Hellow fellow monks, I have responded to 3 or 4 other posts, but this is my first post. So, please be nice, thanks.

I am writing this, because of something I read in Programming Perl. It went something like this: Other languages force you to use a temporary variable to swap variables, but perl doesn't. In perl you can do this: ($a,$b)=($b,$a). I just want you to know, I love the O`Reily series of books, this is not an attack against them, that said, I'll move on.

Now, I'm not exactly sure, but I remember reading somewhere that perl makes an internal copy of the variables to swap. So, that solution is really just ducking the bullet. If you really don't want to use a temporary variable, as in this code: 

$temp = $a;
$a = $b;
$b = $a;

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UPDATE: Thanks Rich, I should have changed that last asignment to $b = $temp. Sorry.

You can use a little ingenuity. Now I have never seen any of these following things in a book, nor have I been taught them in a class (I'm only 15, I can't really take many cs classes), so I don't know how widespread this knowledge is. First of all, I don't really recomend you use all of these solutions (only the first is better than using a temporary variable), but it's nice to understand the concepts.

First, if you want to swap integers, use this: 

$a ^= $b;
$b ^= $a;
$a ^= $b;

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I benchmarked this, and it performed exactly the same as the temperary solution, but it does not waste a variable. (Those monks who wish to see how this works, just click the readmore at the bottom of this post). If you wish to switch strings without a temporary variable, just use this code: 
$a .= $b;
$b = substr($a,0,length($a)-length($b));
substr($a,0,length($b)) = '';

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When I benchmarked this, the temporary variable solution was very slightly faster. Finally, if your variables could be integers, but could also be floating point numbers, you can use this solution: 
$a += $b;
$b = $a - $b;
$a -= $b;

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Now, I know it's easier to just use ($a,$b)=($b,$a), but I wrote this because I was concerned with the language, the book said that other languages force you to use a temporary variable, well they don't. Also, I was recently involved in a robotics competition, in which we used the PBASIC programming language with the BASIC 2SX chip. We had a limited supply of variables, so we had to use solutions like these all the time. So, I'm quite familiar with little optimiztions like this one.
Here is the code: 
$a ^= $b;
$b ^= $a;
$a ^= $b;

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It works by creating a mask of $a and $b, and storing it in $a. This mask, when given the original value of $a, returns $b, and vice versa. Next we store in $b, the value of the mask XORed to $b, returning $a, just what we want. Next, we store in $a the value of the mask XORed to $b, which is now the original value of $a, so it returns $b, and the variables are swapped.

The other examples are relatively easy to follow, so I will leave them out of this post.



The 15 year old, freshman programmer,
Stephen Rawls

In reply to Temporary Variables by srawls

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