Hi! I'm supposed to input a large integer value(long int) and output it like this:

e.g.
input: 123456789
output: 123,456,789

I'm supposed to use recursion for this. I just need suggestions or partial codes - not a full code (I want to learn, not copy). I'm a newbie, and I don't know how to convert long int to strings. I'm not even sure we're supposed to do that yet. Please help. I don't know where to start.

>>I'm a newbie, and I don't know how to convert long int to strings.
Two methods:
1) In c++ use stringstream class

#include <sstream>
#include <string>
#include <iostream>
using namespace std;
int main()
{
   int n = 12345678;
   std::string s;
   std::stringstream str;
   str << n;
   str >> s;  // converts integer to string
   cout << s << "\n";
}

2) use sprintf()

#include <cstring>
#include <iostream>
using namespace std;
int main()
{
    char str[40];
    int n = 123456;
    sprintf(str,"%d", n);
    cout << str << "\n";
}

Alas, the OP want's to know how to insert those obnoxious commas (,) in between digits.

Remember to use integer division and modulo (remainder) to split numbers apart ( / and % ).
For example:

1492 % 10 --> 2
1492 / 10 --> 149

149 % 10 --> 9
149 / 10 --> 14

etc

Then, to convert each digit you peel off into a number, just use the following kind of code:

int number = 7;
char digit = (char)number + '0';

Now think about how recursive functions work. In your case, you'll have to do three somethings, recurse, and then print stuff.

Hope this helps.