C Exercises: Find the sum of all the primes below ten thousand

C Programming Challenges: Exercise-26 with Solution

A prime number is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself. However, 6 is composite because it is the product of two numbers (2 × 3) that are both smaller than 6.
The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.
Write a C program to find the sum of all prime numbers below ten thousand.

C Code:

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
  char *temp;
  unsigned i, j;
  size_t num = 10000;
  unsigned long long sum = 0ULL;

  temp = calloc(num, sizeof *temp);
  for (i = 2; i < num; i++) {
    if (!temp[i]) {
      sum += i;
      for (j = i*2; j < num; j += i) {
        temp[j] = 1;
      }
    }
  }
  free(temp);
  printf("%llu\n", sum);
  return 0;
}

Sample Output:

5736396

Flowchart:

C Programming Code Editor:

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