Functions in C(Part 2-Parameter Passing Methods)

In the previous article, you learnt basics of functions in C. In this article, you will learn about methods of passing values.

Passing values to function

There are two ways to pass arguments/parameters to function calls -- call by value and call by reference.

(i). Pass by value

(ii). Pass by reference

(i).Pass by Value

  In this method of parameter passing, a copy of actual parameters is passed to respective formal parameters. Any changes done in formal parameters are not reflected in actual parameters.

When a function is called, any arguments that are provided by the caller are simply treated as expressions. The value of each expression has the appropriate conversions applied and is then used to initialize the corresponding formal parameter in the called function, which behaves in exactly the same way as any other local variables in the function.

The initialization of the formal parameters is the last time that any communication occurs between the caller and the called function, except for the return value.

Example:

//Following example shows the functioning of call by value method

#include<stdio.h>

void swapCallByValue(int,int);  //function prototype

int main(){

    int num1=10,num2=20;

   printf("Values before swapping by call by value: num1=%d,num2=%d.\n",num1,num2);

    swapCallByValue (num1,num2);

    printf("Values after swapping by call by value: num1=%d,num2=%d.\n\n",num1,num2);

    return 0;

}

//function definition: using third variable

void swapCallByValue (int n1,int n2){

  int temp;

  temp=n1;

  n1=n2;

  n2=temp;

  printf("Inside swapCallByValue : num1=%d,num2=%d\n",n1,n2);

}

Output:

   Values before swapping by call by value: num1=10,num2=20.

   Inside swapCallByValue : num1=20,num2=10

   Values after swapping by call by value: num1=10,num2=20.

(ii).Pass by Reference

In call by reference method of parameter passing, the memory address of actual parameters is passed to formal parameters, hence any change made to formal parameters will also reflected in actual parameters.

This method of parameter passing is use-

(a).If the size of data is large.

(b).When there are more than one value we want to alter.

Example:

//Following example shows the functioning of call by reference method

#include<stdio.h>

void swapCallByReference(int *,int *);  //function prototype

int main(){

    int num1=10,num2=20;

    printf("Values before swapping by call by reference: num1=%d,num2=%d.\n",num1,num2);

    swapCallByReference (&num1,&num2);

    printf("Values after swapping by call by reference: num1=%d,num2=%d.\n\n",num1,num2);

    return 0;

}

//function definition: using third variable

void swapCallByReference (int *n1,int *n2){

  int temp;

  temp=*n1;

  *n1=*n2;

  *n2=temp;

  printf("Inside swapCallByReference : num1=%d,num2=%d\n",*n1,*n2);

}

Output:

   Values before swapping by call by reference: num1=10,num2=20.

   Inside swapCallByReference : num1=20,num2=10

   Values after swapping by call by reference: num1=20,num2=10.

Points to note about Functions

(a).The name of a function is always followed by ( ).

(b).The keyword void is used in the example to show the function has no return

value and no parameters.

(c).When no return type is specified, it is assumed to be int.

(d).The main program is also a function. It is called the driver function.

(e).The program always starts at the beginning of main, regardless of whether it is

the first function defined or not.

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Functions in C (Part 1-Introduction)

 Introduction to Functions

A function is an independent module or block of program code which deals with a particular task. A function can be called or used anywhere in the program by calling the name.

Each function has a name or identifier by which is used to refer to it in a program. A function can accept a number of parameters or values which pass information from outside, and consists of a number of statements and declarations, enclosed by curly braces { }, which make up the doing part of the object.

The main is a special function called at the beginning of the program. All other functions are directly or indirectly called from main. That’s why main function is called driver function.

Reasons for using functions

There are many reasons for using functions.

(i).Reusability: A function can be reused in many different contexts without repeating parts of the program text.

(ii).Manageability: Using functions, program will be divided to separate blocks. Each block will do a specific job Designing, understanding and managing smaller blocks of code is easier.

(iii).Readability : Since each function has few lines of code which is easy to read and understand.

Parts of a function

(i).Function prototype

(ii).Function Definition

(iii).Function Calling

(i).Function Prototype / Function Declaration

If we want to use a function before we define it, we must declare it just like a variable to inform the compiler about the function. This declaration is called the function prototype.

The point at which a name has a type associated with it. No storage is reserved at this point.

Syntax:

returnType functionName(dataType var1, dataType var2,……,dataType varN);

 The variable names are not required when declaring a function prototype.

returnType functionName(dataType, dataType,……,dataType);

Examples:

          int square(int number);

          void display(void);

(ii).Function Definition

The block of code between braces following the function header is called the function body.

The function header defines the name of the function, the function parameters (which specify the number and types of values that are passed to the function when it’s called), and the type for the value that the function returns.

The function body contains the statements that are executed when the function is called, and these have access to any values that are passed as arguments to the function.

The general form of a function looks like this:

Return_type Function_name( Parameters - separated by commas )

{

// Statements...

}

Also a declaration, but at this point some storage is reserved for the named object.

The arguments used in the function definition are called formal parameters.

Example:

int square(int num){

   int result;

   result=num*num;

   return result;

}

(iii).Calling a function

A function is called (i.e. control is passed to the function) by using its name with the usual brackets () to follow it, along with the values which are to be passed to the function.

When a function is called, the statements within the body of that function are executed, and when the function has finished executing, control returns to the point at which that function was called.

    Syntax:

                   functionName(parameters);

The  arguments used when the function is actually called are called actual parameters. In other words, the values that the formal parameters will have on entry to the function.

Example:

   int res=square(number);

Complete example

#include<stdio.h>

 //function prototype

int square(int number);

int main(){

  int number,res;

  printf("Enter an integer:\t");

  scanf("%d",&number);

  //functional calling

  res=square(number); 

  printf("\nSquare of %d is %d.\n",number,res);

  return 0;

}

//function definition

int square(int num){

   int result;

   result=num*num;

   return result;

}

How function works?

Program execution start from main function.  Statements are executed in sequence in the normal way until a function call is found.

When function call is found, control passes to function definition along with parameter values and statement with in the called function are executed.

Execution of the program continues through the function statements until it encounters a return statement or reaches the closing brace marking the end of the function body. The return statement of closing brace ‘}’ indicates the end of function and control passes to the point immediately after the point the function was originally called.

A Function can different forms:-

Type 1: No return type, no argument

   void functionName(void){

          //function body

      }

Type 2 : return type, no argument

   dataType functionName(void){

          //function body

        return variable;

      }

Type 3 : return type, with argument list

dataType functionName(argument list){

          //function body

        return variable;

      }

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