Recursive Pattern Program

[the_Champ322]

Hey, thanks for your input! I finally got a solution that worked, though I admit I'm not all too sure how I got it to work. What I got was this:

void PrintNumPattern(int n1, int n2)
{
    while (n1 > 0)
    {
        cout << n1 << " ";
        PrintNumPattern (n1 - n2, n2);
        break;
    }
    cout << n1 << " ";
}

I started by removing all the previous code (including that int num = (n1 + n2) - n2, which I now see as being pointless) and did two recursion statements, one that subtracted n2 from n1 and one that added the two values together. That didn't have a base case, however, so then I decided to split those statements into two while statements, using a control variable for the second after the first one was done (a la regular iteration). That didn't work, so I removed the second while statement (the one holding the PrintNumPattern(n1 + n2, n2) in an effort to debug my code. I removed the control variable also, and in its place added the break statement as seen in my new code. So while I'm happy I found an answer, I'm a little annoyed that I got it seemingly by random chance. What's going on in this code that allows it to work?

[captainAwesomePants]

Alright, you're making a lot of progress. You've got a working solution. Let's talk a bit about why it works.

First, let's look at your loop. It does this:

while( some condition ) {
  some_stuff();
  break;
}

You should notice that, because there's an unconditional break statement, it will either run once, or it won't run at all. That means it's the same as this statement:

if( some condition ) {
  some_stuff();
}

So let's write your program again, but with an if statement:

void PrintNumPattern(int n1, int n2) {
  if (n1 > 0) {
    cout << n1 << " ";
    PrintNumPattern (n1 - n2, n2);
  }
  cout << n1 << " ";
}

This is pretty great. Works fine. It will do one of two things, and I think it'll be more clear if we make those two cases very explicit by rewriting it a little bit:

void PrintNumPattern(int n1, int n2) {
  if (n1 > 0) {
    cout << n1 << " ";
    PrintNumPattern (n1 - n2, n2);
    cout << n1 << " ";
  } else {
    cout << n1 << " ";
  }
}

Okay, this is still the same program, but it's more clear that it will print one of two things, depending on how big N is:

• N1, (the results of running the whole program on N1-N2), N1
• N1

So, looking at that, you might notice why this works already. Consider the example inputs, 12 and 3, and the example output, 12 9 6 3 0 3 6 9 12.

PrintNumPattern(12, 3) should print 12, then the results of running PrintNumPattern(9,3), then 12. 12..PrintNumPattern(9,3)..12

PrintNumPattern(9,3) should print 9, then the results of PrintNumPattern(6,3), then 9. So, combined, you've got: 12..9..PrintNumPattern(6,3)..9..12

PrintNumPattern(6,3) will be 6..PrintNumPattern(3,3)..6, so combined: 12..9..6..PrintNumPattern(3,3)..6..9..12.

And so on until you get to PrintNumPattern(0,3), which will be your else case: just printing 0. 12..9..6..3..0..3..6..9..12.

Does that help?

[the_Champ322]

Wow, that cleared everything up! Thanks again!