1. Georgia Institute of Technology
Conditionals – part 2
Georgia Institute of Technology

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?
Georgia Institute of Technology

3. Georgia Institute of Technology
Truth Table
Conditional
Operand 1
Operand 2
Result
And
true
false Or
Exclusive Or
An and conditional is only true when both operands are true.
An or conditional is true as long as either operand is true.
An exclusive or conditional is true only if one, but not both, operands are true.
Georgia Institute of Technology

4. 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 (exclusive OR)
Georgia Institute of Technology

5. 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 ^
The conditional operators && and || are ‘short circuit’ operators. They stop evaluation early when the result is obvious from evaluating the first operand.
See for Sun’s tutorial on conditional operators.
Georgia Institute of Technology

6. Georgia Institute of Technology
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”);
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7. Georgia Institute of Technology
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

8. Georgia Institute of Technology
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

9. Georgia Institute of Technology
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

10. 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 <= 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

11. 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
See for information on DeMorgan’s Law
Georgia Institute of Technology

12. 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)
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13. Georgia Institute of Technology

14. Exercise: 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
The image is from foal.jpg in mediasources after doing edge detection with a limit of 10.
Georgia Institute of Technology

15. 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

16. 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
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17. Georgia Institute of Technology
Exercise
Have students walk through a flowchart of a loop with a conditional.
dice < 6
false
Say your name
Set count to 0
true
Clap your hands count times
Jump count times
count < 3
false
true
count = count + 1
Throw the dice
Say, "all done! And the value of count"
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18. 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

19. Testing Edge Detection
String file =
FileChooser.getMediaPath(“butterfly1.jpg”);
Picture p = new Picture(file);
p.explore();
p.edgeDetection(10);
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20. 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?
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21. 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
Georgia Institute of Technology