c# upc-a Boolean Expressions in C#

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19.1 Boolean Expressions
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Except for the simplest control structure, the one that calls for the execution of statements in sequence, all control structures depend on the evaluation of boolean expressions.
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19. General Control Issues
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Using True and False for Boolean Tests
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Use the identifiers True and False in boolean expressions rather than using flags like 0 and 1. Most modern languages have a boolean data type and provide predefined identifiers for true and false. They make it easy they don t even allow you to assign values other than True or False to boolean variables. Languages that don t have a boolean data type require you to have more discipline to make boolean expressions readable. Here s an example of the problem:
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Visual Basic Examples of Using Ambiguous Flags for Boolean Values
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Dim printerError As Integer Dim reportSelected As Integer Dim summarySelected As Integer ... If printerError = 0 Then InitializePrinter() If printerError = 1 Then NotifyUserOfError() If reportSelected = 1 Then PrintReport() If summarySelected = 1 Then PrintSummary() If printerError = 0 Then CleanupPrinter()
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If using flags like 0 and 1 is common practice, what s wrong with it It s not clear from reading the code whether the function calls are executed when the tests are true or when they re false. Nothing in the code fragment itself tells you whether 1 represents true and 0 false or whether the opposite is true. It s not even clear that the values 1 and 0 are being used to represent true and false. For example, in the If reportSelected = 1 line, the 1 could easily represent the first report, a 2 the second, a 3 the third; nothing in the code tells you that 1 represents either true or false. It s also easy to write 0 when you mean 1 and vice versa. Use terms named True and False for tests with boolean expressions. If your language doesn t support such terms directly, create them using preprocessor macros or global variables. The code example is rewritten below using Visual Basic s built-in True and False:
Good Visual Basic Examples of Using True and False for Tests Instead of Numeric Values
Dim printerError As Boolean Dim reportSelected As ReportType Dim summarySelected As Boolean ... If ( printerError = False ) Then InitializePrinter()
For an even better approach to 3 4 making these same tests, see 5 the next code example.
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19. General Control Issues
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If ( printerError = True ) Then NotifyUserOfError() If ( reportSelected = ReportType_First ) Then PrintReport() If ( summarySelected = True ) Then PrintSummary() If ( printerError = False ) Then CleanupPrinter()
Use of the True and False constants makes the intent clearer. You don t have to remember what 1 and 0 represent, and you won t accidentally reverse them. Moreover, in the rewritten code, it s now clear that some of the 1s and 0s in the original Visual Basic example weren t being used as boolean flags. The If reportSelected = 1 line was not a boolean test at all; it tested whether the first report had been selected. This approach tells the reader that you re making a boolean test; it s harder to write True when you mean False than it is to write 1 when you mean 0, and you avoid spreading the magic numbers 0 and 1 throughout your code. Here are some tips on defining True and False in boolean tests:
Compare boolean values to True and False implicitly If your language supports boolean variables, you can write clearer tests by treating the expressions as boolean expressions. For example, write
while ( not done ) ... while ( a = b ) ...
rather than
while ( done = False ) ... while ( (a = b) = True ) ...
Using implicit comparisons reduces the number of terms that someone reading your code has to keep in mind, and the resulting expressions read more like conversational English. The example above could be rewritten with even better style like this:
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