1. 1 Statements: Control Structure Issues (Chapters 15-17 and 19 of Code Complete) Don Bagert CSSE 375, Rose-Hulman October 17, 2006

2. 2 Outline  Selection Statements  Loops  Some Other Statements

3. 3 Selection Statements

4. 4 Definition  A selection statement is a statement where a selection of which statements to execute out of two or more possibilities is determined through the use of a condition of some kind. Examples: if and switch statements in Java and C++

5. 5 Some Selection Statement Issues – 1/3  In general, case (switch) statements should be used whenever possible if three of more cases are involved. However, in most languages, case statements are limited to the comparison of an “integral” variable to a series of values or value ranges, so a nested if-then-else must often be used The Boston rail yard: A view of multiple train switches.

6. 6 Some Selection Statement Issues – 2/3  In a nested if-then-else, “lining up” all of the elses aids readability. Example (in Java): if (NumberInput == 0) NumberOfZeroesCounted++; else if (NumberInput > 0) NumberOfPositivesCounted++; else NumberOfNegativesCounted++;

7. 7 Some Selection Statement Issues – 3/3  Using an “else do nothing” can be useful, especially with nested if statements. Example (using an outline in PDL syntax): if... then if... then... else... With which then will the else be associated?

8. 8 Loops

9. 9 Some Loop Issues – 1/3  A pre-test loop (such as the while statement in Java) is most generic of the loop statements, since it can be used to represent any loop.  However, if the loop definitely needs to be executed at least once (e.g. in a bubblesort) a post-test loop (such as the do-while in Java) should be used instead.

10. 10 Some Loop Issues – 2/3  When for loops should be used varies from language to language  Visual Basic Example (a simple format): for ListIndex = 1 to ListSize  Java and C++ Example (more complex): for (i = 0; i < ListSize && !found; i++)

11. 11 Some Loop Issues – 3/3  Entry from the beginning of and exit from either the beginning or the end of the loop is useful in many ways  Therefore (in my opinion), the use of features such as the “break” in Java should be avoided if at all possible

12. 12 Some Other Statements

13. 13 Goto and Return Statements  The goto is a very powerful statement which you will probably find in frequent use in certain types of legacy code  The return statement has some features of the goto. In my opinion, it should generally not be used for procedures (void functions in Java and C++) and only as the last statement of a function. Example: Return (FunctionValue);

14. 14 Recursion  Recursion should be used only when the complexity is significantly less than that of using a loop  Infinite recursion must be avoided in the same way as infinite loops are

15. 15 Quiz Exercise 5: Coding Standards for Control Statements