C++ :: Finding Largest Prime Factor Of A Number?

I am trying to find the largest prime factor of a number. But when I am trying to determine if a number is a prime number in the function:

int is_prime(int number), I am unable to exit the loop.

Here is the code:

#include<iostream>
using namespace std;
int is_prime(int number) //the problem is in this function {
int num = number;
int factor=0;
do{
num++;
for(int i=1;i<(num+1);i++){

[code].....

So when the program runs, it first divides 20 by 2, to get 10, then divides 10 by 2 to get 5. Since, // condition 1 is not met, it passes 2 to the function int is_prime(int number). The function is able to return 3, but cannot exit the loop when num is 4.

I think the problem lies in the function: int is_prime(int number).