Nested Loops in Kotlin: for Loops Inside for Loops

A nested loop has one loop inside of another. These are typically used for working with two dimensions such as printing stars in rows and columns as shown below. When a loop is nested inside another loop, the inner loop runs many times inside the outer loop. In each iteration of the outer loop, the inner loop will be re-started. The inner loop must finish all of its iterations before the outer loop can continue to its next iteration.

Example:

for (row in 0 until 3) {
    for (col in 0 until 5) {
        print("*")
    }
    println()
}

// This code prints a rectangle with 3 rows and 5 columns, filled with '*'

Iterating through multidimensional arrays

Nested loops are also helpful when we want to iterate through a multi dimensional array, for example:

val arr = arrayOf(intArrayOf(1, 2), intArrayOf(3, 4), intArrayOf(5, 6))

for (i in 0 until arr.size) {
    for (j in 0 until arr[i].size) {
        println(arr[i][j])
    }
}

This outputs each sub-element in arr one at a time. Note that for the inner loop, we are checking the .length of arr[i], since arr[i] is itself an array.

Breaking nested loops

In a nested loop, a break statement only stops the loop it is placed in. Therefore, if a break is placed in the inner loop, the outer loop still continues. For example:

val arr = arrayOf(intArrayOf(1, 2), intArrayOf(3, 4), intArrayOf(5, 6))

for (i in 0 until arr.size) {
    for (j in 0 until arr[i].size) {
        if (arr[i][j] == 3) {
            break
        }
        println(arr[i][j])
    }
}

// prints the numbers 1, 2, 5 and 6 on different lines

When i is 1 and j is 0, we execute the break. This means we stop the inner loop and go back to the outer loop to continue from the next iteration, which is i = 2. And as you can see, we print all the elements of row 2.

However, if the break is placed in the outer loop, all of the looping stops. For example:

val arr = arrayOf(intArrayOf(1, 2), intArrayOf(3, 4), intArrayOf(5, 6))

for (i in 0 until arr.size) {
    if (arr[i][0] == 3) {
        break
    }
    for (j in 0 until arr[i].size) {
        println(arr[i][j])
    }
}
// prints the numbers 1 and 2 on different lines

Assignment
Follow the Coding Tutorial and let's write some nested loops.

Hint
Look at the examples above if you get stuck.