Question

How to check if remove digits from left to right from a prime number remain prime at each stage in C

Answer 1

#include <stdio.h>
#include <stdbool.h>
#include <math.h>
  
bool isPrime(int n) {
    if (n == 0) return false;
    if (n == 1) return false;
  
    for (int i = 2; i <= sqrt(n); i++) {
        if (n % i == 0) {
            return false;
        }
    }
    
    return true;
}

int truncate_left(int num) {
    int total = (int)log10(num); // total - 1 = 3
     
    int first_digit = num / pow(10, (total));
     
    return num - first_digit * pow(10, (total));
}

bool remain_prime(int num) {
    if (!isPrime(num)) {
        return false;
    }
    
    printf("%d\n", num);
    
    while (num > 9) {
        num = truncate_left(num);
        if (!isPrime(num)) {
            return false;
        }
        printf("%d\n", num);
    }
    
    return true;
}
  
int main(void) {
    int num = 3797;
          
    puts(remain_prime(num) ? "yes" : "no");
    
    return 0; 
}
  
  
  
  
/*
run:
      
3797
797
97
7
yes

*/

 

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