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 <iostream>
#include <cmath>

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;
    }
     
    std::cout << num << "\n";
     
    while (num > 9) {
        num = truncate_left(num);
        if (!isPrime(num)) {
            return false;
        }
        std::cout << num << "\n";
    }
     
    return true;
}
   
int main(void) {
    int num = 3797;
           
    std::cout << (remain_prime(num) ? "yes" : "no");
}
   
   
   
   
/*
run:
       
3797
797
97
7
yes
 
*/
   

 

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