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Control Structures

Control Structures. special statements whose purpose is to change the order of execution in a program. Sequential structures – Statements executed sequentially Selection structures Repetition structures (loop). Selection. Given hours worked and pay rate, calculate total pay

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Control Structures

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  1. Control Structures • special statements whose purpose is to change the order of execution in a program. • Sequential structures – Statements executed sequentially • Selection structures • Repetition structures (loop) decision.ppt

  2. Selection • Given hours worked and pay rate, calculate total pay • How about if you work overtime? • How do you indicate if your work overtime? • How about double overtime? decision.ppt

  3. Selection • Single-selection structure • The if structure • selects or ignores a single action. • Double-selection structure • The if/else structure • selects between two different actions. • Multiple-selection structure • The switch structure • selects one of many possible actions (to be discussed later) decision.ppt

  4. What is truth? • 0 is false • anything other than 0 is true decision.ppt

  5. Relational Operators decision.ppt

  6. examples: int  x = 4; int  y = 6 ; EXPRESSION                 VALUE x < y       true  x + 2 < y             false  x != y                true  x + 3 >= y            true  y == x                false  y == x + 2true ‘r’ < ‘t’ true decision.ppt

  7. Beware:== vs = • = assignment • == test for equality • x = 5; • if (x == 5) decision.ppt

  8. Java Operator Precedence decision.ppt

  9. Comparing strings • str1.compareTo(str2) • Compares two string lexicographically • A negative integer value if str1 < str2 • 0 if str1 is equivalent to str2 • A positive integer if str1 > str2 decision.ppt

  10. Example >String str1 = "Hello"; >String str2 = "Hi"; >System.out.println(str1.compareTo(str2)); -4 > System.out.println(str2.compareTo("Hi")); 0 > System.out.println(str2.compareTo("Big")); 6 decision.ppt

  11. equals – tests objects for equality • str1.equals(str2) • str1.equalsIgnoreCase(str2); • if (s.equalsIgnoreCase("yes")) decision.ppt

  12. Logical Operators decision.ppt

  13. && - Logical AND decision.ppt

  14. || - Logical OR decision.ppt

  15. ! Not decision.ppt

  16. DeMorgan's Law: not(P and Q) is equivalent to (not P) or (not Q),  !(P && Q) = (!P) || (!Q)     not(P or Q) is equivalent to (not P) and (not Q),  !(P || Q) = (!P) && (!Q) Note: left expression is equivalent to right expression with! added and the relational and logical operators reversed decision.ppt

  17. Java Operator Precedence decision.ppt

  18. Examples: • Do not depend on operator precedence while using expressions containing side-effects • x>3 && x++ • 5 > 2 && 4 > 7 false • 5 > 2 || 4 < 7 true • !(4 < 7) false • if ((sex == 'F’) && (cityCode == 18) && (gpa >= 3.8)) • if ((zipCode.equals("48002")) || (zipCode.equals("48003")) || • (zipCode.equals( "48004")) • !(hours > 40)  same as: hours <=  40      decision.ppt

  19. Short-Circuit Evaluation • evaluation proceeds from left to right • the computer stops evaluating sub-expressions as soon as the truth value of the entire expression can be determined • Exercise caution when the operands following the first operand contain side effects (see notes) • Rule of thumb: • first true in OR => Entire expression is true • first false in AND => Entire expression is false decision.ppt

  20. Short-Circuit Examples: • int age = 25; int weight = 150; • (age > 50) && (weight > 140)  // false • Evaluation can stop after determining that (age > 50) is false, it is already determined that the entire expression will be false. • (weight >= 150)||(age > 40)  // true • Evaluation can stop after determining that (weight >= 50) is true, it is already determined that the entire expression will be true. • (age < 50) && (weight > 140)  // true • After determining that (age < 50) is true, we have to determine the truth value of the second part. • (age < 20) || (weight < 140)  // false • After determining that (age < 20) is false, we have to determine the truth value of the second part. decision.ppt

  21. Exercises • Write an expression for the following: • taxRate is over 25% and income is less than $20000 • temperature is less than or equal to 75 or humidity is less than 70% • age is over 21 and age is less than 60 • age is 21 or 22 • For what value(s) of x would this condition be true? • (x < 1) && (x > 10) • For what value(s) of y would this condition be true? • (y >= 1) && (y <= 10) decision.ppt

  22. Example • x > y + 2 => x > (y + 2) • x + 3 <= y * 10 • a <= b > c < d >= e will be executed as: ( ) > c ( ) < d ( ) >= e ( ) decision.ppt

  23. Relational Operators with Floating Point Types • Do not compare floating-point numbers for equality; approximate values, they are rarely exactly equal. • test for near equality • compute the difference between the 2 numbers and check if the result is less than some max difference. • Example: float x = 1.0; float y = 3.0; float z; ... z = x/y; ... if (x == z * y)... // x will not be 1.0, but 0.99999 if (abs(x - z * y) < 0.00001)... // close enough decision.ppt

  24. Compound Logical Expressions • if (a < b < c) • syntax error • In this expression, c is compared with 1 or 0 (the result of evaluating a < b • int  total = 50; if (80 < total < 90) System.out.println("Finalgrade is B“); • According to the precedence chart, the expression (80 < total < 90) means (80 < total) < 90  because "<" is left associative, (80 < total ) is false (false < 90) is invalid. if ((80 < total) && (total < 90)) //correction decision.ppt

