fork(1) download 
  1. #include <stdio.h>
  2. 	#include <stdlib.h>
  3. 	#include <stdbool.h>
  4. 	#include <limits.h>
  5. 	#include <math.h>
  6.  
  7. 	bool is_prime(unsigned long i) {
  8. 		if (i == 2)
  9. 			return true;
  10. 		if (i % 2 == 0)
  11. 			return false;
  12. 		unsigned int j;
  13. 		double root = sqrtl(i);
  14. 		for (j = 3; j < root; j += 2)
  15. 			if (i % j == 0)
  16. 				return false;
  17. 		return true;
  18. 	}
  19.  
  20. 	int main(void) {
  21.  
  22. 		char buffer[1025];
  23. 		while (fgets(buffer, 1025, stdin) != NULL) {
  24. 			unsigned long num;
  25. 			if (sscanf(buffer, "%lu", &num) != 1) {
  26. 				fprintf(stderr, "input a number!\n");
  27. 				continue;
  28. 			}
  29.  
  30. 			if (is_prime(num) == true) {
  31. 				printf("%lu is prime.\n", num);
  32. 			} else {
  33. 				unsigned long i;
  34. 				for (i = num - 1; i >= 2; i--) {
  35. 					if (is_prime(i) == true) {
  36. 						printf("%lu < %lu < ", i, num);
  37. 						break;
  38. 					}
  39. 				}
  40. 				for (i = num + 1; i <= ULONG_MAX; i++) {
  41. 					if (is_prime(i) == true) {
  42. 						printf("%lu\n", i);
  43. 						break;
  44. 					}
  45. 				}
  46. 			}
  47. 		}
  48.  
  49. 		return EXIT_SUCCESS;
  50. 	}
Success #stdin #stdout 0s 10320KB
comments ()
stdin 
270
541
993
649
2010741
stdout 
269 < 270 < 271
541 is prime.
991 < 993 < 997
647 < 649 < 653
2010733 < 2010741 < 2010881
https://ideone.com/zSa6L9
language:
C (gcc 8.3)
created:
7 years ago
visibility:
public

Share or Embed source code

Discover > Sphere Engine API

The brand new service which powers Ideone!

Discover > IDE Widget

Widget for compiling and running the source code in a web browser!