How to swap without a third variable?

Question

I have to swap two variables with a number value without using a third variable. What is the simple solution?

possible duplicate of [Swap the values of two variables without using third variable](http://stackoverflow.com/questions/756750/swap-the-values-of-two-variables-without-using-third-variable). Also [Swap two variables without using a temp variable](http://stackoverflow.com/questions/804706/swap-two-variables-without-using-a-temp-variable) — Michael Mrozek, Mar 17 '11 at 20:31
http://stackoverflow.com/questions/804706/swap-two-variables-without-using-a-temp-variable — Inisheer, Mar 17 '11 at 20:32
@Will I've seen some use things like these as trick questions during interviews. I don't know what it's meant to *prove*, but I've seen it.. — code_dredd, Jun 02 '17 at 22:18

score 20 · Accepted Answer · answered Mar 17 '11 at 20:28

20

Let us see one of the method namely by using arithmetic operators. Consider 2 variables say x=50 and y=70 and let us see how to swap the value of two variables that is make x=70 and y=50 without using third variable. This can be done by using following arithmetic operations namely
x= x + y
y= x - y
x= x - y
Which gives
• x= x + y gives x= 70 + 50 an so x is equal to 120
• y= x - y gives y = 120 - 70 which makes the value of y as 50
• x= x - y gives x= 120 - 50 and thus value of x becomes 70

answered Mar 17 '11 at 20:28

Student

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What if x or y was very large in proportion to the other? – jonsca Mar 17 '11 at 20:34
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that's a good solution, but have to be careful with overflows – Fede Mar 17 '11 at 20:34
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Answer `x -= y = (x += y) - y;` and tell the teacher that's one of the lamest questions ever. – Amadan Mar 17 '11 at 20:40
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@Amadan Except when people use things like these as trick questions during interviews, as if that actually proved anything... :/ – code_dredd Jun 02 '17 at 22:17

score 14 · Answer 2 · answered Mar 17 '11 at 20:35

14

You can achieve it with XOR

int A = ...;
int B = ...;
A = A ^ B;
B = A ^ B;
A = A ^ B;

answered Mar 17 '11 at 20:35

Fede

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@Bertrand, I just made a "mind test" and still believe it's ok. If A equals B, in the first assignment, the result of A would be 0, then B is assigned 0 XOR B which is B, and then A is assigned 0 XOR B which is B again (which was equal to the starting A). – Fede Mar 17 '11 at 20:57
A=1, B=1. Step 1: A=A^B=1^1=0. Step2: B=A^B=0^1=1. Step3: A=A^B=0^1=1. Looks like it works to me. – Jim Mischel Mar 17 '11 at 21:42

score 6 · Answer 3 · answered Mar 17 '11 at 20:32

6

Depending on the type of variable, you can use Interlocked.Exchange. This uses an atomic operation to do the swap.

answered Mar 17 '11 at 20:32

Reed Copsey

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score 4 · Answer 4 · answered Mar 17 '11 at 20:34

4

 int x = 15;
 int y = 5;

 x = x + y;
 y = x - y;
 x = x - y;

answered Mar 17 '11 at 20:34

MBU

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score 2 · Answer 5 · answered Mar 17 '11 at 20:29

2

Another popular way is the XOR swapping strategy. http://en.wikipedia.org/wiki/XOR_swap_algorithm

answered Mar 17 '11 at 20:29

WorldIsRound

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score 1 · Answer 6 · edited Sep 16 '16 at 03:19

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Here we have this in MIPS assembler. The first solution is long and bad. The second one with XOR is better.

addi $t0, $0, -5
addi $t1, $0, 15

add $t0, $t0, $t1
sub $t1, $t1, $t0
nor $t1, $0, $t1
addi $t1, $t1, 1
sub $t0, $t0, $t1

####

xor $t0, $t0, $t1
xor $t1, $t0, $t1
xor $t0, $t0, $t1

edited Sep 16 '16 at 03:19

Peter Mortensen

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answered Mar 17 '11 at 20:30

Jakob Alexander Eichler

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How to swap without a third variable?

6 Answers6

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