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Swapping Variable Values

Don’t Actually Do This

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:

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

But If You Still Want to Know How

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.

And For Strings…

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.