How do you swap two integer variables without using any if conditions, casting, or additional variables?

Question

There are two integer variables. Can you swap those integer variables without using any if conditions, without casting, and without using additional variables? For example:

int a = 10;
int b = 5;

a > b always. The answer should be a == 5 and b == 10

No, I have no idea how to slow down a problem or make it go faster. — BoltClock, Jul 16 '10 at 06:29
seems to be a homework assignment if that's the case you should tag it as such — Rune FS, Jul 16 '10 at 06:29
Duplicate: http://stackoverflow.com/questions/804706/swap-two-variables-without-using-a-temp-variable — Naveen, Jul 16 '10 at 06:30
Isn't it lovely, when someone upvotes a homework duplicate? :P — Palantir, Jul 16 '10 at 06:36
definately not a homework - too complex for that. OK for interview. — Imre L, Jul 16 '10 at 06:39
Just out of interest from somebody from another camp, is a solution like `list($a, $b) = array($b, $a)` possible in C#? — deceze, Jul 16 '10 at 06:46
I see no indication of language here, so it can't be answered satisfactorily. — David Thornley, Jul 20 '10 at 18:19

score 14 · Accepted Answer · answered Jul 16 '10 at 06:35

14

If you think you are being clever by not using 3rd variable then do some performance tests and you see that the much faster way is to use 3rd int to store the variable temporarily.

Anyways, i solved the problem with XOR bitwise operator:

a ^= b;
b ^= a;
a ^= b;

answered Jul 16 '10 at 06:35

Imre L

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More about swapping with xor is here http://en.wikipedia.org/wiki/XOR_swap_algorithm – Developer Marius Žilėnas Jan 06 '14 at 08:56
This fails to work if `a == b`. – Axoren Nov 22 '14 at 16:57
What language are you using? – Imre L Nov 24 '14 at 13:08
@Axoren: no, it does not fail. – Doc Brown Jan 20 '15 at 12:39
My mistake. It fails if `a` and `b` are references to the same variable, not if they're the same value. For example: `int c; int *a = &c; int *b = &c;`. In this case, `*a ^= *b` will cause both `*a` and `*b` to become 0 at the same time, leading to the value being lost. This is still important to note if attempting to implement a swap method that works with the variable's addresses. – Axoren Jan 20 '15 at 20:24
In fact, all of the answers to this question fail under that case. I think that should be part of an interview question. – Axoren Jan 20 '15 at 20:26
1

Your example "fails" because you have the variables pointing to same memory location. This is not in the scope of question – Imre L Jan 22 '15 at 09:42

score 11 · Answer 2 · answered Jul 16 '10 at 06:31

11

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

answered Jul 16 '10 at 06:31

Doc Brown

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score 4 · Answer 3 · answered Jul 16 '10 at 06:31

4

It's a little trick.

int a = 5;
int b= 10;
a = a+b;
b = a-b; /* Really (a+b) - b i.e. a */
a = a-b; /* Really (a+b) - a i.e. b */

answered Jul 16 '10 at 06:31

Matt Mitchell

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Pranay Rana · Answer 4 · 2010-07-16T07:59:17.537

3

yes you can do it By using plus/minus operation.

Example:
num1 = num1 + num2;                
num2 = num1 - num2;                
num1 = num1 - num2;

edited Jul 16 '10 at 07:59

answered Jul 16 '10 at 06:28

Pranay Rana

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Better to use addition/subtraction to avoid overflows and rounding problems. That said, it's better to just use a temp variable, but still. – Dan Puzey Jul 16 '10 at 06:33
Multiply/Divide does not work if num2 is 0. – pascal Jul 16 '10 at 07:56
thanks for the info ans updated now – Pranay Rana Jul 16 '10 at 08:00

score 3 · Answer 5 · edited Jul 16 '10 at 08:20

3

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

That's it!

edited Jul 16 '10 at 08:20

abatishchev

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answered Jul 16 '10 at 06:33

Mayur

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score 3 · Answer 6 · answered Jul 16 '10 at 06:37

3

simple try this

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

and that's it

answered Jul 16 '10 at 06:37

Johnny

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How do you swap two integer variables without using any if conditions, casting, or additional variables?

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