# C program to find the sum of first n odd numbers starting from 1

### C program to find the sum of n odd numbers starting from 1:

In this tutorial, we will learn how to find the total sum of first ’n’ odd numbers using a *C program*. For example , the sum of first 3 odd numbers is *1 + 3 + 5 = 9*. The user will enter the value of *n* and our program will find out the sum and print out the answer.

We will discuss two different ways to find out the sum.

### 1. Using a for or while loop :

We can find out the sum by using one *for* or *while* loop. The loop will run from *1* to *n* and for each number it will check if it is divisible by *2* or not. If not, means it is an odd number, so add it to a final sum variable. Finally, print out the result. The program will explain below each step how it works.

### 2. A better approach : By finding the sum of all n numbers with a difference between each numbers mathematically :

The mathematical formulae to find the sum of first *n* numbers starting from *a* with a difference *d* between each is :

Sn = (n/2) [2a + (n-1)*d]

For example, *sum of first 5 numbers starting from 3 * with a difference between each *2* is :

//here n = 5, a = 3 , d = 2 (5/2)[2*3 + (5-1)*2] = (5/2)[6 + 8] = (5/2) * 14 = 35

Manually , we can verify it like :

3 + 5 + 7 + 9 + 11 = 15 + 20 = 35

Our problem is to find the sum of first *n* odd numbers starting from 1. In this case , *a = 1*, *d = 2* ( since the difference between two odd numbers is 2). So, if we put these values in the above formulae, it will become :

(n/2)[2*a + (n-1)*d] = (n/2)[2*1 + (n-1)*2] = (n/2)[2 + 2n - 2] = (n/2) * 2n = n*n = n^2

That means the sum of first *n* odd number is square of *n*.

### C program explains above solutions :

In the C program below, we will explain you how to solve the problem using above two approaches. Let’a take a look :

#include //4 int findUsingWhile(int n){ //5 int i = 1; int sum = 0; int totalOdd = 0; //6 while(totalOdd != n){ if(i%2 != 0){ sum += i; totalOdd++; } i++; } //7 return sum; } //8 int findUsingFor(int n){ //9 int i; int sum = 0; int totalOdd = 0; //10 for(i=1; totalOdd != n;i++){ if(i%2 != 0){ sum += i; totalOdd ++; } } //11 return sum; } //12 int usingMathematics(int n){ return n*n; } int main(){ //1 int n; //2 printf("Enter the value of 'n' : "); scanf("%d",&n); //3 printf("Total sum using while loop : %d\n",findUsingWhile(n)); printf("Total sum using for loop : %d\n",findUsingFor(n)); printf("Total sum mathematically : %d\n",usingMathematics(n)); }

### Explanation :

*The commented numbers in the above program denote the step number below :*

1. Create one integer variable n to store the value of user input number.

2. Ask the user to enter a number. Read and store it in *n* .

3. Calculate the sum of first *n* odd numbers by using one *for loop*, using one *while loop* and by using the mathematical function shown above. For each case, print out the result. All these results should be same.

4. *findUsingWhile* function takes the value of *n* as parameter and it calculates the sum using one *while* loop.

5. Create one variable *i* to use in the loop, *totalOdd* to total odd numbers found and *sum* to store the sum of all odd numbers.

6. Run one *while loop*. It will run continuously till total number of odd numbers becomes equal to the input *n*. Inside the loop, check if current value of *i* is odd or not. If odd, add it to the sum and increment the value of *totalOdd*. For each iteration, increment the value of *i* means we are checking it for each numbers continuously.

7. After the loop is completed, return the value of *sum*.

8. *findUsingFor* is similar to the above function. It takes the value of *n* as input and calculates the sum using a *for loop*.

9. Create three variables similarly as explained in the above function.

10. Similar to *while* function above, this function runs one *for loop* to find out the sum. It will run from *i = 1* . Each time , value of *i* is increased by one. If the value of *totalOdd* becomes equal to *n* , this loop will end. Inside the loop, check if *i* is odd or even. If odd, add its value to the final sum and increment the value of *totalOdd*.

11. Finally, after the loop is completed, return the value of *sum*.

12. This is the third approach. Simply return the value of *n*n*. It should be the required sum as explained above.

### Sample Output :

Enter the value of 'n' : 9 Total sum using while loop : 81 Total sum using for loop : 81 Total sum mathematically : 81 Enter the value of 'n' : 1 Total sum using while loop : 1 Total sum using for loop : 1 Total sum mathematically : 1 Enter the value of 'n' : 3 Total sum using while loop : 9 Total sum using for loop : 9 Total sum mathematically : 9 Enter the value of 'n' : 11 Total sum using while loop : 121 Total sum using for loop : 121 Total sum mathematically : 121