# C++ Exercises: Compute the sum of the specified number of Prime numbers

## C++ Basic: Exercise-75 with Solution

Write a C++ program to compute the sum of the specified number of prime numbers.

For example when n = 7,

s = 2 + 3 + 5 + 7 + 11 + 13 + 17 = 58.

**Visual Presentation:**

**Sample Solution**:

**C++ Code :**

```
#include <iostream> // Header file for input/output stream operations
using namespace std;
int main() {
const int MAX = 1000000; // Constant for the maximum value
const int sqrtMAX = 1000; // Constant for square root of the maximum value
int n; // Variable to store the user input for the count of prime numbers
int b[MAX + 1] = {0}; // Array to mark non-prime numbers
int i, j; // Loop control variables
int sum; // Variable to store the sum of prime numbers
int count; // Variable to count the number of prime numbers found
b[0] = 1; // Mark 0 as non-prime
b[1] = 1; // Mark 1 as non-prime
cin >> n; // Input the number of prime numbers to be found
// Mark multiples of 2 as non-prime numbers
for (i = 4; i <= MAX; i += 2)
b[i] = 1;
// Sieve of Eratosthenes: Mark multiples of odd numbers as non-prime
for (i = 3; i <= sqrtMAX; i += 2)
for (j = i + i; j <= MAX; j += i)
b[j] = 1;
// If input is 0, exit the program
if (n == 0)
return 0;
sum = 0; // Initialize sum to 0
count = 0; // Initialize count of prime numbers found to 0
// Loop through the array to find and sum the first 'n' prime numbers
for (i = 2; count < n; i++) {
if (b[i] == 0) { // If the number is not marked as non-prime
count++; // Increment the count of prime numbers found
sum += i; // Add the prime number to the sum
}
}
// Display the sum of the first 'n' prime numbers
cout << "Sum of the first " << n << " Prime numbers is: " << sum;
return 0; // Indicating successful completion of the program
}
```

Sample Output:

Sample Input: 7 Sum of the first 7 Prime numbers is: 58

**Flowchart: **

**C++ Code Editor:**

**Previous:** Write a C++ program that accepts various numbers and compute the difference between the highest number and the lowest number. All input numbers should be real numbers between 0 and 1,000,000. The output (real number) may include an error of 0.01 or less.

**Next:** Write a C++ program that accept an integer (n) from the user and outputs the number of combinations that express n as a sum of two prime numbers.

**What is the difficulty level of this exercise?**

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