Swapping using two variables not three?

Question

Possible Duplicate:
Swapping two variable value without using 3rd variable

we have

int a=4;
int b=7;

can i swap these no.s without using third variable?

score 3 · Accepted Answer · edited Sep 23 '10 at 18:44

3

a=a+b;
b=a-b;
a=a-b;

edited Sep 23 '10 at 18:44

Hogan

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answered Sep 23 '10 at 11:57

fredley

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Sure it is -- see gspr's answer. – Hogan Sep 23 '10 at 12:01
@Hogan realised that very quickly and changed answer, sorry for confusion. – fredley Sep 23 '10 at 12:01
@yang , how can you say? – Guri Sep 23 '10 at 12:08
@Guri Jeff is correct, this is susceptible to overflow. The XOR method would be better. – fredley Sep 23 '10 at 12:24
@fredley: Only in programming languages with broken arithmetic. There *are* actually programming languages which *can* add two numbers correctly. – Jörg W Mittag Sep 23 '10 at 14:31
1

@fredley: I find it kind of sad that the fact that 2 billion plus 2 billion equals 4 billion is *still* considered an exciting and exotic feature in 2010. – Jörg W Mittag Sep 23 '10 at 15:23

score 2 · Answer 2 · edited May 23 '17 at 12:34

The precise implementation of course depends on the programming language you're using, but check out XOR swap.

An example in C could be

#include <stdio.h>

/* Swaps the content pointed to by a and b. The memory pointed to is
   assumed non-overlapping! */

void swap(int* a, int* b)
{
  *a = (*a)^(*b);
  *b = (*a)^(*b);
  *a = (*a)^(*b);
}

int main(int argc, char** argv)
{
  int a = 4;
  int b = 7;
  printf("a=%d, b=%d\n", a, b);
  swap(&a, &b);
  printf("a=%d, b=%d\n", a, b);
  return 0;
}

Important: As Prasoon Saurav commented on the question itself, this answer to another question is more correct than mine, seeing as it is important that the two variables reside at non-overlapping locations in memory. My example does not check for this.

ratty · Answer 3 · 2010-09-23T12:27:33.200

1

a=a+b; //a=11,b=7

b=a-b; //a=11,b4

a=a-b; //a=7,b=4

or

a=a*b;

b=a/b;

a=b/a;

but careful with this method, overflow or underflow is possible for some combinations.

edited Sep 23 '10 at 12:27

answered Sep 23 '10 at 12:01

ratty

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All liable to overflow/underflow with certain number combinations. – dty Sep 23 '10 at 12:07

score 0 · Answer 4 · answered Sep 23 '10 at 11:59

0

Sure: a,b = b,a works in various programming languages.

In others you can use the xor trick.

answered Sep 23 '10 at 11:59

sepp2k

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Swapping using two variables not three?

4 Answers4