Nisi's work blog » CPU http://wiki.nisi.ro Nisi's work blog - programing tips Mon, 21 Nov 2011 08:43:00 +0000 en hourly 1 http://wordpress.org/?v=3.1 Interchanging 2 variables without the use of a third http://wiki.nisi.ro/2009/03/interchanging-2-variables-without-the-use-of-a-third/ http://wiki.nisi.ro/2009/03/interchanging-2-variables-without-the-use-of-a-third/#comments Mon, 09 Mar 2009 21:17:53 +0000 admin http://wiki.nisi.ro/?p=29 One of the interesting problems witch every programmer is faced when target device do not have so many memory availble is to interchange 2 variables widthout using a third one:

Theory:

a b a XOR b
0 0 0
0 1 1
1 0 1
1 1 0

.

a 0101
b 0011
a = a XOR b 0110
b = a XOR b 0101
a = a XOR b 0011
a = a ^ b;    // a will contain a xor b = 6
b = a ^ b;    // b will receive 6 xor 3 = 5
a = a ^ b;    // a will become 6 xor 5 = 3

other methods:
a = a * b
b = a / b
a = a / b
or
a = a + b
b = a – b
a = a – b
Every one is ok to use but if you want to program a microcontroller first one is perfect becouse use basic CPU operations XOR and is very fast.
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