Passing Parameters to Functions

Passing Parameters to Functions

There are two ways in which arguments or parameters can be passed to the called function.

Call by value The values of the variables are passed by the calling function to the called function.

Call by reference The addresses of the variables are passed by the calling function to the called function.

Call by Value

In this method, the called function creates new variables to store the value of the arguments passed to it. Therefore, the called function uses a copy of the actual arguments to perform its intended task.

If the called function is supposed to modify the value of the parameters passed to it, then the change will be reflected only in the called function. In the calling function, no change will be made to the value of the variables. This is because all the changes are made to the copy of the variables and not to the actual variables. To understand this concept, consider the code given below. The function add() accepts an integer variable num and adds 10 to it. In the calling function, the value

of num = 2. In add(), the value of num is modified to 12 but in the calling function, the change is not reflected.

#include <stdio.h>

void add(int n);

int main()

{

int num = 2;

printf("\n The value of num before calling the function = %d", num);

add(num);

printf("\n The value of num after calling the function = %d", num);

return 0;

}

void add(int n)

{

n = n + 10;

printf("\n The value of num in the called function = %d", n);

}

Output

The value of num before calling the function = 2

The value of num in the called function = 12

The value of num after calling the function = 2

Following are the points to remember while passing arguments to a function using the call-by-value method:

* When arguments are passed by value, the called function creates new variables of the same data type as the arguments passed to it.

* The values of the arguments passed by the calling function are copied into the newly created variables.

* Values of the variables in the calling functions remain unaffected when the arguments are passed using the call-by-value technique.

Pros and cons

The biggest advantage of using the call-by-value technique is that arguments can be passed as variables, literals, or expressions, while its main drawback is that copying data consumes additional

storage space. In addition, it can take a lot of time to copy, thereby resulting in performance penalty, especially if the function is called many times.

Call by Reference

When the calling function passes arguments to the called function using the call-by-value method, the only way to return the modified value of the argument to the caller is explicitly using the return statement. A better option is to pass arguments using the call-by-reference technique.

In this method, we declare the function parameters as references rather than normal variables.

When this is done, any changes made by the function to the arguments it received are also visible in the calling function.

To indicate that an argument is passed using call by reference, an asterisk (*) is placed after the type in the parameter list.

Hence, in the call-by-reference method, a function receives an implicit reference to the argument, rather than a copy of its value. Therefore, the function can modify the value of the variable and that change will be reflected in the calling function as well. The following code illustrates this

concept.

#include <stdio.h>

void add(int *);

int main()

{

int num = 2;

printf("\n The value of num before calling the function = %d", num);

add(&num);

printf("\n The value of num after calling the function = %d", num);

return 0;

}

void add(int *n)

{

*n = *n + 10;

printf("\n The value of num in the called function = %d", *n);

}

Output

The value of num before calling the function = 2

The value of num in the called function = 12

The value of num after calling the function = 12

Advantages

The advantages of using the call-by-reference technique of passing arguments include:

* Since arguments are not copied into the new variables, it provides greater time and space-efficiency.

* The function can change the value of the argument and the change is reflected in the calling function.

* A function can return only one value. In case we need to return multiple values, we can pass those arguments by reference, so that the modified values are visible in the calling function.

Disadvantages

However, the drawback of using this technique is that if inadvertent changes are caused to variables in called function then these changes would be reflected in calling function as original values would have been overwritten.Consider the code given below which swaps the value of two integers. Note the value of integers

in the calling function and called function.

//This function swaps the value of two variables

#include <stdio.h>

void swap_call_val(int, int);

void swap_call_ref(int *, int *);

int main()

{

int a=1, b=2, c=3, d=4;

printf("\n In main(), a = %d and b = %d", a, b);

swap_call_val(a, b);

printf("\n In main(), a = %d and b = %d", a, b);

printf("\n\n In main(), c = %d and d = %d", c, d);