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17. Unusual Control Structures
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require any further cleanup once it detects an error, not returning immediately means that you have to write more code. The following is a good example of a case in which returning from multiple places in a routine makes sense:
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C++ Example of a Good Multiple Return from a Routine
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COMPARISON Compare ( int value1, int value2 ) { if ( value1 < value2 ) { return Comparison_LessThan; } else if ( value1 > value2 ) { return Comparison_GreaterThan; } else { return Comparison_Equal; } }
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Other examples are less clear-cut, as the next section illustrates.
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Use guard clauses (early returns or exits) to simplify complex error processing Code that has to check for numerous error conditions before performing its nominal actions can result in deeply indented code and can obscure the nominal case, as shown here:
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Visual Basic Code That Obscures the Nominal Case
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If file.validName() Then If file.Open() Then If encryptionKey.valid() Then If file.Decrypt( encryptionKey ) Then ' lots of code ... End If End If End If End If
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5 This is the code for the nominal case.
Indenting the main body of the routine inside four if statements is aesthetically ugly, especially if there s much code inside the innermost if statement. In such cases, the flow of the code is sometimes clearer if the erroneous cases are checked first, clearing the way for the nominal path through the code. Here s how that might look:
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Simple Visual Basic Code That Uses Early Exits to Clarify the Nominal Case
' set up, bailing out if errors are found If Not file.validName() Then Exit Sub If Not file.Open() Then Exit Sub If Not encryptionKey.valid() Then Exit Sub If Not file.Decrypt( encryptionKey ) Then Exit Sub ' lots of code ...
The simple code above makes this technique look like a tidy solution, but production code often requires more extensive housekeeping or cleanup when an error condition is detected. Here is a more realistic example:
More Realistic Visual Basic Code That Uses Early Exits to Clarify the Nominal Case
' set up, bailing out if errors are found If Not file.validName() Then errorStatus = FileError_InvalidFileName Exit Sub End If If Not file.Open() Then errorStatus = FileError_CantOpenFile Exit Sub End If If Not encryptionKey.valid() Then errorStatus = FileError_InvalidEncryptionKey Exit Sub End If If Not file.Decrypt( encryptionKey ) Then errorStatus = FileError_CantDecryptFile Exit Sub End If ' lots of code ...
2 This is the code for the nominal case.
With production-size code, the Exit Sub approach creates a noticeable amount of code before the nominal case is handled. The Exit Sub approach does avoid the deep nesting of the first example, however, and, if the code in the first example were expanded to show setting an errorStatus variable, the Exit Sub approach would do a better job of keeping related statements together. When all the dust
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settles, the Exit Sub approach does appear more readable and maintainable, just not by a very wide margin.
Minimize the number of returns in each routine It s harder to understand a routine if, reading it at the bottom, you re unaware of the possibility that it returned somewhere above. For that reason, use returns judiciously only when they improve readability.
17.2 Recursion
In recursion, a routine solves a small part of a problem itself, divides the problem into smaller pieces, and then calls itself to solve each of the smaller pieces. Recursion is usually called into play when a small part of the problem is easy to solve and a large part is easy to decompose into smaller pieces. Recursion isn t useful very often, but when used judiciously it produces exceptionally elegant solutions. Here s an example in which a sorting algorithm makes excellent use of recursion:
Java Example of a Sorting Algorithm That Uses Recursion
void QuickSort( int firstIndex, int lastIndex, String [] names ) { if ( lastIndex > firstIndex ) { int midPoint = Partition( firstIndex, lastIndex, names ); QuickSort( firstIndex, midPoint-1, names ); QuickSort( midPoint+1, lastIndex, names ) } }
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7 Here are the recursive calls.
In this case, the sorting algorithm chops an array in two and then calls itself to sort each half of the array. When it calls itself with a subarray that s too small to sort ( lastIndex <= firstIndex ), it stops calling itself. In general, recursion leads to small code and slow execution and chews up stack space. For a small group of problems, recursion can produce simple, elegant solutions. For a slightly larger group of problems, it can produce simple, elegant, hard-to-understand solutions. For most problems, it produces massively complicated solutions in those cases, simple iteration is usually more understandable. Use recursion selectively.
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