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

Last update on May 27 2022 12:24:19 (UTC/GMT +8 hours)

C Programming Practice: 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 programming to find the sum of all the primes 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:

Flowchart:

C Programming Code Editor:

Contribute your code and comments through Disqus.

Previous: Write a C programming to find the product xyz.
Next: Write a C programming to find the greatest product of four adjacent numbers in the same direction (up, down, left, right, or diagonally) in the 20×20 grid.

What is the difficulty level of this exercise?

Test your Programming skills with w3resource's quiz.

C Programming: Tips of the Day

Static variable inside of a function in C

The scope of variable is where the variable name can be seen. Here, x is visible only inside function foo().

The lifetime of a variable is the period over which it exists. If x were defined without the keyword static, the lifetime would be from the entry into foo() to the return from foo(); so it would be re-initialized to 5 on every call.

The keyword static acts to extend the lifetime of a variable to the lifetime of the programme; e.g. initialization occurs once and once only and then the variable retains its value - whatever it has come to be - over all future calls to foo().

Ref : https://bit.ly/3fOq7XP