Swap two variables without using a temp variable

Question

I'd like to be able to swap out two variables without the use of a temp variable in C#. Can this be done?

decimal startAngle = Convert.ToDecimal(159.9);
decimal stopAngle = Convert.ToDecimal(355.87);

//swap each:
//startAngle becomes: 355.87
//stopAngle becomes: 159.9

Answer 1

Not in C#. In native code you might be able to use the triple-XOR swap trick, but not in a high level type-safe language. (Anyway, I've heard that the XOR trick actually ends up being slower than using a temporary variable in many common CPU architectures.)

You should just use a temporary variable. There's no reason you can't use one; it's not like there's a limited supply.

Answer 2

yep, use this code:

startAngle = startAngle+stopAngle;
stopAngle = startAngle-stopAngle;
startAngle = startAngle-stopAngle;

Answer 3

for binary types you can use this funky trick:

a %= b %= a %= b;

As long as a and b are not the exact same variable (e.g. aliases for the same memory) it works.

Answer 4

Yes, use this code:

    stopAngle = Convert.ToDecimal(159.9);
    startAngle = Convert.ToDecimal(355.87);

The problem is harder for arbitrary values. :-)

Answer 5

<deprecated>

You can do it in 3 lines using basic math - in my example I used multiplication, but simple addition would work also.

float startAngle = 159.9F;
float stopAngle = 355.87F;

startAngle = startAngle * stopAngle;
stopAngle = startAngle / stopAngle;
startAngle = startAngle / stopAngle;

Edit: As noted in the comments, this wouldn't work if y = 0 as it would generate a divide by zero error which I hadn't considered. So the +/- solution alternatively presented would be the best way to go.

</deprecated>

---

To keep my code immediately comprehensible, I'd be more likely to do something like this. [Always think about the poor guy that's gonna have to maintain your code]:

static bool Swap<T>(ref T x, ref T y)
{
    try
    {
        T t = y;
        y = x;
        x = t;
        return true;
    }
    catch
    {
        return false;
    }
}

And then you can do it in one line of code:

float startAngle = 159.9F
float stopAngle = 355.87F
Swap<float>(ref startAngle, ref stopAngle);

Or...

MyObject obj1 = new MyObject("object1");
MyObject obj2 = new MyObject("object2");
Swap<MyObject>(ref obj1, ref obj2);

Done like dinner...you can now pass in any type of object and switch them around...

Answer 6

At the risk of downvotes, the right way to swap two variables is:

decimal startAngle = Convert.ToDecimal(159.9);
decimal stopAngle = Convert.ToDecimal(355.87);

decimal tempDecimal = startAngle;
startAngle = stopAngle;
stopAngle = tempDecimal;

In other words, use a temporary variable.

There you have it. No clever tricks, no maintainers of your code cursing you for decades to come, no entries to The Daily WTF, and no spending too much time trying to figure out why you needed it in one operation anyway since, at the lowest level, even the most complicated language feature is a series of simple operations.

Just a very simple, readable, easy to understand, t = a; a = b; b = t; solution.

In my opinion, developers who try to use tricks to, for example, "swap variables without using a temp" or "Duff's device" are just trying to show how clever they are (and failing miserably).

I liken them to those who read highbrow books solely for the purpose of seeming more interesting at parties (as opposed to expanding your horizons).

Solutions where you add and subtract, or the XOR-based ones, are less readable and most likely slower than a simple "temp variable" solution (arithmetic/boolean-ops instead of plain moves at an assembly level).

Do yourself, and others, a service by writing good quality readable code.

That's my rant. Thanks for listening :-)

As an aside, I'm quite aware this doesn't answer your specific question (and I'll apologise for that) but there's plenty of precedent on SO where people have asked how to do something and the correct answer is "Don't do it".

Answer 7

For completeness, here is the binary XOR swap:

int x = 42;
int y = 51236;
x ^= y;
y ^= x;
x ^= y;

This works for all atomic objects/references, as it deals directly with the bytes, but may require an unsafe context to work on decimals or, if you're feeling really twisted, pointers. And it may be slower than a temp variable in some circumstances as well.

Answer 8

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

Answer 9

int a = 4, b = 6;

a^= b^= a^= b;

Works for all types including strings and floats.

Answer 10

Beware of your environment!

For example, this doesn’t seem to work in ECMAscript

y^=x^=y^=x;

But this does

x^=y^=x;y^=x;

My advise? Assume as little as possible.