1. Boolean expressions Conditional statements and expressions

2. Penny guessing game
I weighed a pound of pennies and found there was exactly 160.
160 is an integer.
int exact = 160;
But nothing stops me from changing it.
It is also a constant that I never want to change.
final int exact = 160;
So let’s write a guessing game.

3. Penny guessing game import java.util.Scanner;
public class GuessingGame {
public static void main ( String s[] ) {
final int exact = 160;
Scanner in = new Scanner( System.in );
//prompt for a guess
System.out.print( “Guess the # of pennies in a pound. “ );
int guess = in.nextInt();
//check the answer
if (guess==exact)
System.out.println( “You are correct!” );
else
System.out.println( “Nope.” );
in.close(); }

4. Penny guessing game … //check the answer if (guess==exact)
System.out.println( “You are correct!” );
else
System.out.println( “Nope.” );
== is a boolean (true or false) operator
It tests for equality.
A==B is a boolean expression
The result of this expression is either true or false.

5. Penny guessing game
Let’s accept a guess that’s 160 ± 10%.

6. Penny guessing game Let’s accept a guess that’s 160 ± 10%.
So let’s define two more constants – the minimum and the maximum that we’ll accept.

7. Penny guessing game Let’s accept a guess that’s 160 ± 10%.
So let’s define two more constants – the minimum and the maximum that we’ll accept.
final int exact = 160;
final int max = exact + exact/10;
final int min = exact - exact/10;

8. Penny guessing game
Now we must change the check to include the acceptable range. …
//check the answer
if (guess==exact)
System.out.println( “You are correct!” );
else
System.out.println( “Nope.” );

9. Penny guessing game
Now we must change the check to include the acceptable range. …
//check the answer
if (guess==exact)
System.out.println( “You are correct!” );
else if (guess<min)
System.out.println( “Nope.” );
else if (guess>max)
else
System.out.println( “Close enough.” );

10. Penny guessing game … //check the answer if (guess==exact)
System.out.println( “You are correct!” );
else if (guess<min)
System.out.println( “Nope.” );
else if (guess>max)
else
System.out.println( “Close enough.” );
Note that we write “Nope.” if the first boolean expression or the second boolean expression is true.
Wouldn’t it be nice if we had a boolean operator for or?

11. Penny guessing game … //check the answer if (guess==exact)
System.out.println( “You are correct!” );
else if (guess<min | guess>max)
System.out.println( “Nope.” );
else
System.out.println( “Close enough.” );
boolean operator for or

12. Penny guessing game … //check the answer if (guess==exact)
System.out.println( “You are correct!” );
else if (guess>=min & guess<=max)
System.out.println( “Close enough.” );
else
System.out.println( “Nope.” );
boolean operator for and

13. Operators used in boolean expressions
Unary (i.e., op A):
! not
Binary (i.e., A op B):
<, >, <=, >= lt, gt, le, ge
==, != equals, not equal
& and
| or
&& and
|| or
This is also the precedence from highest to lowest.

14. Truth table for AND

15. Truth table for OR

16. Truth table for NOT

17. Comparing strings
== doesn’t work (as you expect) so please don’t use it.
Example:
String city1 = “Philadelphia”;
String city2 = “Phoenix”;
I want to say:
if (city1==city2) but I can’t!

18. Comparing strings Use .equals() instead:
if (city1.equals(city2)) …
.equals() is case sensitive (i.e., NYC is not the same as NyC)
.equalsIgnoreCase() is not case sensitive
if (city1.equalsIgnoreCase(city2)) …

19. if’s and blocks
Sometimes we wish to execute more than one statement for an if statement.
if (x==y)
singleStatement;
if (x==y) {
statement1;
statement2; …
statementn; } {
s1;
s2; …
sn; }
This is called a block.

20. if examples if (x == 2 & y < 10) x = 10; if (x == 2) { }

21. if examples
if (x==y) {
x = 0;
y = 1;
} else {
x = 1;
y = 0; }

22. if examples if (x==1) { x = 2; y = 3; } else if (x==2) { x = 3; y = 4;

23. if-else form in general
if (conds1)
one-statement; or { block-of-statements }
else if (conds2) …
else if (condsn)
else

24. Boolean type and variables
boolean b;
b = true;
b = false;
int x = 10;
int y = 9;
b = (x==y);
b = (x>y);

25. Boolean I/O boolean answer = in.nextBoolean();
You must type in either true or false and hit return (enter).
System.out.println( answer ); //works as too

26. Conditional operator ?:
Many programs contain code like the following:
if (n > 0)
average = sum / n;
else
average = 0;

27. Conditional operator ?:
Many programs contain code like the following:
int apples = 6;
if (apples != 1)
System.out.printf( "I have %d apples \n", apples );
else
System.out.printf( "I have %d apple \n", apples );

28. Conditional operator ?:
(boolean-expr) ? exprif-true : exprif-false
evaluated if-and-only-if (iff) boolean-expr is true
evaluated iff boolean-expr is false

29. Conditional operator ?:
if (n > 0) average = sum / n; else average = 0; average = (n > 0) ? sum / n : 0;

30. Conditional operator ?:
int apples = 6; if (apples != 1) System.out.printf( "I have %d apples \n", apples ); else System.out.printf( "I have %d apple \n", apples ); System.out.printf( "I have %d apple%s \n", apples, (apples != 1) ? "s" : "" );

31. Conditional operator ?:
Problem: We have 2 int’s, A and B. We want another int, C, to be set to the maximum of A and B.
How can we do this with if-else?
How can we do this with ?:?

32. Conditional operator ?:
Problem: We have 2 int’s, A and B. We want another int, C, to be set to the maximum of A and B.
How can we do this with if-else?
if (A>B)
C = A;
else
C = B;
How can we do this with ?:?

33. Conditional operator ?:
Problem: We have 2 int’s, A and B. We want another int, C, to be set to the maximum of A and B.
How can we do this with if-else?
if (A>B)
C = A;
else
C = B;
How can we do this with ?:?
C = (A>B) ? A : B;