Nested Loops in C#: for, while, do-while

A loop within another loop is called nested loop. This is how a nested loop looks like:

Outer-Loop 
{
	// body of outer-loop
	Inner-Loop
	{
		// body of inner-loop
	}
... ... ...
}

As you can see, the outer loop encloses the inner loop. The inner loop is a part of the outer loop and must start and finish within the body of outer loop.

On each iteration of outer loop, the inner loop is executed completely.

Nested for loop

A for loop inside another for loop is called nested for loop.

For example:

for (int i=0; i<5; i++)
{
	// body of outer for loop
	for (int j=0; j<5; j++)
	{
		// body of inner for loop
	}
	// body of outer for loop
}

Example 1: Nested for Loop

using System;

namespace Loop
{
	class NestedForLoop
	{
		public static void Main(string[] args)
		{
			int outerLoop = 0, innerLoop = 0;
			for (int i=1; i<=5; i++)
			{
				outerLoop ++;
				for (int j=1; j<=5; j++)
				{
					innerLoop++;
				}
			}
			Console.WriteLine("Outer Loop runs {0} times", outerLoop);
			Console.WriteLine("Inner Loop runs {0} times", innerLoop);
		}
	}
}

When we run the program, the output will be:

Outer Loop runs 5 times
Inner Loop runs 25 times

In this program, the outer loop runs for 5 times. Each time the outer loop runs, the inner loop runs for 5 times making it run 25 times altogether.

Example 2: Nested for Loop to Print Pattern

using System;

namespace Loop
{
	class NestedForLoop
	{
		public static void Main(string[] args)
		{
			for (int i=1; i<=5; i++)
			{
				for (int j=1; j<=i; j++)
				{
					Console.Write(j + " ");
				}
				Console.WriteLine();
			}
		}
	}
}

When we run the program, the output will be:

1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

Nested while loop

A while loop inside another while loop is called nested while loop.

For example:

while (condition-1)
{
	// body of outer while loop
	while (condition-2)
	{
		// body of inner while loop
	}
	// body of outer while loop
}

Example 3: Nested while Loop

using System;

namespace Loop
{
	class NestedWhileLoop
	{
		public static void Main(string[] args)
		{
			int i=0;
			while (i<2)
			{
				int j=0;
				while (j<2)
				{
					Console.Write("({0},{1}) ", i,j);
					j++;
				}
				i++;
				Console.WriteLine();
			}
		}
	}
}

When we run the program, the output will be:

(0,0) (0,1)
(1,0) (1,1)

Nested do-while loop

A do-while loop inside another do-while loop is called nested do-while loop.

For example:

do
{
	// body of outer while loop
	do
	{
		// body of inner while loop
	} while (condition-2);
	// body of outer while loop
} while (condition-1);

Example 4: Nested do-while Loop

using System;

namespace Loop
{
	class NestedWhileLoop
	{
		public static void Main(string[] args)
		{
			int i=0;
			do
			{
				int j=0;
				do
				{
					Console.Write("({0},{1}) ", i,j);
					j++;
				} while (j<2);
				i++;
				Console.WriteLine();

			} while (i<2);
		}
	}
}

When we run the program, the output will be:

(0,0) (0,1)
(1,0) (1,1)

Different inner and outer nested loops

It is not mandatory to nest same type of loop. We can put a for loop inside a while loop or a do-while loop inside a for loop.

Example 5: C# Nested Loop: Different inner and outer loops

using System;

namespace Loop
{
	class NestedLoop
	{
		public static void Main(string[] args)
		{
			int i=1;
			while (i<=5)
			{
				for (int j=1; j<=i; j++)
				{
					Console.Write(i + " ");
				}

				Console.WriteLine();
				i++;
			}
		}
	}
}

When we run the program, the output will be:

1
2 2
3 3 3
4 4 4 4
5 5 5 5 5

In the above program, a for loop is placed within a while loop. We can use different types of loop inside a loop.