1. Georgia Institute of Technology Conditionals – part 2 Barb Ericson Georgia Institute of Technology August 2005

2. Georgia Institute of Technology Learning Goals Understand at a conceptual and practical level –How to use conditionals with two possibilities –How to do simple edge detection –How to use ‘and’, ‘or’, ‘exclusive or’ and ‘not’ in a conditional –What is De Morgan’s Law?

3. Georgia Institute of Technology Edge Detection Loop through all the pixels in the picture –Calculate the average color for the current pixel and the pixel at the same x but y+1. –Get the distance between the two averages –If the absolute value of the distance is greater than some value turn the current pixel black –Otherwise turn the current pixel white

4. Georgia Institute of Technology Edge Detection Algorithm To find areas of high contrast –Try to loop from row = 0 to row = height – 1 Loop from x = 0 to x = width –Get the pixel at the x and y (top pixel) –Get the pixel at the x and (y + 1) bottom pixel –Get the average of the top pixel color values –Get the average of the bottom pixel color values –If the absolute value of the difference between the averages is over a passed limit »Turn the pixel black »Otherwise turn the pixel white

5. Georgia Institute of Technology Use if and else for two possibilities Sometimes you want to do one thing if the expression is true and a different thing if it is false int x = 200; if (x < 128) System.out.println(“<128”); else System.out.println(“>=128”); statement if (expression) true false Statement or block else

6. Georgia Institute of Technology Exercise Have students walk through a flowchart of a loop with a conditional. Say your name Set count to 0 count < 3 Throw the dice dice < 6 Clap your hands count times Jump count times Say, "all done! And the value of count" count = count + 1 true false true

7. Georgia Institute of Technology Edge Detection Exercise Write a method edgeDetection that takes an input limit –And turns all pixels black where the absolute value of the difference between that pixel and the below pixel is greater than the passed limit –And turns all pixels white where the absolute value of the difference between that pixel and the below pixel is less than or equal the passed limit –Pixel has a getAverage() method that returns the average of the three colors at the pixel

8. Georgia Institute of Technology Testing Edge Detection String file = FileChooser.getMediaPath(“butterfly1.jpg”); Picture p = new Picture(file); p.explore(); p.edgeDetection(10); p.explore();

9. Georgia Institute of Technology Challenge Create another method for simple edge detection –This time compare the current pixel with the one to the right (x+1) –How do you need to change the nested loop? –Do you get a different result?

10. Georgia Institute of Technology How many when there is an “And”? I want you to get soup, milk, bread, and yogurt at the store. –How many items will you come home with? I want you to clean your room and mop the floor in the kitchen and wash the dishes. –How many tasks do you need to do? I want a scoop of chocolate scoop and a scoop of vanilla. –How many scoops of ice cream is this?

11. Georgia Institute of Technology How many when there is an “Or” You need to help clean the house –You can clean the bathroom or the kitchen or the living room –How many jobs do you have to do? You want to get an ice cream –The flavors you can pick from are chocolate, vanilla, strawberry, or orange sherbet –How many flavors do you need to pick for a single scoop?

12. Georgia Institute of Technology Truth Table ConditionalOperand 1Operand 2Result Andtrue Andtruefalse Andfalsetruefalse Andfalse Ortrue Ortruefalsetrue Orfalsetrue Orfalse Exclusive Ortrue false Exclusive Ortruefalsetrue Exclusive Orfalsetrue Exclusive Orfalse

13. Georgia Institute of Technology Conditional Exercise When are the following true? When are they false? –You can go out if your room is clean and you did your homework –You can go out if your room is clean or you did your homework –You can go out if either your room is clean or you did your homework but not if both of these is true

14. Georgia Institute of Technology Conditional Operators We can check if several things are true - And –Using && (evaluation stops if the first item is false) –Using & (to always evaluate both operands) We can check if at least one of several things are true - Or –Using || (evaluation stops if the first item is true) –Using | (to always evaluate both operands) We can check if only one and only one of the things is true – Exclusive Or –Using ^

15. Georgia Institute of Technology Conditional Exercise What is the result from the following code? int x = 3; int y = 4; if (x 5) System.out.println(“and is true”); else System.out.println(“and is false”);

16. Georgia Institute of Technology Conditional Exercise What is the result from the following code? int x = 3; int y = 6; if (x 5) System.out.println(“and is true”); else System.out.println(“and is false”);

17. Georgia Institute of Technology Conditional Exercise What is the result from the following code? int x = 4; int y = 4; if (x 5) System.out.println(“or is true”); else System.out.println(“or is false”);

18. Georgia Institute of Technology Conditional Exercise What is the result from the following code? int x = 4; int y = 4; if (x 5) System.out.println(“or is true”); else System.out.println(“or is false”);

19. Georgia Institute of Technology Using && (And) and || (Or) Check that a value is in a range –Is some value between 0 and 255 (inclusive) for valid pixel color values –0 <= x <= 255 is written as –0 <= x && x <= 255 // in Java or –x >= 0 && x <= 255 // is the same Check if at least one of several things is true –Is this black or white? –True if either it is black or it is white

20. Georgia Institute of Technology Not Conditional Operator Use ! To change the value to the opposite –!true = false –!false = true A not conditional operator applied to a complex conditional changes it –!(op1 && op2) = !op1 || !op2 –!(op1 || op2) = !op1 && !op2 This is known as De Morgan’s LawDe Morgan’s Law

21. Georgia Institute of Technology De Morgan’s Law Exercise What is equivalent to the following? !(x > 4 && x < 8) !(y > 2 || y < 10) !(x == 2 && y == 4) !(y != 2 && x != 3) !(x == 3 || x == 5) !(y == 2 || y < 5)

22. Georgia Institute of Technology Summary Use if and else if you have two possibilities to deal with if (test) { // statements to execute when the test is true } else { // statements to execute when the test is false } Complex conditionals –Use ‘and’ to test for more than one thing being true –Use ‘or’ to test if at least one thing is true –Use ‘exclusive or’ to test that one and only one thing is true –Use ‘not’ to change the result from true to false and false to true