1. Selection Chapter 6

2. Objectives • Expand on introduction to structures • Examine if statement in more detail • Study use of switch statement to implement multialternative selections • Observe use of boolean expressions for modeling logical circuits • Study architecture of typical computer systems • First look at class mutator methods C++ An Introduction to Computing, 3rd ed.

3. Introductory Example • Consider Big 10 Conference mascots • Univ. of Illinois Fighting Illini • Ohio State Univ. Buckeyes . . . • We seek a mascot() function • Given name of a university • Returns name of mascot C++ An Introduction to Computing, 3rd ed.

4. Objects, Operations Operations • Compare university to "Michigan State";if true, return "Spartans" • Compare university to "Ohio State";if true, return "Buckeyes" . . . C++ An Introduction to Computing, 3rd ed.

5. Algorithm • if university is "Illinois" return "Fighting Illini", • Otherwise if university is "Ohio State" return "Buckeyes", • Otherwise if … • Note function source code, Figure 6.1 • Driver program, Figure 6.2 • Sample run C++ An Introduction to Computing, 3rd ed.

6. Single branchif (boolean_exp) statement Dual branchif (boolean_exp) statement1 else statement2 Multibranchif (boolean_exp) statementelse if (boolean_exp) statementelse if … Selection: the if Statement C++ An Introduction to Computing, 3rd ed.

7. Selection: the if Statement • Note multibranch if • Appears to be a different version • Actually is an if statement with anotherif statement as the statement of the elseif (boolean_exp) statementelse if (boolean_exp) statementelse if … C++ An Introduction to Computing, 3rd ed.

8. Cond1 F T Cond2 Stmt1 T F . . . Stmt2 CondN T F StmtN StmtN+1 The Multi-branch if • The if’s final form has a nested if as Statement2: if (Cond1) Stmt1 else if (Cond2) Stmt2 ... else if (CondN) StmtN else StmtN+1 C++ An Introduction to Computing, 3rd ed.

9. Multibranch if • Note which else goes with which if C++ An Introduction to Computing, 3rd ed.

10. In a nested if statement, an else is matched with the nearest preceding unmatched if. The Dangling else Problem • Consider:if (x > 0) if (y > 0) z = sqrt (x) + sqrt(y); else cerr << " * * Unable to Compute * *"; • Which if does the else go with? C++ An Introduction to Computing, 3rd ed.

11. Warning: Confusing = and == • True and false in C++ • An integer value of 0 interprets as false • A non zero value interprets as true • Assignments are expressions x = 7; • The value is assigned to the variable … and • The expression has a value (the value assigned) • What happens when you writeif (x = 7) … C++ An Introduction to Computing, 3rd ed.

12. Warning: Confusing = and == • When you writeif (x = 7) … • The value is assigned to the variable • The expression has that value (in this case non zero) • The value of the expression is used to select the true or false branch of the if statement • The program will • Compile and run without crashing • But will probably not execute as expected C++ An Introduction to Computing, 3rd ed.

13. Selection: The switch Statement • Consider an extension to the problem of Chapter 4 • Temperature Conversions • Generalize the problem • User chooses which conversion is to be performed C++ An Introduction to Computing, 3rd ed.

14. Objects, Operations Operations: • Display prompts, MENU on the screen • Read temperature (a double) from the keyboard • Read conversion (a char) from the keyboard • Select conversion function corresponding to conversion, apply it to temperature C++ An Introduction to Computing, 3rd ed.

15. Algorithm • Display menu via cout • Read conversion from cin • Display prompt for temperature via cout • Read temperature from cin • If conversion is 'A' or 'a' Convert from F to C, store in resultOtherwise if conversion is 'B' Convert from C to F, store in result. . .Otherwise display an error message C++ An Introduction to Computing, 3rd ed.

16. Drawback The multi-branch if has non-uniform execution time: Menu option 'A' requires 1 comparison Menu option 'B' requires 2 comparisons ... Menu option 'F' requires 6 comparisons • Computations that are “later” in the if take longer. • There are situations where the time to select one of many statements must be uniform. C++ An Introduction to Computing, 3rd ed.

17. Coding and Testing • C++ switch statement provides more convenient way to do this • Note source code, Figure 6.3 • Sample run for Figure 6.3 • Note convenience of the switch statement C++ An Introduction to Computing, 3rd ed.

18. Keywords Int or char expression Each StatementList usually ends with a break or return statement. Each caseList is one or more cases of this form: case ConstantValue : The switch Statement • Form:switch (expression) { case_list1: statement_list1; case_list2: statement_list2; … default: statement_listn+1;} C++ An Introduction to Computing, 3rd ed.

