Without using a third variable at all, swap X and Y values

Question

var x=10;
var y=5;

Solution 1

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

Solution 2

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

the two solutions are equal to each other but the second one is faster, why?

Can anyone show me the optimized opcode generated for both solutions by JS engine?

Watch out if you are expecting integer arithmetic and x and y are of v.large magnitude - since x + y could exceed the integer range. — GavinBrelstaff, Nov 20 '14 at 17:35
Apart from that, speed might depend on using the += and -= operators (it used to in C) x += y; y = x - y; x -= y; — GavinBrelstaff, Nov 20 '14 at 17:37
As expected, the assertion that one is faster is simply wrong: http://jsperf.com/two-ways-of-swapping — T.J. Crowder, Nov 20 '14 at 17:41
No, this isn't a duplicate of http://stackoverflow.com/questions/16201656/how-to-swap-two-variables-in-javascript. Actually **read** the question above, it's about optimization and why would is faster than the other. (It's wrong, but not a duplicate.) — T.J. Crowder, Nov 20 '14 at 17:42
This question appears to be off-topic because it is based on a mistaken assumption. — T.J. Crowder, Nov 20 '14 at 17:43
you are right.. same speed, and this is the generated opCode movl $10, -4(%rbp) movl -4(%rbp), %eax addl %eax, -8(%rbp) movl -4(%rbp), %eax movl -8(%rbp), %edx subl %eax, %edx movl %edx, %eax movl %eax, -4(%rbp) movl -4(%rbp), %eax subl %eax, -8(%rbp) movl $0, %eax — nullException, Nov 20 '14 at 17:47
@nullException: Well, that's what it is with one specific JavaScript compiler. There are several different ones. — T.J. Crowder, Nov 20 '14 at 17:48

score 0 · Answer 1 · edited May 23 '17 at 11:57

0

Well, in Ecmascript 6 you can do:

var x=10;
var y=5;

[x,y] = [y,x]
console.log(x,y) // 5, 10

If not, here is already a good way: How to swap two variables in JavaScript

As for the 2 methods you have, doubt they are different, you should first try to check whether there is a big time difference with multiple tests

edited May 23 '17 at 11:57

Community

1
1

answered Nov 20 '14 at 17:33

juvian

15,875
2
37
38

score 0 · Answer 2 · answered Nov 20 '14 at 18:50

I can't find any meaningful difference in performance for either solution or using XOR. Non on the test are consistently faster so I assume the premise of the question is incorrect.

JSLitmus.test('Solution1', function() {
    var x = 10;
    var y = 5;    
    x = x + y;
    y = x - y;
    x = x - y;
});

JSLitmus.test('Solution2', function() {
    var x = 10;
    var y = 5;
    x = y + x, y = x - y, x = x - y;
});

JSLitmus.test('Solution3', function() {
    var x = 10;
    var y = 5;
    x ^= y;
    y ^= x;
    x ^= y;
});

http://jsfiddle.net/6L0d6rpn/

Without using a third variable at all, swap X and Y values

2 Answers2