A simple program in C++ that finds the next prime number above a given number.

findNextPrime

#include <iostream>

int findNextPrime(int n);   //given a number n, find the next closest prime number above n
bool isPrime(int n);        //given a number n, determine if it is prime

int main()
{
    int input;

    std::cout << "Please enter a prime number: ";
    std::cin >> input;

    std::cout << "\nThe next prime number after " << input << " is " << findNextPrime(input) << ". \n";
}

//given a number n, find the next closest prime number above n
int findNextPrime(int n)
{
    int nextPrime = n;
    bool found = false;

    //loop continuously until isPrime returns true for a number above n
    while (!found)
    {
        nextPrime++;
        if (isPrime(nextPrime))
            found = true;
    }

    return nextPrime;
}

//given a number n, determine if it is prime
bool isPrime(int n)
{
    //loop from 2 to n/2 to check for factors
    for (int i = 2; i <= n/2; i++)
    {
        if (n % i == 0)     //found a factor that isn't 1 or n, therefore not prime
            return false;
    }

    return true;
}

/*
SAMPLE OUTPUT
Please enter a prime number: 6
The next prime number after 6 is 7.
Please enter a prime number: 10
The next prime number after 10 is 11.
Please enter a prime number: 11
The next prime number after 11 is 13.
Please enter a prime number: 788
The next prime number after 788 is 797.
*/

I think there is just one problem with isPrime(): if n == 1; isPrime(1) will return true, but 1 is not a prime number.

Please correct me if I am wrong. Thanks for a great prime check function!

Also, nextprime would be nearly twice as fast if you test only odd numbers (instead of ++ use +=2 once the number is >=3). Also in isPrime: check if even (n & 1)==0, then (if odd) check only odd divisors. Also, you can stop at sqrt(n), for large n this makes an important difference.

the for loop in isPrime(): for (int i = 2; i * i <= n; ++i) is enough

as I said 1 yr ago...
FWIW, I made these improvements (only odd trial divs <= sqrt(n), also use of cmd line argv[1] if given) in a fork, see https://gist.github.com/m-f-h/8c81e4014ff9b44abb3440a0f7eb2aa9
(I'm new to Gist, don't know whether/how it's possible to propose to merge with this...)

for loop in isPrime():
To save time and complexity you can use this
for (int i = 2; i * i <= n; ++i) is enough

I don't know why you say the same thing than @majorli on May 2020.
What is worse, it seems my improvement from May 2020:
check parity and then " for (int i = 3; i * i <= n; i+=2) ",
(please see here: https://gist.github.com/m-f-h/8c81e4014ff9b44abb3440a0f7eb2aa9 )
which makes it twice as fast, apparently didn't make it's way in :-( .
Do I have to do some "commit" / merge request? if so, how?
(I don't see any button or link to do that on the page where I see my improved version, i.e., the above link.)
Thanks and hope that helps!