## How to find the sum of first n numbers of the Fibonacci series in C#:

In this post, we will learn how to find the *sum* of first *n* numbers of the *Fibonacci* series in *C#*. The program will take the value of *n* as the input and print the sum.

The *Fibonacci* series starts with *0* and *1* and each value in the series is equal to the sum of *previous two numbers* of the series. The first *10* numbers of the series are *0, 1, 1, 2, 3, 5, 8, 13, 21, 34*.

### Method 1: C# program to find the sum of first n numbers of the Fibonacci series by using a for loop:

In this method, we will use a *for loop* to find the sum of the first *n* numbers of the *Fibonacci* series.

```
using System;
namespace Program
{
class Program
{
static void Main(string[] args)
{
int n;
int CurrentValue = 1, PrevValue = 0, Sum = 1, CurrentSum;
Console.WriteLine("Enter the value of n: ");
n = Convert.ToInt32(Console.ReadLine());
if(n == 0) Sum = 0;
for (int i = 2; i < n; i++)
{
CurrentSum = PrevValue + CurrentValue;
PrevValue = CurrentValue;
CurrentValue = CurrentSum;
Sum += CurrentValue;
}
Console.WriteLine("Sum of first {0} numbers of the Fibonacci series is: {1}", n, Sum);
}
}
}
```

In this example,

- The
*for loop*is used to print the sum. *PrevValue*and*CurrentValue*are integer variables to hold the previous and current values of the series. These are assigned as*0*and*1*at the start of the program.- Inside the loop, it finds the final
*Sum*. On each iteration, it calculates the sum of the previous two values of the series to find the current value. This value is added to*Sum*.

If you run this program, it will print output as like below:

```
Enter the value of n:
6
Sum of first 6 numbers of the Fibonacci series is: 12
```

### Method 2: By using a separate method:

Let’s use a separate method to find the sum of first *n* numbers of the *Fibonacci series*:

```
using System;
namespace Program
{
class Program
{
static int FindSumNFibonacci(int n)
{
int CurrentValue = 1, PrevValue = 0, Sum = 1, CurrentSum;
if (n == 0) Sum = 0;
for (int i = 2; i < n; i++)
{
CurrentSum = PrevValue + CurrentValue;
PrevValue = CurrentValue;
CurrentValue = CurrentSum;
Sum += CurrentValue;
}
return Sum;
}
static void Main(string[] args)
{
int n;
Console.WriteLine("Enter the value of n: ");
n = Convert.ToInt32(Console.ReadLine());
Console.WriteLine("Sum of first {0} numbers of the Fibonacci series is: {1}", n, FindSumNFibonacci(n));
}
}
}
```

*FindSumNFibonacci*is a different method to find the sum of first*n*numbers of the Fibonacci series. It takes the value of*n*as its parameter and returns the sum.- This is similar to the above program. It uses the
*for loop*to calculate the sum. The variable*Sum*holds the sum. - At the end of the program, it returns the sum, i.e. the value of
*Sum*.

It will give similar result.

```
Enter the value of n:
7
Sum of first 7 numbers of the Fibonacci series is: 20
```

### Method 3: By using a while loop:

Let’s replace the *for loop* with a *while loop*:

```
using System;
namespace Program
{
class Program
{
static int FindSumNFibonacci(int n)
{
int CurrentValue = 1, PrevValue = 0, Sum = 1, CurrentSum, i = 2;
if (n == 0) Sum = 0;
while (i < n)
{
CurrentSum = PrevValue + CurrentValue;
PrevValue = CurrentValue;
CurrentValue = CurrentSum;
Sum += CurrentValue;
i++;
}
return Sum;
}
static void Main(string[] args)
{
int n;
Console.WriteLine("Enter the value of n: ");
n = Convert.ToInt32(Console.ReadLine());
Console.WriteLine("Sum of first {0} numbers of the Fibonacci series is: {1}", n, FindSumNFibonacci(n));
}
}
}
```

The only difference is that the value of *i* is initialized at the start of the method and it’s value is incremented at the end of each iteration of the loop. You will get similar result.

### Method 4: Recursive method to find the sum of first n numbers of the Fibonacci series:

A recursive method calls itself again and again to find a result. We can use a *recursive method* to find the sum of first *n* numbers of a *Fibonacci series*. Let’s write down the program:

```
using System;
namespace Program
{
class Program
{
static int GetSumNFibonacciSeries(int First, int Second, int n, int Sum)
{
if (n == 0) return Sum;
return GetSumNFibonacciSeries(Second, First + Second, n - 1, Sum + First);
}
static void Main(string[] args)
{
int n;
Console.WriteLine("Enter the value of n: ");
n = Convert.ToInt32(Console.ReadLine());
Console.WriteLine("Sum of first {0} numbers of the Fibonacci series is: {1}", n, GetSumNFibonacciSeries(0, 1, n, 0));
}
}
}
```

*GetSumNFibonacciSeries*method is used to find the sum of first*n*numbers of the Fibonacci series.- It takes
*4*parameters.*First*is the first value of the series,*Second*is the second value of the series in the beginning. These values are updated on each recursive call.*n*is the value of*n*entered by the user.*Sum*is to hold the final sum. It is*0*in the beginning. - On each iteration, it calls itself recursively with updated
*First*,*Second*,*n*and*Sum*values. - The value of
*n*is decremented by*1*on each recursive call. It returns the value of*Sum*once the value of*n*become*0*.

It will give similar result.

```
Enter the value of n:
8
Sum of first 8 numbers of the Fibonacci series is: 33
Enter the value of n:
9
Sum of first 9 numbers of the Fibonacci series is: 54
```

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