How to find the nearest prime number?

Question

The goal of this method is to find the nearest prime number less than the value passed. Any value less than 3 is undefined behavior so I just return -1. The issue lies when I pass in values 5, 20, 100, and 1, this is the output:

Passed in 5 Expected 3: 5
Passed in 20 Expected 19: 19
Passed in 100 Expected 97: 97
Passed in 1 Expected -1: -1

Here is the method:

int Hash::nearestPrime(int num){
        if(num <= 3){
                return -1;
        }
        for(int i = 2; i < num; i++){
                if(num == 3){
                        return 3;
                }
                if(num % i == 0 && num != i){
                        num--;
                        i = 2;
                }
        }
        return num;
}

Answer 1

You could loop and decrement until the next one is found, by having a function determine if a value is prime.

#include <iostream>

int prime (int n)
{
  int i;
  for (i = 2; i < n; i++)
    if (n % i == 0) return 0;
  return 1;
}

int run (unsigned int n)
{
  while (!prime (--n));
  return n;
}

int main ()
{
  std::cout << run (3);
  return 0;
}

Answer 2

Your method is right but with simple mistake.

Assign i = 1 instead of i = 2 inside the if condition. Because if you assign 2 then for i++ next checking value will start from 3 . But you need to check it from 2 .

int Hash::nearestPrime(int num){
        if(num <= 3){
                return -1;
        }
        for(int i = 2; i < num; i++){
                if(num == 3){
                        return 3;
                }
                if(num % i == 0 && num != i){
                        num--;
                        // mistake in this line
                        i = 1;
                }
        }
        return num;
}