In this article, i have explained about what is fibonacci in C and how to create program for fibonacci series in C, with and without recursion. Let's understand about it and create it's program in C.

**Fibonacci series** is a series of natural numbers where next number is equivalent to the sum of previous two numbers i.e. fn = fn-1 + fn-2.In fibonacci sequence each item is the sum of the previous two. So, you wrote a recursive algorithm, for example, recursive function example for up to 5

```
fibonacci(5) = fibonacci(4) + fibonacci(3)
fibonacci(3) = fibonacci(2) + fibonacci(1)
fibonacci(4) = fibonacci(3) + fibonacci(2)
fibonacci(2) = fibonacci(1) + fibonacci(0)
```

The first two numbers in the Fibonacci sequence are either 1 and 1, or 0 and 1, and each subsequent number is the sum of the previous two numbers.

First Term = 0

Second term = 1

Third Term = First + Second = 0+1 =1

Fourth term = Second + Third =1+1 = 2

Fifth Term = Third + Fourth = 2+1 = 3 and so on...

So for example fibonacci series upto 10 numbers will look like

`1 1 2 3 5 8 13 21 34 55`

Let's take a look at it's program.

### Program for Fibonacci in C

In the below code, we are using `while`

loop to add the last 2 numbers and print the result until 'nth' number.

```
#include <stdio.h>
int main()
{
int i, n, firstTerm=0, secondTerm=1, sum=0;
printf("\nEnter number of terms required in Fibonacci Series: ");
scanf("%d",&n);
// Showing the first two term of the Fibonacci Series
printf("\nFibonacci Series is:\n\n\n %d %d ", firstTerm, secondTerm);
//start i =2
i=2;
//i starts from 2, as the first two terms of the series have already been shown
while (i<n)
{
sum=firstTerm+secondTerm;
firstTerm=secondTerm;
secondTerm=sum;
++i;
printf("%d ",sum);
}
return 0;
}
```

Output:

```
Enter number of terms required in Fibonacci Series: 8
Fibonacci Series is:
0 1 1 2 3 5 8 13
```

### Fibonacci program in C using recursion

In the Recursive way to print the Fibonacci series, we will keep calling same function 'Fibonacci' until nth number and print the sum of last 2 numbers.

```
#include<stdio.h>
int Fibonacci(int);
int main()
{
int n, i = 0, c;
scanf("%d",&n);
printf("Fibonacci series\n");
for ( c = 1 ; c <= n ; c++ )
{
//function is called recursively using for loop
printf("%d\n", Fibonacci(i));
i++;
}
return 0;
}
//fibonacci series function
int Fibonacci(int n)
{
if ( n == 0 )
return 0;
else if ( n == 1 )
return 1;
else
return ( Fibonacci(n-1) + Fibonacci(n-2) );
}
```

Output:

```
Enter number of terms required in Fibonacci Series: 8
Fibonacci series
0 1 1 2 3 5 8 13
```

Recursion method is less efficient as it involves function calls which uses stack, also there are chances of stack overflow if function is called frequently for calculating larger Fibonacci numbers.

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