On Fri, 2009-07-10 at 13:42 +0100, Dermot wrote: > Why is $E getting assigned the value from $k?
my $E = binary($k); I said $E will be assigned the string that represents $k, not it's value. > $E is initialised and > then assigned the return value of the binary(18) during the first > invocation (0). Does the subroutine continue and concatenate $b to $E > and then return (1) ? or does it wait until binary exhausts $n? It "waits" until: $n == 0 || $n == 1; > > In my own groping/nonscientific sort of way, what I see emerging is a > pattern where my result (10100) is nearly the reverse of the correct > answer (100101) minus the leading 1. If that is correct I don't know > why the string is reversed, Here's a breakdown of what's happening in binary(37). Note that I'm mixing arithmetic and string manipulations here. 37 = int(37/2)*2 + 37%2 = (18)*2 + 1 = ( int(18/2)*2 + 18%2 )*2 + 1 = ( (9)*2 + 0 ) + 1 = ( ( int(9/2)*2 + 9%2 )*2 + 0 )*2 + 1 = ( ( (4)*2 + 1 )*2 + 0 )*2 + 1 = ( ( ( int(4/2)*2 + 4%2 )*2 + 1 )*2 + 0 )*2 + 1 = ( ( ( (2)*2 + 0 )*2 + 1 )*2 + 0 )*2 + 1 = ( ( ( ( int(2/2)*2 + 2%2 )*2 + 0 )*2 + 1 )*2 + 0 )*2 + 1 = ( ( ( ( (1)*2 + 0 )*2 + 0 )*2 + 1 )*2 + 0 )*2 + 1 = ( ( ( ( "1" . "0" ) . "0" ) . "1" ) . "0" ) . "1" = ( ( ( ( "10" ) . "0" ) . "1" ) . "0" ) . "1" = ( ( ( "100" ) . "1" ) . "0" ) . "1" = ( ( "1001" ) . "0" ) . "1" = ( "10010" ) . "1" = "100101" In the first part: $n = int($n/2)* + $n%2 but $k = int($n/2) and $b = $n%2 so $n = int($n/2)*2 + $n%2 = $k*2 + $b Addition is commutative, so this could be written as: $n = $b + $k*2 but string concatenation is not. The return string must be $E, the string representing $k; multiplied by 2 by concatenating "0" at it's end; and then that "0" being replaced by "$b". In decimal, to multiply a number by 10, add an zero to it's end. (It's true. Ask anyone on the streets. When asked, "How do you multiply a number by ten?" they'll reply, "Stick a zero at its end.") In binary, to multiply a number by two, stick an zero on its end. If $e is the string representing $k, to reconstruct $n, we have to first multiply $E by 2: my $return_string = $E . "0"; Now we have to add $b to this by replacing the last character in the return string with "$b": $return_string = substr( $return_string, 0, -1 ) . $b; It's just simpler to do it all in one step: my $return_string = $E . $b; -- Just my 0.00000002 million dollars worth, Shawn Programming is as much about organization and communication as it is about coding. -- To unsubscribe, e-mail: beginners-unsubscr...@perl.org For additional commands, e-mail: beginners-h...@perl.org http://learn.perl.org/
