Swap two variables without using a temporary varia

Question

I'd like to be able to swap two variables without the use of a temporary 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

If you want to swap 2 string variables:

a = (a+b).Substring((b=a).Length);

An helper method accordingly:

public static class Foo {
    public static void SwapString (ref string a, ref string b) {
       a = (a+b).Substring((b=a).Length);
    }
}

Usage would be then:

string a="Test 1";
string b="Test 2";
Foo.SwapString(a, b);

Answer 2

For the sake of future learners, and humanity, I submit this correction to the currently selected answer.

If you want to avoid using temp variables, there are only two sensible options that take first performance and then readability into consideration.

• Use a temp variable in a generic Swap method. (Absolute best performance, next to inline temp variable)
• Use Interlocked.Exchange. (5.9 times slower on my machine, but this is your only option if multiple threads will be swapping these variables simultaneously.)

Things you should never do:

• Never use floating point arithmetic. (slow, rounding and overflow errors, hard to understand)
• Never use non-primitive arithmetic. (slow, overflow errors, hard to understand) Decimal is not a CPU primitive and results in far more code than you realize.
• Never use arithmetic period. Or bit hacks. (slow, hard to understand) That's the compiler's job. It can optimize for many different platforms.

Because everyone loves hard numbers, here's a program that compares your options. Run it in release mode from outside Visual Studio so that Swap is inlined. Results on my machine (Windows 7 64-bit i5-3470):

Inline:      00:00:00.7351931
Call:        00:00:00.7483503
Interlocked: 00:00:04.4076651

Code:

class Program
{
    static void Swap<T>(ref T obj1, ref T obj2)
    {
        var temp = obj1;
        obj1 = obj2;
        obj2 = temp;
    }

    static void Main(string[] args)
    {
        var a = new object();
        var b = new object();

        var s = new Stopwatch();

        Swap(ref a, ref b); // JIT the swap method outside the stopwatch

        s.Restart();
        for (var i = 0; i < 500000000; i++)
        {
            var temp = a;
            a = b;
            b = temp;
        }
        s.Stop();
        Console.WriteLine("Inline temp: " + s.Elapsed);


        s.Restart();
        for (var i = 0; i < 500000000; i++)
        {
            Swap(ref a, ref b);
        }
        s.Stop();
        Console.WriteLine("Call:        " + s.Elapsed);

        s.Restart();
        for (var i = 0; i < 500000000; i++)
        {
            b = Interlocked.Exchange(ref a, b);
        }
        s.Stop();
        Console.WriteLine("Interlocked: " + s.Elapsed);

        Console.ReadKey();
    }
}

Answer 3

With tuples

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

(startAngle, stopAngle) = (stopAngle, startAngle);

Answer 4

Here is some different process to swap two variables

//process one
a=b+a;
b=a-b;
a=a-b;
printf("a= %d  b=  %d",a,b);

//process two
a=5;
b=10;
a=a+b-(b=a);
printf("\na= %d  b=  %d",a,b);

//process three
a=5;
b=10;
a=a^b;
b=a^b;
a=b^a;
printf("\na= %d  b=  %d",a,b);

//process four
a=5;
b=10;
a=b-~a-1;
b=a+~b+1;
a=a+~b+1;
printf("\na= %d  b=  %d",a,b);

Answer 5

int a = 4, b = 6;
a ^= b ^= a ^= b;

Works for all types including strings and floats.

Answer 6

var a = 15;
var b = -214;
a = b | !(b = a);

This works great.