Function recursion in C programming

Calling of same function from its function body is known as function recursion. It is alternative of loop. Any c program which is possible using loop it must be possible using function recursion. Simple example:

Find the sum of all even numbers from 0 to 20 using function recursion. Program:

int main(){

int total;

total=sum(2);

printf("%d",total);

return 0;

}

int sum(int i){

static int even=0;

if(i<=20){

even=even+i;

sum(i+2); //calling same function

}

return even;

}

Output:

It is very difficult to understand the execution as well as to write the function recursion program.

How to write function recursion program in easier way:

I have already told it is very difficult to write function recursion program directly. If any person is writing such program directly he may be memorized that program. I am telling a very nice trick: how to write function recursion program in easier way? As we know any c program which possible using loop it must be possible using function recursion.

Steps for how to write a function recursion program:

Step1: First of all write same program using while loop and function. (Except main function)

Step 2: In that function make all local variable static.

Step 3: Replace while keyword by if.

Step 4: The increment or decrement of variable which is used for condition checking, replace with function call and pass the parameter of that incremented or decremented variable. Now understand by example:

Find the sum of all even numbers from 0 to 20 using function recursion.

Step 1: Write the same program using while loop and function. Here function is sum.

int main(){

int total;

total=sum(2);

printf("%d",total);

return 0;

}

int sum(int i){

int even=0;

while(i<=20){

even=even+i;

i=i+2;

}

return even;

}

Step 2: Make local variable even as static variable

int sum(int i){

static int even=0;

while(i<=20){

even=even+i;

i=i+2;

}

return even;

}

Step 3: Replace while keyword by if keyword.

int sum(int i){

int even=0;

if(i<=20){

even=even+i;

i=i+2;

}

return even;

}

Step 4: Since here variable i has used in condition checking. So replace the statement i=i+2 by sum (i+2).

int sum(int i){

int even=0;

if(i<=20){

even=even+i;

sum(i+2);

}

return even;

}

Following only three simple change you can write any difficult function recursion program.

int main(){

int total;

total=sum(2);

printf("%d",total);

return 0;

}

int sum(int i){

int even=0;

if(i<=20){

even=even+i;

sum(i+2);

}

return even;

}

One more example:

Write a program to find a factorial of given number using function recursion.

Step 1: write same program using while loop and function.

int main(){

int fact,num;

scanf("%d",&num);

fact=factorial(num);

printf("%d",fact);

return 0;

}

int factorial(int num){

int fact=1; //make it static

while(num>0){ //replace while by if

fact=fact*num;

num--; // replace by function call as factorial(num-1);

}

return fact;

}

After following step1, step 2, step 3:

int main(){

int fact,num;

scanf("%d",&num);

fact=factorial(num);

printf("%d",fact);

return 0;

}

int factorial(int num){

static int fact=1;

if(num>0){

fact=fact*num;

factorial (num-1);

}

return fact;

}

Note1: In step 3 while calling function don’t pass the parameter using unary operator like factorial (num--);

Note2: write the function in such a way parameter of calling function must be used in conditional statement. For example in the above program:

int factorial(int num)

num is function parameter and num is also used in the conditional if statement.