c# upc-a Comparison of the Approaches in Visual C#

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Comparison of the Approaches
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Each of the four methods has something to be said for it. The goto approach avoids deep nesting and unnecessary tests but of course has gotos. The nested-if approach avoids gotos but is deeply nested and gives an exaggerated picture of the logical complexity of the routine. The status-variable approach avoids gotos
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17. Unusual Control Structures
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and deep nesting but introduces extra tests. The try-finally approach avoids both gotos and deep nesting, but isn t available in all languages. The try-finally approach is the most straightforward in languages that provide try-finally and in code bases that haven t already standardized on another approach. If try-finally isn t an option, the status-variable approach is slightly preferable to the first two because it s more readable and it models the problem better, but that doesn t make it the best approach in all circumstances. Any of these techniques works well when applied consistently to all the code in a project. Consider all the trade-offs, and then make a project-wide decision about which method to favor.
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gotos and Sharing Code in an else Clause
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One challenging situation in which some programmers would use a goto is the case in which you have two conditional tests and an else clause and want to execute code in one of the conditions and in the else clause. Here s an example of a case that could drive someone to goto:
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C++ Example of Sharing Code in an else Clause with a goto
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if ( statusOk ) { if ( dataAvailable ) { importantVariable = x; goto MID_LOOP; } } else { importantVariable = GetValue(); MID_LOOP: // lots of code ... }
This is a good example because it s logically tortuous it s nearly impossible to read as it stands, and it s hard to rewrite correctly without a goto. If you think you can easily rewrite it without gotos, ask someone to review your code! Several expert programmers have rewritten it incorrectly. You can rewrite the code in several ways. You can duplicate code, put the common code into a routine and call it from two places, or retest the conditions. In most languages, the rewrite will be a tiny bit larger and slower than the original, but it will be extremely close. Unless the code is in a really hot loop, rewrite it without thinking about efficiency.
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17. Unusual Control Structures
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The best rewrite would be to put the // lots of code part into its own routine. Then you can call the routine from the places you would otherwise have used as origins or destinations of gotos and preserve the original structure of the conditional. Here s how it looks:
C++ Example of Sharing Code in an else Clause by Putting Common Code into a Routine
if ( statusOk ) { if ( dataAvailable ) { importantVariable = x; DoLotsOfCode( importantVariable ); } } else { importantVariable = GetValue(); DoLotsOfCode( importantVariable ); }
Normally, writing a new routine is the best approach. Sometimes, however, it s not practical to put duplicated code into its own routine. In this case you can work around the impractical solution by restructuring the conditional so that you keep the code in the same routine rather than putting it into a new routine. Here s how it looks:
C++ Example of Sharing Code in an else Clause Without a goto
if ( ( statusOk && dataAvailable ) || !statusOk ) { if ( statusOk && dataAvailable ) { importantVariable = x; } else { importantVariable = GetValue(); } // lots of code ... }
Anot her approach to this problem 6 is to use a decision table. For 7 details, see 18, 8 Table-Driven Methods.
5 CROSS-REFERENCE
This is a faithful and mechanical translation of the logic in the goto version. It tests statusOK two extra times and dataAvailable one, but the code is equivalent. If retesting the conditionals bothers you, notice that the value of statusOK doesn t need to be tested twice in the first if test. You can also drop the test for dataAvailable in the second if test.
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