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In this tutorial, you will learn to write C Recursion with the help of an example.

A function that calls itself is known as a recursive function. And, this technique is known as recursion.

```
void recursion() {
recursion(); /* function calls itself */
}
int main() {
recursion();
}
```

The C programming language supports recursion, i.e., a function to call itself. But while using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go into an infinite loop.

Recursive functions are very useful to solve many mathematical problems, such as calculating the factorial of a number, generating Fibonacci series, etc.

## Number Factorial

The following example calculates the factorial of a given number using a recursive function −

```
#include <stdio.h>
unsigned long long int factorial(unsigned int i) {
if(i <= 1) {
return 1;
}
return i * factorial(i - 1);
}
int main() {
int i = 12;
printf("Factorial of %d is %d\n", i, factorial(i));
return 0;
}
```

When the above code is compiled and executed, it produces the following result −

Factorial of 12 is 479001600

## Fibonacci Series

The following example generates the Fibonacci series for a given number using a recursive function −

```
#include <stdio.h>
int fibonacci(int i) {
if(i == 0) {
return 0;
}
if(i == 1) {
return 1;
}
return fibonacci(i-1) + fibonacci(i-2);
}
int main() {
int i;
for (i = 0; i < 10; i++) {
printf("%d\t\n", fibonacci(i));
}
return 0;
}
```

When the above code is compiled and executed, it produces the following result −

0 1 1 2 3 5 8 13 21 34

### Advantages and Disadvantages of Recursion

Recursion makes the program elegant. However, if performance is vital, use loops instead as recursion is usually much slower.

Recursion is an important concept. It is frequently used in data structure and algorithms. For example, it is common to use recursion in problems such as tree traversal.