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C++ Example of Testing the Most Common Case First
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This test, the most common, is now done first.
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} else if ( IsPunctuation( inputCharacter ) ) { characterType = CharacterType_Punctuation; } else if ( IsDigit( inputCharacter ) ) { characterType = CharacterType_Digit; } else if ( IsControl( inputCharacter ) ) { characterType = CharacterType_ControlCharacter; } if ( IsLetter( inputCharacter ) ) { characterType = CharacterType_Letter;
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6 This test, the least common, is now done last
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Make sure that all cases are covered Code a final else clause with an error message or assertion to catch cases you didn t plan for. This error message is intended for you rather than for the user, so word it appropriately. Here s how you can modify the character-classification example to perform an other cases test:
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15. Using Conditionals
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This is also a good example of how you can use a chain of if-then-else tests instead of deeply nested code. For details on this technique, see Section 19.4, Taming Dangerously Deep Nesting.
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C++ Example of Using the Default Case to Trap Errors
if ( IsLetter( inputCharacter ) ) { characterType = CharacterType_Letter; } else if ( IsPunctuation( inputCharacter ) ) { characterType = CharacterType_Punctuation; } else if ( IsDigit( inputCharacter ) ) { characterType = CharacterType_Digit; } else if ( IsControl( inputCharacter ) ) { characterType = CharacterType_ControlCharacter; } else { DisplayInternalError( "Unexpected type of character detected." ); }
Replace if-then-else chains with other constructs if your language supports them A few languages Visual Basic and Ada, for example provide case statements that support use of strings, enums, and logical functions. Use them. They are easier to code and easier to read than if-then-else chains. Here s how the code for classifying character types would be written using a case statement in Visual Basic;
Visual Basic Example of Using a case Statement Instead of an if-thenelse Chain
Select Case inputCharacter Case "a" To "z" characterType = CharacterType_Letter Case " ", ",", ".", "!", "(", ")", ":", ";", " ", "-" characterType = CharacterType_Punctuation Case "0" To "9" characterType = CharacterType_Digit Case FIRST_CONTROL_CHARACTER To LAST_CONTROL_CHARACTER characterType = CharacterType_Control Case Else DisplayInternalError( "Unexpected type of character detected." ) End Select
15.2 case Statements
The case or switch statement is a construct that varies a great deal from language to language. C++ and Java support case only for ordinal types taken one value at
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15. Using Conditionals
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a time. Visual Basic supports case for ordinal types and has powerful shorthand notations for expressing ranges and combinations of values. Many scripting languages don t support case statements at all. The following sections present guidelines for using case statements effectively.
Choosing the Most Effective Ordering of Cases
You can choose from among a variety of ways to organize the cases in a case statement. If you have a small case statement with three options and three corresponding lines of code, the order you use doesn t matter much. If you have a long case statement for example, a case statement in an event-driven program order is significant. Here are some ordering possibilities:
Order cases alphabetically or numerically If cases are equally important, putting them in A-B-C order improves readability. A specific case is easy to pick out of the group. Put the normal case first If you have one normal case and several exceptions, put the normal case first. Indicate with comments that it s the normal case and that the others are unusual. Order cases by frequency Put the most frequently executed cases first and the least frequently executed last. This approach has two advantages. First, human readers can find the most common cases easily. Readers scanning the list for a specific case are likely to be interested in one of the most common cases. Putting the common ones at the top of the code makes the search quicker.
In this instance, achieving better human readability also supports faster machine execution. Each case represents a test that the machine performs at run time. If you have 12 cases and the last one is the one that needs to be executed, the machine executes the equivalent of 12 if tests before it finds the right one. By putting the common cases first, you reduce the number of tests the machine must perform and thus improve the efficiency of your code.
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