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C Exercises: Get the 1001st prime number

C Programming Practice: Exercise-23 with Solution

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 programming 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:

7927

Flowchart:

C Programming Flowchart: Get the 1001st prime number.

C Programming Code Editor:

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Previous: Write a C programming to find the difference between the sum of the squares of the first one hundred natural numbers and the square of the sum.
Next: Write a C programming to find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

What is the difficulty level of this exercise?

C Programming: Tips of the Day

C Programming - Why do all the C files written by my lecturer start with a single # on the first line?

In the very early days of pre-standardised C, if you wanted to invoke the preprocessor, then you had to write a # as the first thing in the first line of a source file. Writing only a # at the top of the file affords flexibility in the placement of the other preprocessor directives.

From an original C draft by the great Dennis Ritchie himself:

12. Compiler control lines

[...] In order to cause [the] preprocessor to be invoked, it is necessary that the very first line of the program begin with #. Since null lines are ignored by the preprocessor, this line need contain no other information.

That document makes for great reading (and allowed me to jump on this question like a mad cat).

I suspect it's the lecturer simply being sentimental - it hasn't been required certainly since ANSI C.

Ref : https://bit.ly/2Mb8OVZ