# Fibonacci series in C

We have learned about the Fibonacci series in mathematics. For a quick recap, **A Fibonacci series is the sequence of numbers following a certain pattern i.e., the next number should be the sum of the before two numbers.**

## Clear understanding on Fibonacci series:

The first two numbers are 0 and 1. Adding the previous two elements, it becomes:

0 |

1 |

1 (0+1) |

2 (1+1) |

3 (1+2) |

5 (2+3) |

8 (3+5) |

13 (5+8) |

21 (8+13) |

34 (13+21) |

55 (21+34) |

To execute a program to print the Fibonacci series in C, we have 2 prominent programs:

- Using Recursion
- Non-recursive

## Non-Recursive:

## Upto the given number of terms:

```
#include<stdio.h>
void main()
{
int num1 = 0, num2 = 1, sum, i, n;
printf (“Enter the number of elements you need: “);
scanf (“%d”,&n);
printf (“\n%d %d”,num1,num2);
for (i=2;i<n;i++)
{
sum = num1+num2;
printf (“ %d”,sum);
num1 = num2;
num2 = sum;
}
}
```

**Output:**

```
Enter the number of elements you need: 4
0 1 1 2
```

## Mechanism:

We initialized the first two numbers num1 and num2, to 0 and 1 and printed the 2 numbers. Now, inside the loop, the sum of the first two numbers is calculated and is printed as it’s the rule, and then num1 and num2 are updated with num2 and sum, and the loop goes on till it reaches n numbers, printing the sequence.

## Fibonacci series upto the given number:

```
#include<stdio.h>
void main()
{
int num1 = 0, num2 = 1;
int sum = 0,n;
printf(“Enter the number upto which the series is needed: “);
scanf(“%d”,&n);
printf(“Fibonacci series: %d %d”,num1,num2);
sum = num1 + num2;
while(sum<=n)
{
printf(“%d, “, sum);
num1 = num2;
num2 = sum;
sum = num1 + num2;
}
}
```

**Output:**

```
Enter the number upto which the series is needed: 100
Fibonacci series: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89,
```

## Mechanism:

First, we tool the input of upto which number the series need to be printed. We print the first two elements 0 and 1 and then we calculate the sum as 0 + 1. Now, we open a while loop with condition, sum should be less than n because, when sum becomes n, it reaches n, it needs to stop at that point. Inside the loop, we print the sum and keep on updating the values of num1 and num2 as num2 and sum.

## Using Recursion:

## Upto the given number of terms:

```
#include<stdio.h>
void Fib(int n)
{
static int num1=0,num2=1,sum;
if(n>0)
{
sum = num1 + num2;
num1 = num2;
num2 = sum;
printf ("%d ",sum);
Fib (n-1);
}
}
void main()
{
int n;
printf("Enter the number of elements: ");
scanf ("%d",&n);
printf ("Fibonacci Series: ");
printf ("%d %d ",0,1);
Fib (n-2);
}
```

**Output:**

```
Enter the number of elements: 4
Fibonacci Series: 0 1 1 2
```

## Mechanism:

We use a function Fib () taking the parameter n->number of elements. We take the number of elements as the input, print the first two numbers as 0 and 1. Two numbers are printed already; we need more n-2 elements, so we give the n-2 parameters to the function. In the function body, we calculate the sum, print the sum, then update the values of num1, num2 as num2, sum. Now, we call the function will n-1. Again the whole process continues till the value of n becomes 0. When n becomes 0, there will be no executable part in the function hence terminating the execution.