A nested loop means a loop inside another loop. The inner loop runs completely for every single iteration of the outer loop.

Table of Contents

Basic Syntax

// Outer loop
for(int i = 1; i <= 3; i++) {
// Inner loop
for(int j = 1; j <= 3; j++) {
printf("%d,%d ", i, j);
}
printf("\n");
}

Output:

1,1 1,2 1,3
2,1 2,2 2,3
3,1 3,2 3,3

How it works:

i=1 → inner loop runs j=1,2,3 → prints 1,1 1,2 1,3
i=2 → inner loop runs j=1,2,3 → prints 2,1 2,2 2,3
i=3 → inner loop runs j=1,2,3 → prints 3,1 3,2 3,3

Total iterations = Outer iterations × Inner iterations = 3 × 3 = 9

Types of Nested Loops

Combination	Use Case
`for` inside `for`	Matrices, tables, patterns
`while` inside `while`	Complex data traversal
`for` inside `while`	Mixed control flows
`do-while` inside `for`	Input validation loops

Practical Example 1: Multiplication Table

#include <stdio.h>
int main() {
for(int i = 1; i <= 5; i++) {
for(int j = 1; j <= 5; j++) {
printf("%3d ", i * j);
}
printf("\n");
}
return 0;
}

Output:

  1   2   3   4   5
2   4   6   8  10
3   6   9  12  15
4   8  12  16  20
5  10  15  20  25

Practical Example 2: Triangle Pattern

#include <stdio.h>
int main() {
for(int i = 1; i <= 5; i++) {
for(int j = 1; j <= i; j++) {
printf("* ");
}
printf("\n");
}
return 0;
}

Output:

*
* *
* * *
* * * *
* * * * *

Notice: Inner loop runs i times, not a fixed number.

Practical Example 3: Sum of All Elements in 2D Array

#include <stdio.h>
int main() {
int matrix[3][4] = {
{1, 2, 3, 4},
{5, 6, 7, 8},
{9, 10, 11, 12}
};
int sum = 0;
for(int i = 0; i < 3; i++) {
for(int j = 0; j < 4; j++) {
sum += matrix[i][j];
}
}
printf("Total sum: %d\n", sum);
return 0;
}

Output:

Total sum: 78

Practical Example 4: Find Prime Numbers (Break in Nested Loops)

#include <stdio.h>
int main() {
printf("Prime numbers from 2 to 50:\n");
for(int n = 2; n <= 50; n++) {
int isPrime = 1;
for(int i = 2; i <= n/2; i++) {
if(n % i == 0) {
isPrime = 0;
break;  // Breaks inner loop only
}
}
if(isPrime) {
printf("%d ", n);
}
}
return 0;
}

Output:

Prime numbers from 2 to 50:
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47

Important Rules

Rule	Explanation
Break	Only exits the innermost loop
Continue	Skips to next iteration of innermost loop
Indentation	Always indent inner loops for readability
Performance	Be careful—complexity multiplies (O(n²) or higher)

Breaking Out of Multiple Loops

#include <stdio.h>
int main() {
// Method 1: Using a flag variable
int found = 0;
for(int i = 0; i < 5 && !found; i++) {
for(int j = 0; j < 5; j++) {
if(i == 2 && j == 3) {
found = 1;
break;  // Exits inner loop
}
printf("%d,%d ", i, j);
}
}
printf("\n\nMethod 2 - Using goto:\n");
// Method 2: Using goto (controversial but effective)
for(int i = 0; i < 5; i++) {
for(int j = 0; j < 5; j++) {
if(i == 2 && j == 3) {
goto exit_all;
}
printf("%d,%d ", i, j);
}
}
exit_all:
return 0;
}

Common Pitfalls

Mistake	Problem	Fix
Wrong loop bounds	Off-by-one errors	Use `<=` or `<` carefully
Missing braces	Only first line in inner loop	Always use `{}`
Same variable name	Conflict/unexpected behavior	Use `i`, `j`, `k` distinct names
Infinite loops	Outer or inner loop never ends	Verify update statements

Quick Summary

Nested loop = loop containing another loop
Total iterations = outer iterations × inner iterations
Use cases: matrices, patterns, searching, sorting
Break/continue affect only the innermost loop
Performance declines quickly as nesting depth increases

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Basic Syntax

Types of Nested Loops

Practical Example 1: Multiplication Table

Practical Example 2: Triangle Pattern

Practical Example 3: Sum of All Elements in 2D Array

Practical Example 4: Find Prime Numbers (Break in Nested Loops)

Important Rules

Breaking Out of Multiple Loops

Common Pitfalls

Quick Summary

Complete C Programming Guide + Compilers Collection

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3. C log() Function

4. Mastering Date and Time in C

5. Mastering time_t Type in C

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7. C log() Function (Alternate Guide)

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9. C tan() Function

10. Random Numbers in C (Secure vs Predictable)

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Function Parameters in C

Function Declarations in C

Function Calls in C

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Pass-by-Reference in C

C strstr() Function

C Preprocessor & Macros

C Structures, Memory, Scope & Linkage

C Scope, Storage Classes & Typedef

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Online Compilers

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