Logical Operators and Control Statements

1. Logical Operators and Control Statements

2. Boolean • Boolean takes true or false values. • Used in deciding states. • Commonly used in conditional statements and loops. • A logical operation is a formula that has a true or false result. • Boolean variables are defined with “bool” keyword. • bool bVal = true;

3. Example

4. Boolean Operators • Used for constructing Boolean expressions. • and  && • or  || • not  ! • equal  == • Not equal  != • Comparison  <, >, <=, >=

5. And (&&) Both values are to be “true”. • true && true = true • false && true = false • true && false = false • false && false = false

6. Or (||) At least one value is to be “true”. • true || true = true • false || true = true • true || false = true • false || false = false

7. Not (!) Reverses the value. • !true = false • !false = true

8. Equal (==) Returns true if two values are equal. • 1 == 2 • 1 == 0 • 42 == 42 • variable1 == otherVariable

9. Not Equal (!=) Returns true if two values are not equal. • 1 != 2 • 1 != 0 • 42 != 42 • a != variable

10. Comparison (>,<,>=,<=) • 1 < 2 • 0 > 1 • 42 <= 42 • age>= 18

11. Operator Priority • Parantheses • Not (!) • Comparison(<, >, <=, >=) • Equals(==) • Not Equals(!=) • And(&&) • Or(||) Highest Lowest

12. Example

13. Common Mistakes • Writing logical expressions without taking operator priorities in to account. • Writing just one equals(=) instead of two (==).

14. Control statements

15. if – else • Decides which code block will run depending on the result of logical expression. • Logical expressions retuns boolean values. If return value is true then code blocks within th if statement will be executed. Otherwise else statement will be executed.

16. if – else • A simple if case decides whether corresponding code block will be executed or not. • A structure that is composed of ifandelse will execute two different code blocks depending on the logical expression. • if/elsestatement can be used in nested forms to express more complex stuations.

17. If case • If control statements has two forms: if ( expression ) { codes; } • or if ( expression ) code;

18. Example

19. if – else if – else • Common if use case : • We can also write if – else as: No semicolons here!!!! if ( expression ) statement; No semicolons here!!!! if (expression ) statement1; else statement2; No semicolons here!!!!

20. if – else if – else if ( boolean expression is true ) statement ; else if (boolean expression is true) statement ; else statement ;

21. Multiple if – else if – else • When there are multiple conditions with one line of statements, we can use if-else structure seen below: if (boolean expression is true ) statement; else if (boolean expression is true ) statement; else if (boolean expression is true ) statement; else statement;

22. if – else if – else blocks if (boolean expression is true ) { statement blocks; } else if (boolean expression is true ) { statement blocks; } else { statement blocks; }

23. Example: Comparing two numbers

24. Exercise • Meteorogy department asks for a program that converts humudity values to a human readable format. • Program will write to output the following results depending input ranges between 0 and 100. • %20 and lower “too dry" • %21 - %40: “dry" • %41 - %60: “little dry" • %61 - %80: “little moist" • %81 and higher: “moist“ • Write a program that takes a humudiy value from user and writes it’s corresponding human readable output.

25. Nested if conditions • C# compiler matches else condition to nearest if condition. Because of this, using {} paranthesis makes life easier and your code readable. if (humudity < 20) if (temperature <= 0) Console.WriteLine(“A cold and dry day.") if (ruzgar < 10) Console.WriteLine(“Wonderful, no wind!"); else Console.WriteLine(“low moist and higher than 0 degrees"); else if (humudity < 60) if (temperature <= 0) Console.WriteLine(“cold and moderate moisture."); else Console.WriteLine(“Higher than 0, moderate moisture."); ?

26. Nested if Conditions if (humudity < 20) { if (temperature <= 0) { Console.WriteLine("A cold and dry day.") if (ruzgar < 10) { Console.WriteLine(“Wonderful, no wind!"); } } else { Console.WriteLine(“low moist and higher than 0"); } } else if (humudity < 60) { if (temperature <= 0) { Console.WriteLine(“cold and moderate moist."); } else{ Console.WriteLine(“Hidher than 0, moderate moist"); } }

27. switch – case conditions Selection control using multiple values

28. switch - case • Used if the value of a variable is used for controlling the program flow. • May execute different code blocks for each different value of the variable. • C#language offers this functionality with switch-case structure.

