Control-of-Flow Guidelines

A good way to keep yourself out of trouble when it comes to debugging and code maintenance is to employ good control-of-flow coding practices. This might seem like a no-brainer, but experience has shown that the concept definitely warrants reinforcement.

If Statements and Case/Switch Constructs

For simple decisions in your code that have two or fewer possible alternatives, If statements are ideal. They are a quick and easy way to guide the flow of your code. If you need to make a selection based on a long list of possible alternatives, however, If statements can get messy and intertwined. If you are choosing the path of execution based on the value of a single variable, we strongly recommend using a Case construct (Switch construct, in C#). Consider Listings 4.16 and 4.17 that use multiple If statements.

Listing 4.16. Large If Statement Block
public int Foo(int someVar) 
{
     if(someVar==10 || someVar==20 || somevar==30) 
          {//do something here} 
     else if(someVar==15 || someVar==25 || someVar==35) 
          {//do something here} 
     else if(someVar==40 || someVar==50 || someVar==60) 
          {//do something here} 
     else 
          {//do something here} 
}

Listing 4.17. Large If Statement Block (Visual Basic .NET)
Public Function Foo(someVar As Integer) As Integer 
     If someVar = 10 Or someVar = 20 Or someVar = 30 Then 
          ' do something here 
     ElseIf someVar = 15 Or someVar = 25 Or someVar = 35 Then 
          ' do something here 
     ElseIf someVar = 40 Or someVar = 50 Or someVar = 60 Then 
          ' do something here 
     Else 
          ' do something here 
     End If 
End Function

Although you can tell what is going on upon close examination of the code, it might not be apparent when you are scanning it for bugs. A more concise and organized way of accomplishing this task is to use a Case/Switch construct as shown in Listings 4.18 and 4.19.

Listing 4.18. Switch Block (C#)
public int Foo(int someVar) 
{
     switch(someVar) 
     {
          case 10: case 20: case 30: 
               //do something here 
               break; 
          case 15: case 25: case 35: 
               //do something here 
               break; 
          case 40: case 50: case 60: 
               //do something here 
               break; 
          default: 
               //do something here 
               break; 
     } 
}

Listing 4.19. Case Statement Block (Visual Basic .NET)
Public Function Foo(someVar As Integer) As Integer 
     Select Case someVar 
          Case 10, 20, 30 
               ' do something here 
          Case 15, 25, 35 
               ' do something here 
          Case 40, 50, 60 
               ' do something here 
          Case Else 
               ' do something here 
     End Select 
End Function

Function and Loop Exit Points

Some of the most elusive bugs in applications arise from improper use of function and loop exit points. As a general rule, a function or loop should have only one exit point. Why should this be so? If a function or loop has more than one exit point, then it is more difficult to track down which exit point your code is using when you are debugging. An exit point could be hidden in your code, leaving you scratching your head when you try to determine why your code is not working properly. Consider the following function that violates the single exit point rule (shown in Listings 4.20 and 4.21).

Listing 4.20. Code with Multiple Exit Points
public int Foo(int someVar) 
{
      int i = 0; 

      for(i=0;i<=100;i++) 
      {
            //do something to someVar 
            someVar = i; 
            //see whether you need to get out early 
            if(someVar==50) 
                 {break;} 

            if((someVar * 2) % 10 == 0) 
                 {break;} 
      } 

      if(i < 100) 
           {return 1;} 

      return 0; 
}

Listing 4.21. Code with Multiple Exit Points (Visual Basic .NET)
Public Function Foo(someVar As Integer) As Integer 
      Dim i As Integer 

      For i = 1 To 100 
           ' do something to someVar 
           someVar = i 

           ' see whether you need to get out early 
           If someVar = 50 Then 
                 Exit For 
           End If 

           If (someVar * 2) Mod 10 = 0 Then 
                 Exit For 
           End If 
      Next i 

      If i < 100 Then 
           ' loop ended prematurely, so get out 
           Return 1 
      End If 

      Return 0 
End Function

You would need to set breakpoints and step through your code line by line to determine what is going on here. A more effective approach is to use status flags. Status flags can be included in loop definitions to determine when the loop should end. Status flags can also be used to set return codes at the single function exit point. Listings 4 .22 and 4.23 demonstrate this concept.

Listing 4.22. Code with a Single Exit Point
public int Foo2(int someVar) 
{
      int status = 0; 

      for(int i=0;i<=100;i++) 
      {
            //do something to someVar 
            someVar = i; 

            //see whether you need to get out early 
            if(someVar==50) 
                   {status = 1;} 

            if((someVar * 2) % 10 == 0) 
                   {status = 1;} 

            //single place where loop can terminate early 
            if(status==1) {break;} 
      } 

      return status; 
}

Listing 4.23. Code with Single Exit Point (Visual Basic .NET)
Public Function Foo(someVar As Integer) As Integer 
     Dim i As Integer = 0 
     Dim status As Integer = 0 

     For i = 1 To 100 
          ' do something to someVar 
          someVar = i 

          ' see whether you need to get out early 
          If someVar = 50 Then 
                status = 1 
           End If 

           If (someVar * 2) Mod 10 = 0 Then 
                status = 1 
           End If 

           ' single place where loop can terminate early 
           If status = 1 Then Exit For 
     Next i 

     Return status 
End Function

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