Control Statements

1. Control Statements Ch 5, 6

2. Control Structures • Sequence • Statements executed one by one • { … } // block • Selection • One of several statements is selected • if (if…else), switch • Repetition (Loop) • Same statements executed multiple times • while, do…while, for (foreach)

3. Block • Several statements bracketed by { and } • Statements within a block executed sequentially • Treated as a single statement • Can be used wherever one statement is legal

4. Selection Statements • if • Single-selection • if…else • Double-selection • switch • Multiple-selection

5. ifExamples if(grade >= 60) Console.WriteLine("Passed");

6. UML Activity Diagram • Describes execution sequence

7. if…else Examples if(grade >= 60) Console.WriteLine("Passed"); else Console.WriteLine("Failed");

8. UML Activity Diagram

9. ifSyntax if (<boolean_expression>) <yes_statement>else <no_statement> • Each alternative is only ONEstatement! • To have multiple statements in either branch, use a block • else is optional

10. Boolean Expression • Gives a Boolean result (true or false) • A value of type bool bool isGraduate = true; • Comparison operation grade >= 60 • Logical operation grade >= 60 && isGraduate

11. Comparison Operators

12. Logical Operators • Combine multiple Boolean expressions VB C# Example in C# && grade >= 60 && isGraduate AND OR || m=="MIS" || m=="CS" NOT ! ! isGraduate

13. Nested if Statement if(grade >= 90) Console.WriteLine("A"); else if(grade >= 80) Console.WriteLine("B"); else if(grade >= 70) Console.WriteLine("C"); else Console.WriteLine("F");

14. Equivalent To // Nested if in a different form if(grade >= 90) Console.WriteLine("A"); else if(grade >= 80) Console.WriteLine("B"); else if(grade >= 70) Console.WriteLine("C"); else Console.WriteLine("F");

15. Not Equivalent To Multiple if if(grade >= 90) Console.WriteLine("A"); if(grade >= 80) Console.WriteLine("B"); if(grade >= 70) Console.WriteLine("C"); else Console.WriteLine("F");

16. switch Statement

17. switch Example

18. More on switch • Needs a break before end of each case • The default is optional • Must use exact values in cases • Can't test ranges, such as >=93, 90 to 93 • Can every switch be translated into if? • Can every if be translated into switch?

19. Select Case in VB is more general! SELECT CASE grade CASE >= 93 letterGrade = "A" CASE 90 to 93 letterGrade = "A-" … CASE ELSE letterGrade = "F" END SELECT

20. Loop Statements • while, for (foreach) • Loop body executed ZERO or many times • do…while • Loop body executed ONE or many times

21. whileExample // find the first power of 3 // larger than 100 int product = 3; while (product <= 100) product = 3 * product;

22. UML Activity Diagram

23. Formulating Algorithms with Loops • Counter-controlled loop • Use a variable (called counter) to control the number of iterations • Sentinel-controlled loop • Use a special sentinel value to indicate "end of data entry"

24. Counter-Controlled Loop(Fig. 5.7)

26. Results

27. Sentinel-Controlled Repetition(Fig. 5.8)

30. Results

31. Three Key Items of a Loop • Initialization • Set initial state of the data before the loop • Where to start • Loop Continuation Condition • Determine whether continue to next iteration • When to stop • Update • During each iteration, some data must change • How to approach the end

32. Infinite Loop • A loop that can never terminate • A common error • Carefully design the three key items to avoid it

33. for Loop for (initialization; loopContinuationCondition; increment) statement;

34. while and for • The previous for is equivalent to the following while, unless there is a continue initialization; while ( loopContinuationCondition ) { statement; increment; }

35. Exercise • Translate the previous while examples into for

36. do…while Loop • Similar to while • Tests loop-continuation after performing body of loop • Loop body executes at least once • Can the previous while examples be changed to do…while?

37. Alter the Flow of Control • break • Forces loop to exit immediately • continue • Forces loop to skip rest of loop body and start another iteration immediately

38. A break Example

39. Result

40. A continue Example

41. Result

42. Summary • Control structures • Sequence, selection, repetition • Selection • if (if…else), switch • switch can't test ranges • Repetition (Loop) • while, for (foreach) • do…while must go thru the loop body at least once • Alter the flow of control • break, continue

43. Aside: Other Topics • Integer Division • Type Casting • Conditional Operator • Compound Assignment Operators • Increment and Decrement Operators

44. Integer Division • Results are integers. • Remainder discarded ! Not rounded ! 7 / 5→1, not 1.4, not 2. • % returns the remainder 7 % 5→2 • m / n * n + m % n→ m 7 / 5 * 5 + 7 % 5→7

45. Division in VB / C# VB C# Division (real) / / Division (integer) \ / Modulus MOD %

46. Division Example (by Instructor) Enter first real number: 7 Enter second real number: 5 x = 7, y = 5 x / y = 1.4 x / y * y = 7 Enter first integer: 7 Enter second integer: 5 m = 7, n = 5 m / n = 1 m % n = 2 m / n * n = 5 m / n * n + m % n = 7

47. Type Conversion • Explicit conversion • Explicitly request type conversion average = (double)total / gradeCounter; • Implicit conversion • Performed implicitly when needed • Promotion (from a smaller type to a larger type) only

48. Conditional Operator • The only ternary operator Console.WriteLine(grade >=60 ? "Passed": "Failed"); • A Boolean expression and two possible values • If the Boolean expression is true, take the first value; • Otherwise, take the second value

49. Compound Assignment • Shorthand of assignment expression

50. Increment/Decrement