Difference Between Call By Value and Call By Reference

In C programming, functions can receive arguments in two primary ways: Call by Value and Call by Reference. These mechanisms determine how the arguments are passed to functions and how modifications to those arguments inside the function reflect in the calling function.

The main difference between Call by Value and Call by Reference is that in Call by Value, a copy of the actual parameter is passed to the function, meaning any changes made inside the function do not affect the original data. On the other hand, Call by Reference passes the address of the argument, allowing the function to modify the actual value of the argument.

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In this article, we will dive deep into the differences between Call by Value and Call by Reference methods. We will explore what each method is, explore examples, discuss when each should be used, and much more.

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Tabular Difference Between Call by Value and Call by Reference in C

For better understanding, let's explore the difference between Call by Value and Call by Reference in a tabular format:

Also Read: Call by Value and Call by Reference in C++ article for a better understanding!

What is Call by Value in C?

In Call by Value, when a function is called, the actual parameters (the arguments) are passed to the function by making a copy of the values. The function works with these copies, and any changes made inside the function do not affect the original arguments. Essentially, the values of the variables are "isolated" inside the function, and any modification within the function will only alter the local copy of the data.

This method is simple and ensures that the original data remains unaffected. It is commonly used when the function only needs to read the value and not modify the original argument.

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Example of Call by Value in C

#include <stdio.h>

void addTen(int num) {
    num = num + 10;
    printf("Value inside the function: %d\n", num);
}

int main() {
    int x = 5;
    addTen(x);
    printf("Value outside the function: %d\n", x);
    return 0;
}

Output:

Value inside the function: 15
Value outside the function: 5

Explanation: In this example, x is passed to the addTen() function by value. The function modifies its local copy of x, but the original x in the main() function remains unchanged.

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When should you use the call by value in C?

Call by Value is useful when:

• You do not want the function to modify the original data.
• You are working with simple, small data types like integers or characters.
• You need to preserve the original value for other operations after the function call.

This method is also suitable for situations where data integrity is important, and you want to avoid side effects.

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What is Call by Reference in C?

In Call by Reference, instead of passing a copy of the argument, the memory address (or reference) of the actual parameter is passed to the function. This allows the function to access and modify the original data directly. Since the function operates on the actual data, changes made within the function will affect the original variable in the calling function.

Call by Reference is commonly used when you need to modify the original data inside the function or when working with large data structures like arrays or structures, as it avoids the overhead of copying large amounts of data.

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Example of Call by Reference in C

#include <stdio.h>

void addTen(int *num) {
    *num = *num + 10;
    printf("Value inside the function: %d\n", *num);
}

int main() {
    int x = 5;
    addTen(&x);
    printf("Value outside the function: %d\n", x);
    return 0;
}

Output:

Value inside the function: 15
Value outside the function: 15

Explanation: In this example, the address of x is passed to the addTen() function. Inside the function, we dereference the pointer *num to modify the value of x. As a result, the change is reflected in the original variable.

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When should you use the call by reference in C?

Call by Reference is preferred when:

• You need to modify the original data.
• You are working with large data structures like arrays or structures, and passing by reference is more memory-efficient.
• You want to return multiple values from a function using pointers.

It is also ideal for recursive functions where you need to modify the arguments and reflect those changes in the recursive calls.

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Code Example Demonstrating Both Call by Value and Call by Reference

#include <stdio.h>

void modifyByValue(int num) {
    // This function will modify only the local copy of 'num'
    num = num + 5;
    printf("Inside modifyByValue: num = %d\n", num); // Changes only local 'num'
}

void modifyByReference(int *num) {
    // This function will modify the original 'num' using its reference
    *num = *num + 5;
    printf("Inside modifyByReference: num = %d\n", *num); // Changes the original 'num'
}

int main() {
    int x = 10;  // Initial value of x

    printf("Before calling modifyByValue: x = %d\n", x);
    modifyByValue(x);  // Call by Value - 'x' won't be affected
    printf("After calling modifyByValue: x = %d\n", x);  // 'x' remains the same

    printf("\nBefore calling modifyByReference: x = %d\n", x);
    modifyByReference(&x);  // Call by Reference - 'x' will be affected
    printf("After calling modifyByReference: x = %d\n", x);  // 'x' is changed

    return 0;
}

Explanation of Code:

1. modifyByValue function: This function takes an integer parameter num and modifies its value by adding 5. However, since the function is using Call by Value, it only modifies the local copy of num. The original variable passed in main() remains unaffected.
2. modifyByReference function: This function takes a pointer to an integer (int *num). It dereferences the pointer (*num) and modifies the original value passed from main(). Since it uses Call by Reference, the actual argument is modified.