19. Warning C++ switch statements exhibit drop-through behavior. 1. Expression is evaluated. 2. If Expression == ConstantValuei: Control jumps to the Statement after ConstantValuei. 3. Control continues within the switch statement until: a. The end of the switch is reached; b. A break is executed, terminating the switch; c. A return is executed, terminating the function; or d. An exit() is executed, terminating the program. Note the use of the break statement in Figure 6.3 Note the use of the return statement in Figure 6.4 C++ An Introduction to Computing, 3rd ed.

20. Using switch in a Function • Convert numeric codes to names • University code of 1, 2, … for "freshman", "sophomore", … • Objects • Operations • Return the name of the year corresponding to yearCode C++ An Introduction to Computing, 3rd ed.

21. Using switch in a Function • AlgorithmIf yearCode is 1 return "Freshman"Otherwise, if yearCode is 2 return "Sophomore" . . . • View source code, Figure 6.4 • Driver program, Figure 6.5 • Sample run for Figure 6.5 C++ An Introduction to Computing, 3rd ed.

22. Cases with No Action • In some situations the expression of a switch statement may be a legal value but: • No action takes place for that value • Solution: • Include that value in the case list • But include no statement, only the break;switch (option) {case 1 : doWhatever(); break;case 2 : doSomethingElse(); break; . . .case 9: case 10: break;} C++ An Introduction to Computing, 3rd ed.

23. When to Use switch • Choose switch over if-else-if when: • The equality == comparison is being performed • The same expression is being compared in each condition … and … • The value to which the expression is being compared is int compatible C++ An Introduction to Computing, 3rd ed.

24. Problem • Consider the weighting of different grades. • Tests, quizzes, etc. • Average scores calculated, multiplied by weighting factor, then added to give final weighted average • Program needed to receive averages for different types of grades will compute letter grade • HW = 20%, tests = 50%, final exam = 30% C++ An Introduction to Computing, 3rd ed.

25. Behavior For grade calculation : Enter homework average : 80 Enter test average : 80 Enter final exam : 80 Letter grade given : B C++ An Introduction to Computing, 3rd ed.

26. Objects, Operations Operations • Display prompt on screen • Read three quantities from keyboard • Compute weighted average • Compute letter grade • Display letter grade C++ An Introduction to Computing, 3rd ed.

27. Grade Computation Algorithm • Prompt for homework average, test average, exam score • Enter homeworkAverage, testAverage, examScore • CalculatefinalAverage = HOMEWORK_WEIGHT * homeworkAvarage + TEST_WEIGHT * testAverage + EXAM_WEIGHT * examScore • Calculate and display letter grade corresponding to finalAverage • We will use a function to accomplish steps 3 and 4 above C++ An Introduction to Computing, 3rd ed.

28. Function Objects, Operations Operation: Realize need for selective execution according to table: C++ An Introduction to Computing, 3rd ed.

29. Coding, Testing • Note adjustments for use of switch • Convert weightedAverage to an int • Reduce number of casesswitch int(weightedAverage) / 10 { … • Note function source code, Figure 6.6 • Full program source code Figure 6.7 • Sample run of Figure 6.7 C++ An Introduction to Computing, 3rd ed.

30. Conditional Expression • Form:condition ? expression1 : expression2 • Behavior • condition evaluated • If it is true, expression1 returned as result • If it false, expression2 returned as result • Try it out … what gets printed?double a = 5, b = 10, c = -3;cout << (b * b – 4 * a * c > 0 ? "real root" : "imaginary root") ; C++ An Introduction to Computing, 3rd ed.

31. Boolean Logic and Digital Design • Based on work by George Boole • Note basic rules, pg 316 • Use of these rules became important with invention of digital computer • Circuits C++ An Introduction to Computing, 3rd ed.

32. Boolean Logic and Digital Design • Combine to create half adder circuit • digit1, digit2, sum, and carry are all binary digits (either 0 or 1) • Note source code Figure 6.8, sample run C++ An Introduction to Computing, 3rd ed.

33. OBJECTive Thinking:Mutator Methods • Methods which allow the programmer to change the value of an instance variable • Includes input methods • User can define an object using values read from an istream • In both types of methods we write special validation code • Ensures valid values for instance variables C++ An Introduction to Computing, 3rd ed.

34. Mutators and Input Methods • For the class Name, Figure 6.9 • Note driver code, Figure 6.10 • Contrast accessor and output methods with mutators and input methods • Accessors retrieve attribute values • Mutators change attribute values • Accessors, output methods declared as const since they do not change attribute values • Mutators, input methods NOT declared const C++ An Introduction to Computing, 3rd ed.

35. Mutators and Input Methods • For Sphere classvoid setRadiusAndDensity (double radius, double density); …void readRadiusAndDensity(istream& in); • See source code of Sphere.h, Figure 6.11 • Sphere.cpp, Figure 6.12 • Driver to test class sphere, Figure 6.13 C++ An Introduction to Computing, 3rd ed.