Decimal to Hex in C++

#include <iostream>
#include <math.h>
using namespace std;
int main()
{
    int dec_num , r;
    string hexdec_num="";
    char hex[]={'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'};
    cout << "Input a decimal number : ";
    cin>> dec_num;
    if(dec_num>=0&&dec_num<=15)
    {
        while(dec_num>0)
        {
            r = dec_num % 16;
            hexdec_num = hex[r] + hexdec_num;
            dec_num = dec_num/16;
        }
        cout<<"The hexadecimal number is : "<<hexdec_num<<"\n";
    }
    else
    {
        cout<<dec_num<<" is an invalid input";
    }
}