3.2.6 - Two-dimensional arrays (Python)
A two-dimensional collection models a table or grid. You will use Python lists of rows, access a value with two indexes and traverse the grid to solve a small counting problem.
Rows, columns and indexes
A two-dimensional array needs two indexes because a value has both a row and a column position.
A classroom plan can record 0 for an empty seat and 1 for an occupied seat. A Python equivalent is a list containing one list per row:
seats = [[1, 0, 1], [0, 1, 0]]
print(seats[1][0])
The outer index 1 chooses the second row, [0, 1, 0]; the inner index 0 chooses that row's first item, so the output is 0. Read the indexes in the stated row/column order, not as Cartesian x/y coordinates.
| Row index | Column 0 | Column 1 | Column 2 |
|---|---|---|---|
| 0 | 1 | 0 | 1 |
| 1 | 0 | 1 | 0 |
There are two rows and three columns: row indexes 0–1 and column indexes 0–2. len(seats) is 2 because it counts outer elements (rows); len(seats[0]) is 3 because it counts the first row's elements. To occupy row 1, column 0, assign 1 to that position.
seats[1][0] = 1
One-dimensional arrays use one index; two-dimensional arrays use two. These are the dimensions required by this specification. The rectangular examples here use rows of equal length.
Traversing a two-dimensional array
Nested iteration can visit every cell by choosing each row and then each column within that row.
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