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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; }

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); } }

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.

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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