Recursion function

Recursion

C Programming Recursion

A function that calls itself is known as recursive function and the process of calling function itself is known as recursion in C programming.
Example of recursion in C programming

Write a C program to find sum of first n natural numbers using recursion. Note: Positive integers are known as natural number i.e. 1, 2, 3....n

#include <stdio.h>
int sum(int n);
int main(){
    int num,add;
    printf("Enter a positive integer:\n");
    scanf("%d",&num);
    add=sum(num);
    printf("sum=%d",add);
}
int sum(int n){
    if(n==0)
       return n;
    else
       return n+sum(n-1);    /*self call  to function sum() */
}
Output
Enter a positive integer:
5
15
In, this simple C program, sum() function is invoked from the same function. If n is not equal to 0 then, the function calls itself passing argument 1 less than the previous argument it was called with. Suppose, n is 5 initially. Then, during next function calls, 4 is passed to function and the value of argument decreases by 1 in each recursive call. When, n becomes equal to 0, the value of n is returned which is the sum numbers from 5 to 1.
For better visualization of recursion in this example:
sum(5)
=5+sum(4)
=5+4+sum(3)
=5+4+3+sum(2)
=5+4+3+2+sum(1)
=5+4+3+2+1+sum(0)
=5+4+3+2+1+0
=5+4+3+2+1
=5+4+3+3
=5+4+6
=5+10
=15
Every recursive function must be provided with a way to end the recursion. In this example when, n is equal to 0, there is no recursive call and recursion ends.

Advantages and Disadvantages of Recursion

Recursion is more elegant and requires few variables which make program clean. Recursion can be used to replace complex nesting code by dividing the problem into same problem of its sub-type.
In other hand, it is hard to think the logic of a recursive function. It is also difficult to debug the code containing recursion.

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Function arguments

Function arguments

Types of User-defined Functions in C Programming

For better understanding of arguments and return in functions, user-defined functions can be categorised as:

    1.Function with no arguments and no return value
    2. Function with no arguments and return value
    3.Function with arguments but no return value
    4. Function with arguments and return value.

Let's take an example to find whether a number is prime or not using above 4 cateogories of user defined functions.

Function with no arguments and no return value.

/*C program to check whether a number entered by user is prime or not using function with no arguments and no return value*/

#include <stdio.h>
void prime();
int main(){
    prime();      //No argument is passed to prime().
    return 0;
}
void prime(){ 
/* There is no return value to calling function main(). Hence, return type of prime() is void */
    int num,i,flag=0;
    printf("Enter positive integer enter to check:\n");
    scanf("%d",&num);
    for(i=2;i<=num/2;++i){
        if(num%i==0){
             flag=1;
         }
    }
    if (flag==1)
        printf("%d is not prime",num);
    else
       printf("%d is prime",num); 
    }

Function prime() is used for asking user a input, check for whether it is prime of not and display it accordingly. No argument is passed and returned form prime() function.

Function with no arguments but return value

/*C program to check whether a number entered by user is prime or not using function with no arguments but having return value */

#include <stdio.h>
int input();
int main(){
    int num,i,flag;
    num=input();     /* No argument is passed to input() */
    for(i=2,flag=i;i<=num/2;++i,flag=i){
    if(num%i==0){
        printf("%d is not prime",num);
        ++flag;
        break;
    }
    }
    if(flag==i)
        printf("%d is prime",num);
    return 0;
}
int input(){   /* Integer value is returned from input() to calling function */
    int n;
    printf("Enter positive enter to check:\n");
    scanf("%d",&n);
    return n;
}

There is no argument passed to input() function But, the value of n is returned from input() to main() function.

Function with arguments and no return value

/*Program to check whether a number entered by user is prime or not using function with arguments and no return value */

#include <stdio.h>
void check_display(int n);
int main(){
    int num;
    printf("Enter positive enter to check:\n");
    scanf("%d",&num);
    check_display(num);  /* Argument num is passed to function. */
    return 0;
}
void check_display(int n){    
/* There is no return value to calling function. Hence, return type of function is void. */
    int i,flag;
    for(i=2,flag=i;i<=n/2;++i,flag=i){
    if(n%i==0){
        printf("%d is not prime",n);
        ++flag;
        break;
    }
    }
    if(flag==i)
        printf("%d is prime",n);
}

Here, check_display() function is used for check whether it is prime or not and display it accordingly. Here, argument is passed to user-defined function but, value is not returned from it to calling function.

Function with argument and a return value

/* Program to check whether a number entered by user is prime or not using function with argument and return value */

#include <stdio.h>
int check(int n);
int main(){
    int num,num_check=0;
    printf("Enter positive enter to check:\n");
    scanf("%d",&num);
    num_check=check(num); /* Argument num is passed to check() function. */
    if(num_check==1)
       printf("%d in not prime",num);
    else
       printf("%d is prime",num);
    return 0;
}
int check(int n){  
/* Integer value is returned from function check() */
    int i;
    for(i=2;i<=n/2;++i){
    if(n%i==0)
        return 1;
}
   return 0;
}

Here, check() function is used for checking whether a number is prime or not. In this program, input from user is passed to function check() and integer value is returned from it. If input the number is prime, 0 is returned and if number is not prime, 1 is returned.

