Multidimensional arrays
- JavaScript course
- Beginner
- Lesson 46/133
- 5 min
Learn to create matrix-like structures with nested arrays.
Nested arrays
The elements of an array can themselves be arrays. This is called a nested array. It is used to store data shaped like a two-dimensional grid.
Example: const grid = [[1, 2, 3], [4, 5, 6]]; is a matrix with two rows. The first row is grid[0], the second row is grid[1].
To access a cell you use two indices: grid[0][1] returns the second element of the first row (2).
Traversing with nested loops
To traverse a nested array you use a nested for loop. The outer loop visits rows, the inner loop visits columns.
Example: for (let i = 0; i < grid.length; i++) { for (let j = 0; j < grid[i].length; j++) { console.log(grid[i][j]); } } prints every cell in order.
grid.length gives the row count, grid[i].length gives the column count of one row. In a regular matrix these are the row and column counts.
Practice Print a cell from the matrix array
Create a nested array named matrix with at least two rows. Each row must hold at least three numbers. Then print the third element of the second row (matrix[1][2]) with console.log.
Starter code
// Create the nested array
// Print the element at row 1, column 2Sample solution
const matrix = [[1, 2, 3], [4, 5, 6]];
console.log(matrix[1][2]);Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What value does <code>grid[2][1]</code> return?
const grid = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];- 2
- 5
- 8 Correct answer
- 6
Explanation: grid[2] is the third row [7, 8, 9]; grid[2][1] returns 8.
2 Access a cell with grid[1][2]
const grid = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
const cell = grid[1][____];
console.log(cell);const grid = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
const cell = grid[1][2];
console.log(cell);Explanation: grid[1][2] = 6 (third element of the second row).
3 Match expressions for <code>const grid = [[1, 2, 3], [4, 5, 6]]</code>
| Term | Meaning |
|---|---|
grid.length |
2 |
grid[0] |
[1, 2, 3] |
grid[1][0] |
4 |
grid[0][2] |
3 |