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Arrange tests so that the one that s fastest and most likely to be true is performed first. It should be easy to drop through the normal case, and if there are inefficiencies, they should be in processing the uncommon cases. This principle applies to case statements and to chains of if-then-elses. Here s a Select-Case statement that responds to keyboard input in a word processor:
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Visual Basic Example of a Poorly Ordered Logical Test
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Select inputCharacter Case "+", "=" ProcessMathSymbol( inputCharacter ) Case "0" To "9" ProcessDigit( inputCharacter ) Case ",", ".", ":", ";", "!", " " ProcessPunctuation( inputCharacter ) Case " " ProcessSpace( inputCharacter ) Case "A" To "Z", "a" To "z" ProcessAlpha( inputCharacter ) Case Else ProcessError( inputCharacter ) End Select
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The cases in this case statement are ordered in something close to the ASCII sort order. In a case statement, however, the effect is often the same as if you had written a big set of if-then-elses, so if you get an <;$QS>a<;$QS> as an input character, the program tests whether it s a math symbol, a punctuation mark, a digit, or a space before determining that it s an alphabetic character. If you know the likely frequency of your input characters, you can put the most common cases first. Here s the reordered case statement:
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Select inputCharacter Case "A" To "Z", "a" To "z" ProcessAlpha( inputCharacter ) Case " " ProcessSpace( inputCharacter )
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de Complete
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26. Code-Tuning Techniques
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Case ",", ".", ":", ";", "!", " " ProcessPunctuation( inputCharacter ) Case "0" To "9" ProcessDigit( inputCharacter ) Case "+", "=" ProcessMathSymbol( inputCharacter ) Case Else ProcessError( inputCharacter ) End Select
Since the most common case is usually found sooner in the optimized code, the net effect will be the performance of fewer tests. Here are the results of this optimization with a typical mix of characters:
Code-Tuned Time 0.260 2.56 0.260
Language C# Java Visual Basic
Straight Time 0.220 2.56 0.280
Time Savings -18% 0% 7%
Note: Benchmarked with an input mix of 78 percent alphabetic characters, 17 percent spaces, and 5 percent punctuation symbols.
The Visual Basic results are as expected, but the Java and C# results are not as expected. Apparently that s because of the way switch-case statements are structured in C++ and Java since each value must be enumerated individually rather than in ranges, the C++ and Java code doesn t benefit from the optimization as the Visual Basic code does. This result underscores the importance of not following any optimization advice blindly specific compiler implementations will significantly affect the results. You might assume that the code generated by the Visual Basic compiler for a set of if-then-elses that perform the same test as the case statement would be similar. Here are those results:
Code-Tuned Time 0.330 0.460 1.00
Language C# Java Visual Basic
Straight Time 0.630 0.922 1.36
Time Savings 48% 50% 26%
The results are quite different. For the same number of tests, the VB compiler takes about 5 times as long in the unoptimized case, 4 times in the optimized case. This suggests that the compiler is generating different code for the case approach than for the if-then-else approach.
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26. Code-Tuning Techniques
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The improvement with if-then-elses is more consistent than it was with the case statements, but that s a mixed blessing. In C# and VB both versions of the case statement approach are faster than both versions of the if-then-else approach, whereas in Java both versions are slower. This variation in results suggests a third possible optimization, described in the next section.
Compare Performance of Similar Logic Structures
The test described above could be performed using either a case statement or ifthen-elses. Depending on the environment, either approach might work better. Here is the data from the preceding two tables reformatted to present the codetuned times comparing if-then-else and case performance:
if-thenelse 0.330 0.460 1.00 Time Savings -27% 82% 258% Performance Ratio 1:1 6:1 1:4
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