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Function definition

Function definition

Function definition contains programming codes to perform specific task.
Syntax of function definition
return_type function_name(type(1) argument(1),..,type(n) argument(n))
{
                //body of function
}
Function definition has two major components:

Function definition has two major components:

Function declaration

1. Function declarator

Function declarator is the first line of function definition. When a function is invoked from calling function, control of the program is transferred to function declarator or called function.
Syntax of function declarator
return_type function_name(type(1) argument(1),....,type(n) argument(n))
Syntax of function declaration and declarator are almost same except, there is no semicolon at the end of declarator and function declarator is followed by function body.
In above example, int add(int a,int b) in line 12 is a function declarator.
Function body

2. Function body

Function declarator is followed by body of function which is composed of statements.
Passing arguments to functions
In programming, argument/parameter is a piece of data(constant or  variable) passed from a program to the function.
In above example two variable, num1 and num2 are passed to function during function call and these arguments are accepted by arguments a and b in function definition.
 

Arguments that are passed in function call and arguments that are accepted in function definition should have same data type. For example:

If argument num1 was of int type and num2 was of float type then, argument variable a should be of type int and b should be of type float,i.e., type of argument during function call and function definition should be same.

A function can be called with or without an argument.

Return Statement
Return statement is used for returning a value from function definition to calling function.
Syntax of return statement
return (expression);
          OR
     return;    
For example:
return;
return a;
return (a+b);
In above example, value of variable add in add() function is returned and that value is stored in variable sum in main() function. The data type of expression in return statement should also match the return type of function.
 

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Function call

Function call

Control of the program cannot be transferred to user-defined function unless it is called invoked).
Syntax of function call
function_name(argument(1),....argument(n));
In the above example, function call is made using statement add(num1,num2); from main(). This make the control of program jump from that statement to function definition and executes the codes inside that function.

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Function prototype

Function prototype

Syntax of function prototype

return_type function_name(type(1) argument(1),....,type(n) argument(n));

In the above example,int add(int a, int b); is a function prototype which provides following information to the compiler:

1.    name of the function is add()
2.    return type of the function is int.
3.    two arguments of type int are passed to function.

Function prototype are not needed if user-definition function is written before main() function.

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Funtions

Funtions

What is functions?

C Programming Functions
Function in programming is a segment that groups a number of program statements to perform specific task.
A C program has at least one function main( ). Without main() function, there is technically no C program.
Types of C functions

Basically, there are two types of functions in C on basis of whether it is defined by user or not.

·    Library function
·    User defined function

Library function

Library functions are the in-built function in C programming system. For example:
main()
- The execution of every C program starts from this main() function.
printf()
- prinf() is used for displaying output in C.
scanf()
- scanf() is used for taking input in C.
Visit this page to learn more about library funin C programming language.

User defined function

C provides programmer to define their own function according to their requirement known as user defined functions. Suppose, a programmer wants to find factorial of a number and check whether it is prime or not in same program. Then, he/she can create two separate user-defined functions in that program: one for finding factorial and other for checking whether it is prime or not.

How user-defined function works in C Programming?

#include <stdio.h>
void function_name(){
................
................
}
int main(){
...........
...........
function_name();
...........
...........
}
As mentioned earlier, every C program begins from main() and program starts executing the codes inside main() function. When the control of program reaches to function_name() inside main() function. The control of program jumps to void function_name() and executes the codes inside it. When, all the codes inside that user-defined function are executed, control of the program jumps to the statement just after function_name() from where it is called. Analyze the figure below for understanding the concept of function in C programming. Visit this page to learn in detail about user-defined functions.
 

Remember, the function name is an identifier and should be unique.

Advantages of user defined functions

1.    User defined functions helps to decompose the large program into small segments which makes programmar easy to understand, maintain and debug.
2.    If repeated code occurs in a program. Function can be used to include those codes and execute when needed by calling that function.
3.    Programmar working on large project can divide the workload by making different functions.
4.    This chapter is the continuation to the function Introduction chapter.
5.    Example of user-defined function
6.    Write a C program to add two integers. Make a function add to add integers and display sum in main() function.
7.    /*Program to demonstrate the working of user defined function*/
8.    #include <stdio.h>
9.    int add(int a, int b);           //function prototype(declaration)
10.    int main(){
11.         int num1,num2,sum;
12.         printf("Enters two number to add\n");
13.         scanf("%d %d",&num1,&num2);
14.         sum=add(num1,num2);         //function call
15.         printf("sum=%d",sum);
16.         return 0;
17.    }
18.    int add(int a,int b)            //function declarator
19.    {            
20.    /* Start of function definition. */
21.         int add;
22.         add=a+b;
23.         return add;                  //return statement of function
24.    /* End of function definition. */  
25.    }                                 

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