Kind people,
I am befuddled, and looking for understanding:
int a, b;
a = 7;
b = 12;
a ^= b;
b ^= a;
a ^= b;
should swap the contents of a and b (and it does).
Since ^= associates to the right
a ^= b ^= a ^= b;
should do the same thing (and it does).
However, if I access a and b through pointers, it breaks.
int *p;
int *q;
p = &a;
q = &b;
*p ^= *q ^= *p ^= *q;
puts the original contents of *p into *q, but puts 0 into *p.
Specifically, the following program, compiled with gcc 4.1.2 yields the
results listed below it:
#include <stdlib.h>
#include <stdio.h>
int main()
{
int a, b;
int *p, *q;
p = &a;
q = &b;
a = 55;
b = 27;
printf("before a = %d b = %d\n", a, b);
a ^= b ^= a ^= b;
printf("after a = %d b = %d\n", a, b);
printf("before *p = %d *q = %d\n", *p, *q);
*p ^= *q ^= *p ^= *q;
printf("after *p = %d *q = %d\n", *p, *q);
return 0;
}
before a = 55 b = 27
after a = 27 b = 55
before *p = 27 *q = 55
after *p = 0 *q = 27
So, is this an "I don't understand C as well as I thought I did" issue, or
might it be a compiler issue?
Paul
-------------------------
Paul S. LaFollette, Jr.
Computer & Information Sciences Department
Temple University
+1 215 518 3134
http://lucas.cis.temple.edu/~lafollet
paul dot lafollette at temple dot edu
