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  1. #include <stdlib.h> //for atoi()
  2. #include <stdio.h> //for printf()
  3. #include <unistd.h> //for read()
  4. #include <math.h> //for sqrt()
  5.  
  6.  int isPrime(int num) {
  7.     if(num <= 1)
  8.         return 0;
  9.     if (num <= 3)
  10.         return 1; 
  11.     int range = sqrt(num);
  12.     if (num % 2 == 0 || num % 3 == 0) 
  13.         return 0; 
  14.     for (int i = 5; i <= range; i += 6) 
  15.         if (num % i == 0 || num % (i + 2) == 0) 
  16.             return 0; 
  17.     return 1;
  18. }
  19.  
  20. int main(void) {
  21.   char buffer[255] = {0};
  22.   int read_count = read(STDIN_FILENO, buffer, 255);
  23.   int number;
  24.   if (read_count == 0) {
  25.     printf("Input empty. Assume the starting number is 1.\n");
  26.     number = 1;
  27.   }
  28.   else {
  29.     number = atoi(buffer);
  30.     printf("Starting number is %i.\n", number);
  31.   }
  32.   int distance = 0;
  33.   while(1) {
  34.     number++;
  35.     if (isPrime(number)) {
  36.       distance++;
  37.       if (number % 10 == 1) break;
  38.     }
  39.   }
  40.   printf("Next prime ending in 1 is %i\n", number);
  41.   printf("Distance is %i primes\n", distance);
  42.   return 0;
  43. }
  44.  
Success #stdin #stdout 0s 5428KB
comments (?)
stdin 
7901
stdout 
Starting number is 7901.
Next prime ending in 1 is 7951
Distance is 7 primes
https://ideone.com/lbntUe
Find next prime
language:
C (gcc 8.3)
created:
2 years ago
visibility:
public

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