Need help understanding a recursive function in Xcode

Recursion works like this:
You call and a function, if some parameters allow it, the function will call itself from within, In this case if the number of beers is >0.

Function will do this,
Print # of bottles of beer on the wall.
Remove one bottle.
Take one down...
Than calls itself, at this point the current function "Freezes" It will be the bottom Frame, and the last line of code will be the last one called. Since it was put onto the stack. You can think of last line as a stack of papers.

So put a bottle in the recycling will layer them, the first one being at the bottom last one at the top, than when they are all done layering, they will execute. Starting from the top. Recursion is a technique to reverse the order of something.

 

Change the initial call to singTheSong (2), set a breakpoint on that call, and step through the code. It will become obvious what's happening.

 

Each function call is unique, and none of them finish until the last one - then the whole thing unwraps, which each function call printing the recycling bit as it finishes.

Déjà vu

 

C is procedural. Recursion is nothing special. Think about it as if this code was written this way instead :

Code:

#include <stdio.h>

void singTheSong(int numberOfBottles)
{
    if (numberOfBottles == 0)
   {
        printf("There are simple bo more bottles of beer on the wall.\n");
    }
    else
    {
        printf("%d bottles of beer on the wall. %d bottles of  beer.\n", numberOfBottles, numberOfBottles);
               
        int oneFewer = numberOfBottles - 1;
        printf("Take one down, pass it around, %d bottles of beer on the wall.\n", oneFewer);
               
        if (oneFewer == 0)
        {
            printf("There are simple bo more bottles of beer on the wall.\n");
        }
        else
        {
            printf("%d bottles of beer on the wall. %d bottles of  beer.\n", oneFewer, oneFewer);
               
            int anotherOneFewer = oneFewer - 1;
            printf("Take one down, pass it around, %d bottles of beer on the wall.\n", anotherOneFewer);
               
            if (anotherOneFewer == 0)
            {
                printf("There are simple bo more bottles of beer on the wall.\n");
            }
            else
            {
                printf("%d bottles of beer on the wall. %d bottles of  beer.\n", anotherOneFewer, anotherOneFewer);
               
                int lastOneFewer = anotherOneFewer - 1;
                printf("Take one down, pass it around, %d bottles of beer on the wall.\n", lastOneFewer);
                printf("Put a bottle in the recycling, %d empty bottles in the bin.\n", anotherOneFewer);
	    }
            printf("Put a bottle in the recycling, %d empty bottles in the bin.\n", oneFewer);
        }
        printf("Put a bottle in the recycling, %d empty bottles in the bin.\n", numberOfBottles);
    }
}

int main (int argc, const char * argv[])
{

    singTheSong(2);
    
    return 0;
}

Code:

$ gcc -o test -Wall test.c
$ ./test
2 bottles of beer on the wall. 2 bottles of  beer.
Take one down, pass it around, 1 bottles of beer on the wall.
1 bottles of beer on the wall. 1 bottles of  beer.
Take one down, pass it around, 0 bottles of beer on the wall.
There are simple bo more bottles of beer on the wall.
Put a bottle in the recycling, 1 empty bottles in the bin.
Put a bottle in the recycling, 2 empty bottles in the bin.

See how I replaced all the function calls with basically repeating the function ? That's basically how it's working, except it's using the stack to repeat the code in the function call. This doesn't end the function itself once another is called, it simply runs the code in the function and then comes back and continues where it left off.

Really, calling your own function is the same as calling API functions. Do you expect your code to stop where you call printf() ? No, once the runtime is done executing printf()'s code, it simply returns and continues to execute your own code where it left off. Recursion is the same.

 

Darn I wanted to chime in and sound smart

 

If that's why you post here ...

 

After so many beers, I start talking like that, too.

 

Thanks!

Big thanks for this interpretation!