  25. if Syntax if ( Expression ) Statement NOTE: Statement can be a single statement, a null statement, or a block (compound statement). decision.ppt

  26. if statement is a selection of whether or not to execute a statement (which can be a single statement or an entire block) TRUE expression FALSE statement decision.ppt

  27. if • Single statement EX1: if (number % 2 == 0) System.out.println("Even number!"); EX2: if (i) { //defensive programming System.out.println(iis nonzero"; } decision.ppt

  28. compound statement • Multiple statements where one is allowed if (state == MD) { amt = amt + (amt * .05); cout<<amt; } decision.ppt

  29. if elsesyntax if (Expression ) StatementA else StatementB NOTE: StatementA and StatementB each can be a single statement, a null statement, or a compound statement. decision.ppt

  30. if..else provides two-way selection between executing one of 2 clauses (the if clause or the else clause) TRUE FALSE expression if clause else clause decision.ppt

  31. if (i >= 0) { System.out.println("positive"); } else { System.out.println("negative"); } if (i >= 0) System.out.println("positive"); else System.out.println("negative"); braces may be omitted if there is only one clause: Recommended - leaving the braces in case another statement is added. Example: decision.ppt

  32. => Watch out for null statement if (x > 0); //does nothing if (x >0); //hard to find bug System.out.println("Positive"); decision.ppt

  33. Nested IF Statements • The body of an if-clause or else-clause can contain another if or if-else statement • this is called a nested control structure or a nested if • In general, any problem that involves a multiway branch (more than 2 alternative solutions) can use nested if statements. decision.ppt

  34. if (x > 0) System.out.println(“Positive”); else // nested if else if (x < 0) System.out.println(“Negative”); else System.out.println(“Zero”); decision.ppt

  35. change indentation (remove whitespace) if (x > 0) System.out.println(“Positive”); else if (x < 0) System.out.println(“Negative”); else System.out.println(“Zero”); decision.ppt

  36. version 1:separate ifs if (x > 0) posCount++; if (x < 0) negCount++; if (x==0) zeroCount++; How many branches are tested, best case, worst case? 3 way choice decision.ppt

  37. Version 2: Nested ifs if (x > 0) posCount++; else if (x < 0) negCount++; else zeroCount++; How many branches are tested, best case, worst case? 3 way choice decision.ppt

  38. Version 3: Nested ifs, special indentation (preferred version) if (x > 0) posCount++; else if (x < 0) negCount++; else zeroCount++; How many branches are tested, best case, worst case? 3 way choice decision.ppt

  39. if (cond) statement else if (cond) statement else if (cond) statement else statement evaluated in order, if any expression is true, that statement and that statement only is executed. only difference is whitespace preferred method - shows we are making a choice else if decision.ppt

  40. if (temp <= 32) System.out.println("freezing"); if (temp >= 212) System.out.println("boiling"); if ((temp >= 32) and (temp <= 212)) System.out.println("liquid“); if (temp <= 32) System.out.println("freezing"); else if (temp >= 212) System.out.println("boiling"); else System.out.println("liquid“); Example: decision.ppt

  41. decision.ppt

  42. if (n > 0) if (a > b) z = a; else z = b; if (n > 0) if (a > b) z = a; else z = b; Dangling else:else matches with most recent unmatched if autoformatting in IDEs is a good way to catch many of these kinds of errors decision.ppt

  43. if (hours <= 40) totalPay = hours * payRate; else if (hours <= 60) { regPay = 40 * payRate; otPay = (hours – 40) * payRate * 1.5; totalPay= regPay + otPay; } else { regPay = 40 * payRate; otPay = 20 * payRate * 1.5; doubleOtPay=(hours – 60)* 2 * payRate; totalPay= regPay + otPay + doubleOtPay; } Overtime example decision.ppt

  44. if ((hours < 0 || (hours > 168)) System.out.println("Invalid"); else if (hours <= 40) totalPay = hours * payRate; else if (hours <= 60) { regPay = 40 * payRate; otPay = (hours – 40) * payRate * 1.5; totalPay= regPay + otPay; } else { regPay = 40 * payRate; otPay = 20 * payRate * 1.5; doubleOtPay=(hours – 60)* 2 * payRate; totalPay= regPay + otPay + doubleOtPay; } Overtime example decision.ppt

  45. Grade Example int grade; // Get grade System.out.println( "Enter the grade:“); grade = console.nextInt(); System.out.println( "Your grade of " + grade + " is “); // Print letter grade if (grade >= 90) System.out.println( "A." ); else if (grade > 80) System.out.println( "B." ); else if (grade >= 70) System.out.println( "C." ); else if (grade >= 60) System.out.println( "D." ); else System.out.println( "F." ) decision.ppt

  46. ?: • expression1 ? expression2:expression3 max = (a>=b) ? a : b; if (a >= b) max = a; else max = b; decision.ppt

  47. switch switch (expression) { case value1: statements break; case value2: statements break; … default: statements } decision.ppt

  48. import java.util.*; public class Switch { public static void main (String [] args) { Scanner a = new Scanner(System.in); intyearLevel; System.out.print("Enter year Level: "); yearLevel = a.nextInt(); switch (yearLevel) { case 1: System.out.println("You are a freshman!"); break; case 2: System.out.println("You are a sophomore!"); break; case 3: System.out.println("You are a junior!"); break; case 4: System.out.println("You are a Senior!"); break; default: System.out.println("Invalid entry!"); break; } } } decision.ppt

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