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

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:

5736396

Flowchart:

C Programming Flowchart: Find the thirteen adjacent digits in the 1000-digit number that have the greatest product.

C Programming Code Editor:

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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