Nested loops in C++

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(int row = 0; row < 3; row++) {
	for(int col = 0; col < 5; col++) {
		cout << '*';
	}
	cout << '\n';
}

// 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:

vector<vector<int>> arr = {
	{1, 2},
	{3, 4},
	{5, 6}
};

for (int i = 0; i < 3; i++) {
	for (int j = 0; j < 2; j++) {
		cout << arr[i][j] << "\n";
	}
}

// This outputs each sub-element in arr one at a time

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:

vector<vector<int>> arr = {
	{1, 2},
	{3, 4},
	{5, 6}
};

for (int i = 0; i < 3; i++) {
	for (int j = 0; j < 2; j++) {
		if(arr[i][j] == 3) {
			break;
		}
		cout << arr[i][j] << "\n";
	}
}

// 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:

vector<vector<int>> arr = {
	{1, 2},
	{3, 4},
	{5, 6}
};

for (int i = 0; i < 3; i++) {
	if(arr[i][0] == 3) {
		break;
	}
	for (int j = 0; j < 2; j++) {
		cout << arr[i][j] << "\n";
	}
}

// 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.