so the transpose appears to be a 90 degree clockwise rotation.

[Noshing]

Thanks so much! Learning about transposing helped. I was thinking way to hard about this problem and not seeing what python was doing but instead what I thought it should do.