Nested Loops in C

A nested loop means a loop statement inside another loop statement.

• For a nested loop, the inner loop performs all of its iterations for each iteration of the outer loop.
• If the outer loop is running from i = 1 to 5 and the inner loop is running from j = 0 to 3. Then, for each value of the outer loop variable (i), the inner loop will run from j = 0 to 3.

1. Nested for Loops

Nested for loop refers to any type of loop that is defined inside a 'for' loop.

i = 0: 0 1 2 3 4 
i = 1: 5 6 7 8 9 
i = 2: 10 11 12 13 14 
i = 3: 15 16 17 18 19 

Below is the equivalent flow diagram for nested 'for' loops:

2. Nested while Loops

A nested while loop refers to any type of loop that is defined inside a 'while' loop.

Pattern Printing using Nested While loop
1 
1 2 
1 2 3 
1 2 3 4 
1 2 3 4 5 

Below is the equivalent flow diagram for nested 'while' loops:

3. Nested do-while Loops

A nested do-while loop refers to any type of loop that is defined inside a do-while loop.

1 2 3 
1 2 3 

Below is the equivalent flow diagram for nested 'do-while' loops:

4. Hybrid Nested Loops

A hybrid loop structure is where any type of loop is nested inside another loop type.

i = 1, j = 1
i = 1, j = 2
i = 2, j = 1
i = 2, j = 2

Break Inside Nested Loops

Whenever we use a break statement inside the nested loops it breaks the innermost loop only and program control goes to the outer loop. Breaking the inner loop does not affects the outer loops.

* * * 
* * * 
* * * 

* * * 

In the above program, the first loop will iterate from 0 to 5 but here if i will be equal to 3 it will break and will not print the * as shown in the output. 

Continue Inside Nested Loops

Whenever we use a continue statement inside the nested loops it skips the iteration of the innermost loop only. The outer loop remains unaffected.

0 1 
0 1 

In the above program, the inner loop will be skipped when j will be equal to 2. The outer loop will remain unaffected.

Uses of Nested Loops

• Printing Patterns: Nested loops are often used to print complex patterns such as printing shapes, grids, or tables.
• Searching and Sorting: Nested loops are used in algorithms that involve searching for or sorting elements like bubble sort, insertion sort, matrix searching etc.
• Multi-Dimensional Data: Nested loops are useful when dealing with multidimensional data structures like 2D or 3D arrays, matrices, list of lists.
• Dynamic Programming: Nested loops are commonly used in dynamic Programming for solving problems like knapsack problem or longest common subsequence.