It's been a long time since I've done any C programming (the only C I ever really did was in school 10+ years ago) and I've never done it outside of school. I figure since I look at and study languages that are pretty esoteric, it's only fair to throw in a language that is really widely used. It should make my Objective-C better (which probably isn't saying much). It also gives me a chance to go "a little closer to the metal" I guess. Hey look, a Bus Error!
Anyway, one of the things that I love/hate about C are the little tricks that people often use. Sometimes they're really fun but often they take a bit of thinking to understand if you're not familiar with them. One of those that I ran across again was swapping values without a temp variable.
How Normal People Swap Values
The simplest way of swapping two values is ususally accomplished using a temp variable to hold one of the values for a moment.int a = 2;
int b = 5;
int temp = 0;</p>
<p>temp = a;
a = b;
b = temp;And Another Way
But you can accomplish the same thing using an Exclusive Or (XOR) operation.To review bitwise operations:
& AND 0101 (5) 1100 (12) ---- & 0100 (4)| OR 0101 (5) 1100 (12) ---- | 1101 (13)
^ XOR 0101 (5) 1100 (12) ---- | 1001 (9)
The ^= just combines the XOR with an Assignment.
int a = 2;
int b = 5;</p>
<p>a ^= b;
b ^= a;
a ^= b;
So, I just came up with an example that shows what's happening:
int a = 2; // 0010
int b = 5; // 0101</p>
<p>a ^= b; // 0111 = 0010 ^ 0101
b ^= a; // 0010 = 0101 ^ 0111
a ^= b; // 0101 = 0111 ^ 0010
What are some of the principles in why this works?:
a ^ b == b ^ a
XOR is a reversible binary operation.
a = a ^ b // now a is some other value
b = a ^ b // now b is the original value of a
a = a ^ b // now a is the original value of b
This isn't a proof of any sort, but hopefully it will help you understand why it actually works. If you want even more of an explanation, and a proof, check out the article on Wikipedia: XOR Swap Algorithm