c - Understanding recursion in the beer bottle example

Question

I am practicing recursion in C on my own and I found this example online. However there is one thing I don't understand.

void singSongFor(int numberOfBottles)
{
if (numberOfBottles == 0) {
    printf("There are simply no more bottles of beer on the wall.

");
} 
else {
    printf("%d bottles of beer on the wall. %d bottles of beer.
",
           numberOfBottles, numberOfBottles);
    int oneFewer = numberOfBottles - 1;
    printf("Take one down, pass it around, %d bottles of beer on the wall.

",
           oneFewer);
    singSongFor(oneFewer); // This function calls itself!

    // Print a message just before the function ends
    printf("Put a bottle in the recycling, %d empty bottles in the bin.
",
             numberOfBottles);
   }
}    

Then I use a main method as such:

 int main(int argc, const char * argv[])
{
  singSongFor(4);
  return 0;
}

And the output is as such:

4 bottles of beer on the wall. 4 bottles of beer. Take one down, pass it around, 3 bottles of beer on the wall.

3 bottles of beer on the wall. 3 bottles of beer. Take one down, pass it around, 2 bottles of beer on the wall.

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 simply no 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.

Put a bottle in the recycling, 3 empty bottles in the bin.

Put a bottle in the recycling, 4 empty bottles in the bin.

I understand the first part very well until I come to "There are simply no more bottles of beer on the wall. I don't understand afterwards how the variable number of bottles is incremented from 1 till 4.

Answer 1

The smaller bottles of beer (and their corresponding recycling) are in inner functions. Your function tree looks like this:

4 bottles of beer on the wall. 4 bottles of beer. Take one down, pass it around, 3 bottles of beer on the wall.
|   3 bottles of beer on the wall. 3 bottles of beer. Take one down, pass it around, 2 bottles of beer on the wall.
|   |   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 simply no 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.
|   Put a bottle in the recycling, 3 empty bottles in the bin.
Put a bottle in the recycling, 4 empty bottles in the bin.