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Making Choices (Chap 3) If you wish to defrost, press the defrost button; otherwise press the full power button. Let the dough rise in a warm place until.

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Presentation on theme: "Making Choices (Chap 3) If you wish to defrost, press the defrost button; otherwise press the full power button. Let the dough rise in a warm place until."— Presentation transcript:

1 Making Choices (Chap 3) If you wish to defrost, press the defrost button; otherwise press the full power button. Let the dough rise in a warm place until it has doubled in size.

2 Expressions and Statements Expression statements are formed by adding a semicolon to the end of certain types of expressions. –An assignment expression is an expression involving an assignment. area = width * height; –A method call expression has no assignment. System.out.println(…);

3 Blocks Several statements can be grouped into a block using { }. { x = 1; { y = 2; System.out.println(y); } System.out.println(x); }

4 A block with a declaration { int i = 5 + j; // i is declared in this // block j is from elsewhere … } // end of block, i disappears //can’t access i here

5 Boolean Expressions Any expression that evaluates to true or false. Relational operators,, >=. Equality operators, ==, !=. For example: int i = 3, j = 4; boolean flag; flag = 5 < 6; flag = (i == j); flag = (j + 2) <= 6;

6 Logical Operators Operators that take boolean values as operands. x && y - true if x AND y are both true x || y - true if either x OR y are true, or both !x - true if x is false - read NOT x

7 The if statement if ( BooleanExpression ) Statement BooleanExpr Statement true false

8 Semicolons and the if statement if (temperature < 32) System.out.println("Warning: Below Freezing!"); System.out.println("It's " + temperature + "degrees"); if (temperature < 32) { System.out.println("Warning Warning Warning!"); System.out.println("Warning: Below Freezing!"); System.out.println("Warning Warning Warning!"); }

9 Example: Sort three numbers 17 a 6 b 11 c 6 a b 17 c Before After

10 Pseudocode 1.Place the numbers in boxes a, b, c 2.If the number in a is not larger than the number in b, go to 4. 3. Interchange the numbers in a and b. 4. If the number in b is not larger than the number in c, then go to 8. 5. Interchange the numbers in b and c. 6. If the number in a is not larger than the number in b then go to 8. 7. Interchange the numbers in a and b. 8. Halt

11 a  first b  second c  third t  a a  b b  t a > b b > c t  a b  c c  t t  a a  b b  t Halt No Yes No

12 SortInput.java Dissect SortInput.java Hand simulate an execution.

13 The if-else statement if ( BooleanExpression ) Statement1 else Statement2 BooleanExpr Statement1 true false Statement2

14 Finding the minimum if (x < y) min = x; else min = y;

15 A block in an if-else statement if (temperature < 32) { System.out.println("Warning"); System.out.println(32 - temperature + "(F) below Freezing!"); System.out.println("Warning"); } else { System.out.println("It's " + temperature + "degrees fahrenheit."); }

16 Common Programming Error What is printed by the following if x is 3 and y is 4? if (x < y); System.out.println(x + " is smaller"); if (y < x); System.out.println(y + " is smaller");

17 Common Programming Error What is printed by the following if temperature is 45? if (temperature < 32) System.out.println("It is now"); System.out.print(32 - temperature); System.out.println(" below freezing."); System.out.println("It's " + temperature + "degrees.");

18 Nested if-else statements if (ageOfPerson >= 18) if (ageOfPerson < 65) System.out.println("full fare adult"); if (ageOfPerson >= 18 && ageOfPerson < 65) System.out.println("full fare adult");

19 Nested if-else statements if (temperature < 32) { System.out.println("Warning"); if (temperature < 0) System.out.println((-temperature) + "(F) below zero!"); else System.out.println(32 - temperature + "(F) below Freezing!"); System.out.println("Warning"); else { System.out.println("It is " + temperature); }

20 Common Programming Error if (18 <= age < 65) … This is not legal syntax.

21 if-else-if-else-if-else... if (ageOfPerson < 18) System.out.println("child fare"); else { if (ageOfPerson < 65) System.out.println("adult fare"); else System.out.println("senior fare"); }

22 if-else-if-else-if-else... if (ageOfPerson < 18) System.out.println("child fare"); else if (ageOfPerson < 65) System.out.println("adult fare"); else System.out.println("senior fare");

23 if-else-if-else-if-else... if (ageOfPerson < 18) System.out.println("child fare"); else if (ageOfPerson < 65) System.out.println("adult fare"); else System.out.println("senior fare");

24 The dangling else problem if (Expression1) if (Expression2) Statement1 else Statement2 This indentation is misleading.

25 The dangling else problem if (Expression1) if (Expression2) Statement1 else Statement2 An else always matches the nearest unmatched if.

26 The dangling else problem if (Expression1){ if (Expression2) Statement1 } else Statement2 Use braces to force the grouping you want.

27 The while statement while ( BooleanExpression ) Statement BooleanExpr true Statement false Execution enters the loop Continue with the rest of the program

28 A simple loop class Valentine { public static void main(String[] args) { int howMuch = 0; while (howMuch < 5) { System.out.println("I love you."); howMuch++; }

29 Average - pseudocode - no while 1. Get a number. 2. If the number is 0 go to step 6. 3. Add the number to the running total. 4. Increment the count of numbers read in. 5. Go back to step 1. 6. Divide the running total by the count of number in order to get the average. 7. Print the average.

30 Average - pseudocode - using while Get a number. While the number is not 0 do the following: Add the number to the running total. Increment the count of numbers read in. Get a number. (when the loop exits) Divide the running total by the count of number in order to get the average. Print the average.

31 General form of a while loop InitializationStatement while ( BooleanExpression ) { Statement1 Statement2 … PrepareForNextIteration }

32 Average.java Dissect Average.java Identify parts of the loop - init, test, nextIter Sentinel controlled loop 0 / 0 is NaN

33 Common Programming Error How many times does this loop execute? int count = 13; System.out.println("The multiples of " + "13 between 1 and 100 are:"); while (count != 100) { System.out.println(count); count = count + 13; }

34 The for statement for ( ForInit; BooleanExpr; UpdateExpr ) Statement BooleanExpr true Statement false Execution enters the loop Continue with the rest of the program ForInit

35 SquareRoots.java Dissect SquareRoots.java Redo loop with a while loop. Modify to declare loop index i inside of the for loop.

36 Switch Day of week with if-else Day of week with a switch Day of week grouping weekdays and weekends.

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