Call by reference

Notes

Definition: In C, the technique commonly called call by reference passes the address of a variable to a function using a pointer. The function can then access and modify the original variable through that address.

Why is Call by Reference Used?

Definition: Address-based parameter passing is used when a function needs to modify the original variable of the calling function or when working with large data without making a separate copy of the data.

Pointers and Addresses

Definition: A pointer is a variable that stores the memory address of another variable.

Address-of Operator (&)

Definition: The & operator returns the memory address of a variable.

Dereference Operator (*)

Definition: The * operator is used with a pointer to access the value stored at the address held by that pointer.

How Call by Reference Works

Simple Example

The function receives the address of x and changes the value stored at that address.

#include <stdio.h>

void change(int *p)
{
    *p = 20;                // modify the original variable
}

int main()
{
    int x = 10;

    change(&x);             // pass address of x

    printf("x = %d
", x);

    return 0;
}

Output

x = 20

Call by Value vs Call by Reference

Using Pointer Parameters

A pointer parameter is used when a function needs access to the caller's variable through its address.

Practical Example — Swap Two Numbers

Problem Statement

Write a C program to swap two numbers using a function and pointers. The original values in main() should be changed.

Learning Outcomes

• Understand address-based parameter passing.
• Use pointers as function parameters.
• Modify original variables from inside a function.

Hint

Pass the addresses of the two variables to swap(). Use pointer dereferencing and a temporary variable to exchange their values.

Theory

To modify the caller's variables, pass their addresses to the function. The pointer parameters can then access and update the original values.

Program

#include <stdio.h>

void swap(int *a, int *b)
{
    int temp;

    temp = *a;              // store the value at address a
    *a = *b;                // copy value at b into a
    *b = temp;              // place original a value into b
}

int main()
{
    int x, y;

    printf("Enter two numbers: ");
    scanf("%d %d", &x, &y);

    printf("Before swap: x = %d, y = %d
", x, y);

    swap(&x, &y);         // pass addresses of x and y

    printf("After swap: x = %d, y = %d
", x, y);

    return 0;
}

Expected Output

Enter two numbers: 10 20
Before swap: x = 10, y = 20
After swap: x = 20, y = 10

Note

The function receives the addresses of x and y. Therefore, changes made through *a and *b affect the original variables.

Key Symbols to Remember

Important Points for Exam

• C uses pointers and addresses for the technique commonly called call by reference.
• The address of the variable is passed to the function.
• A pointer parameter receives that address.
• The dereference operator * accesses the original value.
• Changes through the pointer can modify the caller's variable.
• The & operator is used to obtain a variable's address.

Quick Revision

Important Exam Questions

Short Answer Questions

1. What is call by reference?
2. How is call by reference achieved in C?
3. What is a pointer parameter?
4. What is the use of the & operator?
5. What is the use of the * dereference operator?
6. How can a function modify the original variable?

Long Answer Questions

1. Explain call by reference in C with a suitable example.
2. Explain how pointers are used to modify original variables through a function.
3. Write a C program to swap two numbers using pointers.
4. Differentiate between call by value and call by reference.