C Exercises: Get the 1001st prime number

Last update on March 21 2025 12:37:12 (UTC/GMT +8 hours)

23. 1001st Prime Number Variants

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
The first six prime numbers are 2, 3, 5, 7, 11, and 13.
Write a C program to get the 1001st prime number?

C Code:

#include <stdio.h>
int main(void)
{
  char *s;
  size_t i;
  unsigned ctr = 0;
  size_t n = 1000000;
  const unsigned target_val = 1001;
  s = calloc(n, sizeof *s);
  for (i = 2; i < n; i++) {
    if (!s[i]) {
      size_t j;
      ctr++;
      if (ctr == target_val) {
        printf("%lu\n", i);
        break;
      }
      for (j = i*2; j < n; j += i) {
        s[j] = 1;
      }
    }
  }
  free(s);
  return 0;
}

Sample Output:

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to generate prime numbers until the 1001st prime is reached using the Sieve of Eratosthenes.
Write a C program to compute the 1001st prime using an optimized trial division method.
Write a C program to iteratively generate primes until reaching the nth prime, with n as user input.
Write a C program to output the 1001st prime and also list the last 10 primes generated before it.

C Programming Code Editor:

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