Question

I am trying to write a recursive function in Java that prints the numbers one through n. (n being the parameter that you send the function.) An iterative solution is pretty straightforward:

public static void printNumbers(int n){
    for(int i = 1; i <= n; i++){
         System.out.println(i);
         i++;
    }

As a new programmer, I’m having troubles figuring out how a recursive version of this method would work.

Answer 1

You are using a for loop that is iterating from i=1 to n. As you want to do this with recursion and it is easier to pass n instead of i and n, we just reverse the whole thing, so we count down n to 1. To keep the order of the prints, we first call the recursive function and print the number after the execution:

public static void printNumbers ( int n )
{
    if ( n > 0 )
    {
        printNumbers( n - 1 ); // n - 2, if the "i++" within the for loop is intended
        System.out.println( n );
    }
}

For simple iterative -> recursive conversions it is easy to change loops into a format like this:

public static void printNumbers ( int n )
{
    int i = 1;
    while ( i <= n )
    {
        System.out.println( i );
        i++; // i += 2, if the "i++" within the for loop is intended
    }
}

Now you can easily transform that into a recursive function:

public static void printNumbers ( int n, int i )
{
    if ( i <= n )
    {
        System.out.println( i );
        i++; // i += 2, if the "i++" within the for loop is intended
        printNumbers( n, i );
    }
}

Everything else is optimization.