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Functions in C

Swap numbers using function

Swap Two Numbers using a Function — a simple yet essential example to understand call by value and call by reference in C.

C Program: Swap Two Numbers using Function (Call by reference)

Method 1: Call by Reference - Address Passing

#include <stdio.h>
 
// Function declaration
void swap(int *x, int *y);
 
int main() {
    int a, b;
 
    // Input two numbers
    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);
 
    printf("\nBefore swapping: a = %d, b = %d\n", a, b);
 
    // Function call (pass addresses)
    swap(&a, &b);
 
    printf("After swapping: a = %d, b = %d\n", a, b);
 
    return 0;
}
 
// Function definition using call by reference
void swap(int *x, int *y) {
    int temp;
 
    temp = *x;
    *x = *y;
    *y = temp;
}

 
OUTPUT :
Enter two numbers: 10 20

Before swapping: a = 10, b = 20
After swapping: a = 20, b = 10

Explanation

1. Function Parameters
   • The function swap() takes pointers as parameters (int *x and int *y).
   • This allows direct modification of the original variables in main().
2. Logic
   • A temporary variable temp is used to hold one value during the swap.
   • Then values are exchanged using pointer dereferencing (*x and *y).
3. Why Use Pointers?
   • In C, arguments are passed by value by default.
   • To modify the actual variables, we pass their addresses (call by reference).

C Program: Swap Two Numbers using Function (Call by Value)

Method 2: Call by Value

#include <stdio.h>
 
// Function declaration
void swap(int x, int y);
 
int main() {
    int a, b;
 
    // Input two numbers
    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);
 
    printf("\nBefore swapping (in main): a = %d, b = %d\n", a, b);
 
    // Function call
    swap(a, b);
 
    printf("After swapping (in main): a = %d, b = %d\n", a, b);
 
    return 0;
}
 
// Function definition (call by value)
void swap(int x, int y) {
    int temp;
 
    temp = x;
    x = y;
    y = temp;
 
    printf("After swapping (inside function): x = %d, y = %d\n", x, y);
}
 

 
OUTPUT :
Enter two numbers: 10 20

Before swapping (in main): a = 10, b = 20
After swapping (inside function): x = 20, y = 10
After swapping (in main): a = 10, b = 20

Explanation

1. Function Parameters
   • The function receives copies of the variables (x and y).
   • Any changes inside swap() affect only the local copies, not the originals.
2. Key Concept: Call by Value
   • In C, arguments are passed by value by default.
   • Hence, swapping inside the function does not change the values in main().
3. Execution Flow
   • Values are swapped inside the function but remain unchanged outside it.