Recipe 6.4. Swapping Two Integers Without Using a Third


Problem

You want to swap the values of two integer variables without creating a third.

Solution

Sample code folder: Chapter 06\IntegerSwap

Use the exclusive-or bit manipulation function to do the trick.

Discussion

Nowadays efforts to save the space of a single variable in memory seem kind of silly, but this recipe nevertheless demonstrates an interesting technique for swapping two numbers without creating a third variable. More importantly, it demonstrates how bit-manipulation functions can be quite useful in Visual Basic 2005. Here's the sample code:

 Dim result As String Dim firstValue As Integer Dim secondValue As Integer ' ----- Set the initial test values. firstValue = 17 secondValue = 123 result = String.Format("Before swap: {0}, {1}", _    firstValue, secondValue) result &= vbNewLine ' ----- Swap the values at the bit level. firstValue = firstValue Xor secondValue secondValue = firstValue Xor secondValue firstValue = firstValue Xor secondValue result &= String.Format("After swap: {0}, {1}", _    firstValue, secondValue) MsgBox(result) 

The above code loads values into integers firstValue and secondValue, then swaps their values by applying three successive Xor operators on them. The Xor operator combines the two integers on a bit-by-bit basis, resulting in a 1 bit whenever the original bits are different and a 0 when they are the same. Once these three Xor operations have been performed, the original contents of the two integers will have migrated to the opposite locations in memory. Figure 6-4 shows the results displayed by the sample code.

Figure 6-4. Swapping two integers using Xor


See Also

Search for "Xor operator" in Visual Studio Help for more information.




Visual Basic 2005 Cookbook(c) Solutions for VB 2005 Programmers
Visual Basic 2005 Cookbook: Solutions for VB 2005 Programmers (Cookbooks (OReilly))
ISBN: 0596101775
EAN: 2147483647
Year: 2006
Pages: 400

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