Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!julius.cs.uiuc.edu!ux1.cso.uiuc.edu!heal
From: heal@ux1.cso.uiuc.edu (Loren Heal)
Newsgroups: comp.lang.c
Subject: Efficient swap()
Summary: Terse way to #define swap()
Keywords: tricky macro from hell
Message-ID: <1991Jan13.050236.11634@ux1.cso.uiuc.edu>
Date: 13 Jan 91 05:02:36 GMT
Organization: University of Illinois at Urbana
Lines: 23


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Here is a one-line C macro, swap():  The principle is well-known, 
but I've never seen it done this tersely:

#define swap(A,B) (A^=B^=A^=B)

I admit that's not too readable.  It works for integer-compatible 
types, on the principles of the exclusive-OR operation.  Working 
from right to left, First A is xor'd with B, so that A is now A^B, 
B is still B.  Then B is xor'd with the A^B, which leaves the 
original value of A in B.  Then finally A (which is A^B) is xor'd 
with the original value of A, so that A contains the original 
value of B.  I usually use the alternative form

#define sort2(A,B) ((A>=B)?0:((A^=(B^=(A^=B))),1))

Loren Heal, heal@ux1.cso.uiuc.edu, *net!uiucuxc!ux1!heal.
-- 
*----------------------------------------------------------------------*
: Loren E. Heal : leheal@uiuc.edu (from *net, you figure it out)       :
:(Not a spokesman for the University of Illinois at Urbana-Champaign)  :
:(I may work there, but I still own my own mind!)                      :