Output:

• Before calling modifyByValue: x = 10
• Inside modifyByValue: num = 15
• After calling modifyByValue: x = 10

• Before calling modifyByReference: x = 10
• Inside modifyByReference: num = 15
• After calling modifyByReference: x = 15

Note:

• Call by Value (modifyByValue) does not affect the original variable x because only a copy of the value is passed to the function.
• Call by Reference (modifyByReference) modifies the original variable x because the function has access to the variable’s memory address.

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Key Differences Between Call by Value and Call by Reference in C

Here are some of the key differences between Call by Value and Call by Reference methods:

• In the Call by Value method, the function works with a copy of the actual argument, while in Call by Reference method, the function works with the original argument through its reference (memory address).
• The Call by Value method does not modify the original variable, whereas the Call by Reference method allows the function to modify the original value.
• The Call by Value method is simpler and safer because it isolates changes to the function's local scope, avoiding unintended side effects. In contrast, the Call by Reference method is more efficient for large data types since it does not require copying the data.
• The Call by Value method is commonly used when you don’t need to modify the original data. In contrast, the Call by Reference method is used when you need to alter the original data or pass large structures efficiently.

Conclusion

Call by Value and Call by Reference are two different ways of passing arguments to functions in C. Call by Value passes a copy of the argument, preventing any changes to the original data, while Call by Reference passes the memory address, allowing the function to modify the original data. Understanding the differences and when to use each method can significantly impact both the efficiency and behavior of your code.

FAQS

1. Can a function return a value in Call by Reference?

Yes, a function in Call by Reference can modify the value of the argument passed by reference, and thus, the changes will persist outside the function. However, since the function itself does not return a value, the updated value is reflected in the actual argument passed.

2. What are the advantages of Call by Value in C?

Call by Value offers better security since the original values cannot be modified by the called function. It avoids side effects in the caller function, making debugging easier. It also prevents the modification of variables that should remain constant.

3. What are the limitations of Call by Value in C?

The primary limitation is that the changes made inside the function are not reflected outside the function. If you need to modify the original data, Call by Value is not suitable.

4. Can Call by Reference be used with constant variables in C?

No, Call by Reference cannot be used with constant variables because the function will try to modify the passed arguments, but constant variables cannot be modified.

5. Why is Call by Reference used in C?

Call by Reference is used when you want the function to modify the original data. It's more memory-efficient because no copying of the data occurs, particularly with large data structures like arrays and structs.

6. Can an array be passed by Call by Value in C?

Arrays in C are always passed by reference, even when using Call by Value. This means that although you're passing the array "by value" in the function signature, the function still gets access to the array’s memory address, meaning it can modify the array elements.

7. What is the role of pointers in Call by Reference in C?

In Call by Reference, pointers play a crucial role because they allow functions to access and modify the memory location of the arguments. The function accepts pointers to the variables, which means changes to the variables inside the function will directly affect the caller's variables.

8. What is the impact on memory when using Call by Value?

In Call by Value, each argument is copied into the function's local variable, consuming more memory, especially with large data types like arrays. This can be inefficient for large amounts of data.

9. Is Call by Reference faster than Call by Value in C?

Call by Reference is generally faster than Call by Value, especially when working with large data structures, as it doesn't require copying the data into the function's memory space. However, the speed difference might not be noticeable for small data types.

10. Can Call by Value cause problems in recursion?

In Call by Value, since the values of variables are passed as copies, there's no risk of unintended side effects during recursion. Each recursive call works with its own copy of the variables, which makes debugging easier in recursive functions.

11. What happens if both Call by Value and Call by Reference are mixed in a function?

Mixing both methods in a function can lead to confusion and unexpected behavior. It’s usually better to choose one method based on the function's requirements. However, it’s common to see Call by Value for simple data types (like int or float) and Call by Reference for complex data types (like structs or arrays).

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