# 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.

**Pictorial Presentation:**

**Sample Solution**:

**C++ Code :**

```
#include <iostream>
using namespace std;
int main()
{
const int MAX = 1000000;
const int sqrtMAX = 1000;
int n;
int b[MAX+1] = {0};
int i, j;
int sum;
int count;
b[0] = 1;
b[1] = 1;
cin>> n;
for(i=4; i<=MAX; i+=2)
b[i] = 1;
for(i=3; i<=sqrtMAX; i+=2)
for(j=i+i; j<=MAX; j+=i)
b[j] = 1;
if(n == 0)
return 0;
sum = 0;
count = 0;
for(i=2; count<n; i++) {
if(b[i]==0) {
count++;
sum+=i;
}
}
cout << "Sum of the first " << n << " Prime numbers is: " << sum;
return 0;
}
```

Sample Output:

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

**Flowchart: **

**C++ Code Editor:**

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**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?**

## C++ Programming: Tips of the Day

**Why is there no std::stou? **

The most pat answer would be that the C library has no corresponding "strtou", and the C++11 string functions are all just thinly veiled wrappers around the C library functions: The std::sto* functions mirror strto*, and the std::to_string functions use sprintf.

Ref: https://bit.ly/3wtz2qA

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