Hello all! I have never completely understood recursion. I have some understanding of what happens when you have one simple recursive call in a subroutine like the following:

sub factorial 
{
   factorial
   my ($n) = @_;
   return 1 if $n == 0;
   return factorial($n-1) * $n;
}

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The sub factorial is called again and again until $n is equal to 0. Then the 1 is returned and all the past calls are then 'really' executed in reverse order, producing the factorial of the number $n.

OK. But I do not understand how recursion works when there is more than 1 recursive call in a subroutine, like the following: 

sub hanoi 
{
   my ($n, $start, $end, $extra) = @_;
   if ($n == 1) 
   {
      print "Move disk #1 from $start to $end.\n";
   } 
   else 
   {
      hanoi($n-1, $start, $extra, $end);
      print "Move disk #$n from $start to $end.\n";
      hanoi($n-1, $extra, $end, $start);
   }
}

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The output will be:

Move disk #1 from A to C.
Move disk #2 from A to B.
Move disk #1 from C to B.
Move disk #3 from A to C.
Move disk #1 from B to A.
Move disk #2 from B to C.
Move disk #1 from A to C.

[download]

Can anyone explain precisely why this is the output. If the first recursive call were called over and over untill $n==1 and then they 'fell' back and all the calls were executed in reverse order, the third line of output does not make sense. Is the next recursive line then executed? A step-by-step explanation is what I am looking for.

Thank you!

In reply to Recursion Confusion by live4tech

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