Grids and images

Think of a cinema seating chart: rows A, B, C…, and seats 1, 2, 3… in each row. To name a seat, you need two pieces of information: the row and the seat number.

A Python list can hold anything, including other lists. Put each row’s seats in a list, then put the rows in a list, and you have a grid. seats[2][5] means “row 2, seat 5.”

Here’s the exciting part: every photo on your phone is a grid like this, just millions of cells big. Each cell, or pixel, holds three numbers for red, green and blue (remember F01). A photo filter is just a loop that visits every pixel and changes its numbers.

The real words

• A 2D list is a list whose items are lists. grid[r] is a whole row; grid[r][c] is one cell.
• Row first, then column. It’s like reading: top to bottom, then left to right.
• Number of rows: len(grid). Number of columns: len(grid[0]).
• Visiting every cell needs nested loops: the inner loop runs completely for each round of the outer loop.

for r in range(len(grid)):
    for c in range(len(grid[r])):
        # do something with grid[r][c]

• In an image, each cell is a pixel [red, green, blue]. Since each pixel is a little list, you can change it in place: img[r][c] = [255, 0, 0].
• Common filters:
  • Invert: each channel becomes 255 - value.
  • Grayscale: set all three channels to the average of the three.
  • Mirror: swap pixels from the left and right sides of each row.
• ⚠️ Making an empty grid: [[0] * 3] * 2 creates two aliases of the same row (remember lesson 14!). Use a comprehension instead: [[0] * 3 for _ in range(2)].