2. Copyright © 1998-2015 Curt Hill The IF Revisited If part 4 Style and Testing

3. Copyright © 1998-2015 Curt Hill Now what? There are several more issues that need consideration Nesting Testing If problems

4. Copyright © 1998-2015 Curt Hill Nesting Any statement may be the THEN or ELSE statement of an if It is most often a compound statement The interesting one is an if in an if The usual problem is how the elses match the ifs

5. Copyright © 1998-2015 Curt Hill Which else? The else may only match one of these Which one is it? The answer is 3! if(a>b) // 1 if(b>c) // 2 if(c>d)// 3 x = y; else x = z;

6. Copyright © 1998-2015 Curt Hill Matching Elses The general rule is that an else matches the closest, previous, unmatched if The language ignores white space and also the layout on the page –Thus indenting has no relevance to the compiler –It may make the program more (or less) readable

7. Copyright © 1998-2015 Curt Hill A Matched Else How is an if matched? An else is encountered for it It is enclosed within a compound statement An unrelated statement follows

8. Copyright © 1998-2015 Curt Hill Three examples if(a>b) // 1 if(b>c){ // 2 if(c>d)// 3 x = y; } else // matches 2 x = z; if(a>b) // 1 if(b>c) // 2 if(c>d)// 3 x = y; else x = 2*y; else // matches 2 x = z; if(a>b) // 1 x = y; y = x*z; else // error x = z;

9. Copyright © 1998-2015 Curt Hill Testing Ifs complicate our testing With only sequential flow we test it once and it is covered With multiple paths one test is generally inadequate Testing strategies then come into view

10. Copyright © 1998-2015 Curt Hill Every Statement The first strategy is to make sure that in our testing every statement is tested once We generate test data accordingly We have enough test data if each if has both its then and else exercised once This usually means several runs

11. Copyright © 1998-2015 Curt Hill Every statement if(a>b) { // 1 b = a++ * 2; c /= 2; } else // 2 c = a * b / 2; Two tests are needed. One forces a > b and the other a <= b 1 2

12. Copyright © 1998-2015 Curt Hill A path is a unique way through the code 1 2 3 4 There are four paths 1, 3 2, 3 1, 4 2, 4

13. Copyright © 1998-2015 Curt Hill Test Data Again In the above, every statement testing only needs two runs –The paths {1,3} and {2,4} executes every statement However, there may be interactions between 1 and 4 that are not exercised Thus every path testing is always as good or better than every statement –In programs without decisions or loops one set does both

14. Copyright © 1998-2015 Curt Hill How ifs increase paths An if added at the end of a sequence doubles the number of paths An if nested within an if adds one path to the existing number The generalities do not always capture the actual practice Reconsider the quadratic formula evaluation

15. Copyright © 1998-2015 Curt Hill Two different approaches if(discrim < 0) … else if(discrim == 0){ … } else { … } if(discrim < 0){ … } if(discrim == 0){ … } if(discrim > 0){ … }

16. Copyright © 1998-2015 Curt Hill Commentary on above The nested if version is to be preferred If any of the conditions are true then no further ones will be checked –Typically check the most likely one first rather than the straight order I used However, provided that discrim is not changed in the ifs both have three paths

17. Copyright © 1998-2015 Curt Hill Largest of three variables Here are two ways to display the largest of three variables, a, b and c if(a>b) if(a>c) cout << a; else cout c) cout << b; else cout << c; double big; big = a; if(big<b) big =b; if(big<c) big = c; cout << big; Which do you like better? Why?

18. Copyright © 1998-2015 Curt Hill Comparison The first one has two comparisons and one write The second one has two comparisons, one to three assignments, one write and needs one temporary variable Both have four paths Most professional programmers like the second one Easier to categorize where you are

19. Copyright © 1998-2015 Curt Hill Paths and Complexity More paths cause more complexity Complexity promotes errors As programmers we work hard to subdivide the problem to manage the complexity The magic number is 7 ± 2 TMI and cockpit stories

20. Error Checking If an error is found do one of two things –Do no other processing –Correct and continue At least two ways to do the former The latter cannot be done in general way –For any particular program there may be a good correction Copyright © 1998-2015 Curt Hill

21. Do Nothing Further Again two ways –Place rest of method in an else –Use the return statement Big else: if(ed->GetValue().length()<1) // complain else { // rest of method in this Copyright © 1998-2015 Curt Hill

22. Problems with this If there are lots of separate checks then the ifs and else get nested rather deeply –Deep nesting causes problems with complexity Use the return statement: if(ed->GetValue().length() SetValue(“Empty field.”); return; } No else is needed Copyright © 1998-2015 Curt Hill

23. Return Statement Two forms: In a void method, such as an event handler: return ; In a method that returns a value: return expr; –This form will be considered during function definition presentations Return exits the method and optionally returns a value –No else is needed following Copyright © 1998-2015 Curt Hill

24. Correction If there is an obvious correction use that Empty fields imply zero: if(edit1->GetValue().length() SetValue(“0”); } d = edit1->GetValue().ToInt(); However, there is usually no easy fix We will examine this again later Copyright © 1998-2015 Curt Hill

25. Separating Ranges Often we want to classify a value by range For instance a descriptive word for the age of person: infant is less than 1 year preschool is less than 5 years etc This is usually done with a series of ifs –There is a right and wrong way Copyright © 1998-2015 Curt Hill

26. Wrong Way Not nested: if(age =1 && age =5 && age < 12) … This works but there is lots of redundancy If s was set to “preschool” do we want to do another test Copyright © 1998-2015 Curt Hill

27. Right Way Nested with simple conditions: if(age < 1) s = “infant”; else if(age < 5) s = “preschool”; else if(age < 12) … Also works, but the first one to find it eliminates any further tests Copyright © 1998-2015 Curt Hill

28. Not so right Way Nested with ANDed conditions: if(age =1 && age =5 && age < 12) … Also works, but the age>=1 can never be false because of the nesting Copyright © 1998-2015 Curt Hill