Julia Nested Arrays

A nested array is an array whose items are themselves arrays. It stores data in rows, like a small grid.

Build a nested array

Inner arrays sit inside one outer array. The whole value prints with both rows visible.

grid = [[1, 2], [3, 4]]
println(grid)
println(length(grid))

Double indexing reaches one item

The first index picks an inner array. The second index picks an item inside it.

grid = [[1, 2], [3, 4]]
println(grid[2][1])
println(grid[1][2])

Print a grid row by row

A function loops over the outer array. Each round prints one inner array.

function printgrid(grid)
    for row in grid
        println(row)
    end
end
printgrid([[1, 2], [3, 4], [5, 6]])

Example

function total(grid)
    s = 0
    for row in grid
        for value in row
            s += value
        end
    end
    return s
end
grid = [[1, 2], [3, 4]]
println(grid[2][2])
println(total(grid))

Key points

Nested array: an array that holds other arrays as its items.
Build one by writing inner arrays inside an outer array, like [[1, 2], [3, 4]].
Use two indexes to reach one item, first the inner array and then the position inside it.
The first index picks a row in [[1, 2], [3, 4]].
Printing a nested array shows the inner arrays in order.

In this example, the grid holds two inner arrays of two numbers each. The double index grid[2][2] reaches 4 in the second row. The total function adds every value in every row and returns 10.

Note

Julia nested arrays are arrays of arrays, so each inner array can have its own length.

Try it Yourself

Edit the code, then press Run.

Running is simulated · JuliaHub

function total(grid)
    s = 0
    for row in grid
        for value in row
            s += value
        end
    end
    return s
end
grid = [[1, 2], [3, 4]]
println(grid[2][2])
println(total(grid))

Exercise

For grid = [[1, 2], [3, 4]], what is grid[1][2]?

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