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

findNextPrime

// findNextPrime.cpp - return the next larger prime number
// (input taken from command line args or from stdin)

#include <iostream>
int findNextPrime(int n);   // given a number n, find the next larger prime number above n
bool isPrime(int n);        // given a number n, determine whether it is prime

int main(int argc, char**argv)
{
    int input;
    if( argc > 1 ) input = atoi(argv[1]);
    else {
        std::cout << "Please enter a 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 + (n & 1 && n > 1 || !n ? 2 : 1); // start at n+2 for odd n>1 or n=0, else n+1
    while (!isPrime(nextPrime))
        nextPrime += 2;
    return nextPrime;
}

// given a number n, determine if it is prime
bool isPrime(int n)
{
    if ( (n&1)==0 ) return n==2; // if even, then false unless n==2
    // loop over odd trial divisors from 3 to sqrt(n) to check for factors
    for (int i = 3; i*i <= n; i += 2)
    {
        if (n % i == 0)     // found a factor that isn't 1 or n, therefore not prime
            return false;
    }
    return n>1; // prime unless n=1 (no divisors > 1 but not prime)
}

/*
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.
*/

Simplified findNextPrime() to scan only odd numbers (if n > 1), isPrime to check only odd divisors < sqrt(n).

added use of cmd line arg ; changed "enter prime" to "enter number" ; fixed findNextPrime(n=0) .