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Conditionals – part 2

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

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Conditionals – part 2

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

  2. 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? Georgia Institute of Technology

  3. 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 Georgia Institute of Technology

  4. 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 Georgia Institute of Technology

  5. 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”); false if (expression) else true Statement or block Statement or block statement Georgia Institute of Technology

  6. 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 Georgia Institute of Technology

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

  8. 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? Georgia Institute of Technology

  9. 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? Georgia Institute of Technology

  10. 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? Georgia Institute of Technology

  11. Truth Table Georgia Institute of Technology

  12. 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 Georgia Institute of Technology

  13. 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 ^ Georgia Institute of Technology

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

  15. Conditional Exercise • What is the result from the following code? int x = 3; int y = 6; if (x < 4 && y > 5) System.out.println(“and is true”); else System.out.println(“and is false”); Georgia Institute of Technology

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

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

  18. 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 Georgia Institute of Technology

  19. 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 Law Georgia Institute of Technology

  20. 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) Georgia Institute of Technology

  21. 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 Georgia Institute of Technology

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