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How do you swap the values of 2 variables without creating a third temporary variable? There really isn’t a practical reason to do this, other than having some uninformed employer who thinks this is a good question to ask a candidate. So before I explain the technique, here are some reasons to not do this in a real application:

- It’s slower, because you’re doing calculations instead of just storing values.
- It’s harder for the next programmer to interpret what’s going on, and why.
- You’re using extra memory to hold all the calculated values.
- There are hard-to-find errors that only happen in certain conditions.
- It’s just as many lines of code, so why use the slower, more complicated technique?

You can find lots of examples of doing this online, but here’s why it works:

Let’s take 2 variables:

x = 5

y = 10

Now consider some basic math:

x + y = z, which becomes 5 + 10 = 15

As long as you have any 2 of those 3 values, you can calculate the 3rd.

z = x + y

x = z – y If we lose x, we can still calculate it’s original value from z and y.

y = z – x If we lose y, we can still calculate it’s original value from z and x.

So now we have the following sequence to swap the values:

x = x + y, giving us x equal to 15, and y is still equal to 10. The difference is 5, our original value of x, so we can still get our original values.

y = x – y, therefore y equals 5 (15 – 10), the original value of x.

x = x – y, and now x becomes 15 – 5 = 10, the original value of y.

I used +/- here, but we can do the same with multiplication:

x = x * y, now x = 50 and y = 10

y = x / y, so now y = 5 and x = 50

x = x / y, so now x = 10 and y = 5

We can also use XOR:

x = x ^ y, and now x = 15

y = x ^ y, and now y = 5

x = x ^ y, and now x = 10

One of the problems with the mathematical techniques is overflow problems. If your values go over the limit of what your number type can hold, weird things happen. C# won’t always throw an error but you’ll get the wrong results.

int x = 1024

int y = 2147483647

Using the multiplication method, we end up with a “Divide by 0” error.

int x = 2147483647

int y = 1024

Again using multiplication, we end up with:

x = 1024

y = -1

No error, but definitely not the result we wanted.

Now, what if we have strings instead of numbers? Basically, use a temp variable:

x = “Hello”

y = “World”

z = x

x = y

y = z

Attempting to swap 2 strings without a temporary variable is messy, overly complicated, and prone to error. For example, the following seems reasonable:

x = x + y, now x = “HelloWorld”

y = x.Substring(0, x.IndexOf(y)), and y = “Hello”

x = x.Substring(y.Length), and now x = “World”

That works! Well, with our test data of “Hello” and “World”. Trying this against “mxyzptlk” and “tlk”, and we end up with:

x = “tlktlk”

y = “mxyzp”

Now poor Mr. Mxyzptlk has had his name mangled. As soon as we have one string that contains the second, our swap fails. A slight change gives us:

x = x + y, now x = “mxyzptlktlk”

y = x.Substring(0, x.Length – y.Length), and y = “mxyzptlk”

x = x.Substring(y.Length), and now x = “tlk”

Much better, but because strings are immutable, this code keeps creating new strings.

More important, can you tell from a quick glance what that code is doing?

Using a temporary variable is simple, fast, and easily understood. So do it.

I am a computer programmer specializing in building database software with ASP.NET MVC, C#, and SQL Server.