Perhaps a diagram will help.

print "main start\n";
&mySub(4);
print "main end\n";

sub mySub
{
     my ($oldValue, $callDepth, @leftOvers) = @_;
     if (!defined $oldValue) { $oldValue = 0; }
     if (!defined $callDepth) { $callDepth = 0; }

     # Increment call depth each time we get into mySub()
     $callDepth++;
     my $indentSize = $callDepth * 2;
     my $indentSpaces = sprintf("%${indentSize}s", "");

     # Show we've arrived -- indent according to call depth
     print "${indentSpaces}mySub($oldValue) BEGIN\n";

     # Cut value in half and recurse until zero
     my $newValue = int($oldValue / 2);
     if ($newValue > 0)
     {
          &mySub($newValue, $callDepth);
     }

     # Show we've returned -- indent according to call depth
     print "${indentSpaces}mySub($oldValue) END\n";

     # Bye-bye
     return;
}

[download]

Results:

main start
  mySub(4) BEGIN
    mySub(2) BEGIN
      mySub(1) BEGIN
      mySub(1) END
    mySub(2) END
  mySub(4) END
main end

[download]

Diagram of how it works (conceptually, if not implementorily):

  main start   main() mySub()
    ^
    |
    +-- You are here
        You call mySub(4)
  mySub(4) BEGIN   main() mySub(4)
            ^
            |
            +-- You are here
                You call mySub(2)
    mySub(2) BEGIN   main() mySub(4)
            mySub(2)
              ^
              |
              +-- You are here
                  You call mySub(1)
      mySub(1) BEGIN
 
 
 
      mySub(1) END 		  main() mySub(4)
            mySub(2)
              mySub(1)
                ^
                |
                +-- You are here
  
    mySub(2) END   main() mySub(4)
            mySub(2)
              ^
              |
              +-- You are here
  mySub(4) END   main() mySub(4)
            ^
            |
            +-- You are here
  main end   main() mySub()
    ^
    |
    +-- You are here

In reply to Re: recursion basics by marinersk
in thread recursion basics by wrinkles

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