29. Notation switch (variable ) { caseconstant_value1 : statements; break; caseconstant_value2 : statements; break; case constant_value3 : statements; break; default : statements; break; }

30. switch - case • switchis the starting point of the structre. • A variable should be provided after switch. • This variable could be numeric or character. • We should use only constant value in case sections, no expressions. • All cases should end with break keywords. • No need to use { } paranthesis aftes cases.

31. Example • Write a program that takes the course grade (just one letter) from user.Your program will write the following result to the output depending on the grade. • A: “best" • B: “good" • C: “all right" • D: “not bad" • F: “bad"

32. Example Cont.

33. Example • Meteorogy department asks for a program that converts humudity values to a human readable format. • Program will write to output the following results depending input ranges between 0 and 100. • %20 and lower “too dry" • %21 - %40: “dry" • %41 - %60: “little dry" • %61 - %80: “little moist" • %81 and higher: “moist“ • Write a program that takes a humudiy value from user and writes it’s corresponding human readable output by using switch /case control structure.

34. Switch – Case Notes • double,decimal types are not used within switch paranthesis. • Any number of statements can be used within the case element. • Using default: element help us to detect defects. • Forgetting to use breaks is the most common coding bad habbit. Check for breaks after constructing the switch structure.

35. Loops while, do-while and for loops

36. Loops • Loops are used for executing code blocks repeatedly. • Decision of continuing loop is given by boolean expression. If boolean expression is false, then the code block is not executed. • For loops repeats the execution of code blocks in a certain number. Count controlled loop. • while and do-while loops repeats code execution in an unknown number of iterations. Condition controlled loop.

37. Loops • In for and while loops, logical expressions are tested first. If expression is true then the code block inside the loop is executed, else program doesn’t enter the loop. • In do-whileloops code block is executed without checking boolean expression result for once. Then before second iteration boolean expression result is checked, and loop continuation is decided.

38. while loop • Two types of uses: while (logical expression is true) statement; while (logical expression is true) { statement; statement; }

39. Example

40. do-while loop do statement; while(logical expression is true); do { statement; statement; } while(logical expression is true) ;

41. Example

42. For loop • initializationexpressions are executed once before loop starts. If there are multiple initialization expressions, they are separated with a comma ",". • increment,expression are executed after each iteration If there are multiple increment expressions, they are separated with a comma ",". Execution order is from left to right. • After increment operations, logical expression is evaluated and loop terminates if logical expression is false. Else loop enters next iteration. for( initialization; logical expr.(termination);increment) { statement; }

43. for Loop • for (expression1; expression2 ; expresion3) • { • statements; • } • expression1, executed once before for loop starts. It could be one or multiple mathematical or other operations.

44. for Loop • for (expresion1 ;expression2; expresion3) • { • statements; • } • expression2, is a logical operation that returns true or false result. It is not a requirement that expression2 includes variables used in expression1.

45. for Loop • for (expression1; expression2;expression3) • { • statements ; • } • expression3, usually changes result of expression2, but it is not a necessity. It is executed after each iteration.

46. Example for (int counter = 1; counter <= 10; counter ++) { Console.WriteLine(counter.ToString()); }

47. Example int counter = 1; for (; counter <= 10;) { Console.WriteLine(counter.ToString()); counter ++; } int counter = 1; for (;;) { Console.WriteLine(counter.ToString()); counter ++; if (counter>10) break; }

48. Notes • expression1, expression2, and expression3 within the for loop could be empty but, we must use semi colons. • If epression1 and epression3 are not used, for loop behaves like while loop. • We can not know when will the loop terminated if expression2 is not used. In this case loop can be terminated by break expression. • expression1 and expression3 might include more than one expressions. The intention of this type of usage is simplifying program code.

49. for while expression1; while (expression2) { statements; expression3; } Her iki örnek de birbirinin aynısı şekilde çalışır. for (expression1; expression2; expression3) { statements; }

50. Example int counter; for(counter = 0; counter <= 10; counter++) { Console.WriteLine(“The value of counter :” + counter); } int counter; counter = 1 ; while (counter <= 10 ) { Console.WriteLine(“The value of counter :” + counter); counter++ ; }