Two-dimensional arrays

The 2D array: rows and columns, m[i][j] access, nested loops, traversal by rows, by columns or along a diagonal, neighbours of a cell and array of arrays.

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Two-dimensional array: what is it?
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A grid of values organised in rows and columns, where each cell is identified by two indices.
In Python, it is a list of lists: m[i] is row i, and m[i][j] is the cell at row i and column j.
What does this program print?

```python
m = [[1, 2, 3], [4, 5, 6]]
print(m[1][2])
`
- 6
- 3
- 5
- 8
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6
m[1] is the row [4, 5, 6], and its cell at index 2 is 6. 3 would be m[0][2], and 5 would be m[1][1].
In m[i][j], i is the … index and j is the … index.
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In m[i][j], i is the row index and j is the column index.
Read left to right: first the row m[i], then cell j of that row. Swapping the two gives another cell, or an IndexError on a non-square grid.
Row-by-row traversal: what is it?
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A visiting order that finishes a whole row before moving to the next: the outer loop picks the row, the inner one goes through the columns.
Swapping the two loops gives the column-by-column traversal. In C, a grid is stored in memory row after row: the row-by-row traversal is therefore the fastest.

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