Find missing number in the sequence in c#- Learn how to find the missing number among the sequence of numbers from 1 to n with example Programs.

For Example following are the numbers from 1 to 6 as 1,2,3,5,6.

The missing number in the above sequence is 4.

**Input**

1,2,3,5,6

**Output**

Missing Number : 4

### How to calculate missing number?

**Solution:**

total numbers=5

sum of 6 numbers from 1 to 6 =6*(6+1)/2;

=21;

sum of the Numbers in the sequence =17.

missing Number=sum of 6 numbers-sum of the Numbers in the sequence

= 21-17 =4

### Method 1:(Using Mathematical formula)

**Algorithm:**

step1: calculate sum of Numbers : n*(n+1)/2 – say as x

step2: calculate sum of numbers in the sequence – say as y

step3: subtract x-y to get the missing Number.

**C# Programming Example**

```
using System;
class Find_Missing_Number
{
static void Main(string[] args)
{
//array to find the missing number between 1 and 10
// Simplicity, We will take number 1 to 10 i where Number 5 is missing in the sequence.
int[] arr = { 1, 2, 3, 4, 6, 7, 8, 9, 10 };
int missingNumber,Totalsum;
// Accoreding to series rule, calculating sum of total numbers upto 10
//sum of first n natutal numbers=n*(n+1)/2
Totalsum = (arr.Length + 1) * (arr.Length + 2) / 2;
// Missing number is calculating.
foreach (int item in arr)
{
Totalsum = Totalsum - item;
}
missingNumber = Totalsum;
Console.WriteLine("missing number : {0}",missingNumber);
}
}
```

**Output**

missing number : 5

### Method 2:

**Algorithm:**

step1: XOR all the array elements, let the result of XOR be X1.

step2: XOR all numbers from 1 to n, let XOR be X2.

step3: XOR of X1 and X2 gives the missing number.

**C# Programming Example**

```
using System;
class Find_Missing_Number
{
static void Main(string[] args)
{
//array to find the missing number between 1 and 10
// Simplicity, We will take number 1 to 10 i where Number 5 is missing in the sequence.
int[] arr = { 1, 2, 3, 4, 6, 7, 8, 9, 10 };
int missingNumber;
int x1or = 0;
int x2or = 0;
// Accoreding to series rule, calculating sum of total numbers upto 10
//sum of first n natutal numbers=n*(n+1)/2
// Missing number is calculating.
//Perform xor on all the elements of the array
foreach (int item in arr)
{
x1or = x1or ^ item;
}
//perforn xor on 1 to n+1
for (int i = 1; i <= arr.Length+1; i++)
{
x2or = x2or ^ i;
}
missingNumber=(x1or^x2or);
Console.WriteLine("missing number : {0}", missingNumber);
}
}
```

**Output**

missing number : 5