Description
Boolean Expressions
• Evaluate to true or false
Math Java
AND &&
OR ||
NOT !
A = B A == B
A ≤ B A <= B
A ≥ B A >= B
A ≠ B A != B
3
Truth Tables
• A TRUTH TABLE for a compound Boolean
expression shows the results for all possible
combinations of the simple expressions:
x y x AND y x OR y
TRUE TRUE TRUE TRUE
TRUE FALSE FALSE TRUE
FALSE TRUE FALSE TRUE
FALSE FALSE FALSE FALSE
TRUE TRUE
FALSE TRUE
FALSE TRUE
FALSE FALSE
4
Operator NOT
x NOT x
TRUE FALSE
FALSE TRUE
• NOT is an operator to negate the value of a simple or compound
Boolean expression:
• Example. Suppose age = 15. Then:
– Expression age > 16 has a value FALSE, and NOT (age > 16)
has a value TRUE.
– Expression age < 65 has a value TRUE, and NOT (age < 65)
has a value FALSE.
Boolean Expressions:
• Write a test that returns TRUE if x is
between 10 and 20 (inclusive); the test
should return FALSE otherwise
Algorithm:
inRange ←
FALSE
inRange ←
TRUE
false true
X ≥ 10 AND
X ≤ 20 ?
Java:
// assume x has a value
boolean inRange;
if ( (x>=10) && (x<=20) )
{
inRange = true;
}
else
{
inRange = false;
}
AND versus OR
• In the last slide:
– We used: ((x>=10) && (x<=20)) to
test whether x is between 10 and 20.
• What if we used OR || instead of
AND &&
– Suppose x is 7.
– If we had ((x>=10) || (x<=20)):
x<=20 is TRUE, and so the entire expression
is TRUE: but x is not between 10 and 20.
7
Precedence of Operators
• Operators are evaluated left-to-right, with the following
precedence (all operators on the same line are treated
equally):
() (expression)
+ – (to indicate positive or negative values) ! (not)
* / %
+ - (for addition or subtraction of two values,
concatenation)
< > >= <=
== !=
&&
||
= (assignment to variable)
8
Operator Precedence
• What is the order of evaluation in the following
expressions?
a + b + c + d + e a + b * c - d / e
a / (b + c) - d % e
a / (b * (c + (d - e)))
9
Operator Precedence
• What is the order of evaluation in the following
expressions?
a + b + c + d + e a + b * c - d / e
a / (b + c) - d % e
a / (b * (c + (d - e)))
1 2 3 4 3 1 4 2
2 1 4 3
4 3 2 1
10
int i = 10, j = 15, k = 20;
double x = 10.0, y = 2.5, z = 100.0;
What are the results of the following 7 boolean expressions?
Do this on paper first. Only after you are done, write a
program to check your answers.
1. i < j || j < k && x <= y
2. !(j - i < 3) && j % 12 == 3
3. (i / 4) == y
4. (x / 4) == y
5. !(x != i)
6. 'a' != 'b'-1
7. !(!!false || !!true)
Exercise 1
11
Exercise 2
What is the type and value of each of the following
expressions in Java? Do this on paper. Only after you are
done, write a program to check your answers
Expression Type Value
13 * 0.1
(int) 13 * 0.1
13 * (int) 0.1
(int) (13 * 0.1)
13 % 7
2<3== 4<5
12
Exercise 3:
Write a program that has a main method and a
method called isDivisible.
• The method isDivisible takes two integers, n and m
as input paramters and returns true if n is divisible
by m and false otherwise.
• The main method should interact with the user to
get two integers and determine if the 1st is divisible
by the 2nd by calling isDivisible method. It should
print a message explaining the result.
13
Exercise 4
• Write a program that checks if an integer is
divisible by 2 and 3, or just one of 2 or 3, or
neither 2 nor 3. The program should have
the two methods:
– The main method should interact with the user
to get the value of the integer and to display in
which of the above three categories it belongs.
– The isDivisible23 method has one input
parameter, the value of the number. It should
return an integer representing if the number is
divisible by 2 and 3, divisible by one of 2 or 3, or
not divisible by either.
Exercise 5
• The code below is supposed to print the integers from
10 to 1 backwards.
– You need to find 2 logical errors in the code.
– Take the time to follow the logic and find the
errors BEFORE any coding.
– Correct the code and insert it into a main method
to check out your answer.
count = 10;
while (count >= 0)
{
System.out.println(count);
count = count + 1;
}
14
Exercise 6
Write a program that has a main method and a
method called sumOfSquares. The
sumOfSquares method takes integer n as a
input parameter and it computes the value of
the following series: 1 + 2^2 + 3^2 + 4^2 + … +
n^2, and it returns that value. The main method
should prompt the user to input the value for n,
and then it should call sumofSquares method
by passing that value. Finally it should print the
result returned by the sumofSquares method.
15
16
Exercise 7 (a bit harder)
(Game: lottery) Write a program that lets the user guess twodigit number. A program randomly generates an integer from 10
to 99 for the lottery number. The program prompts the user to
enter a two-digit number and determines whether the user wins
according to the following rule:
• If the user’s guess matches the lottery number, the award is
$1,000. (eg. User enters 23 and lottery num is 23)
• If all the digits in the user input match all the digits in the
lottery (but the numbers are not the same), the award is
$300. (eg. User enters 25 and the lottery num is 52)
• If one digit in the user input matches a digit in the lottery,
the award is $100. (e.g. User enters 23 and lottery num is 30)
• Otherwise the user gets nothing
CONTINUE ON THE NEXT PAGE
17
Examples:
Your program: Enter your lottery pick (two digits):
User: 23
Your program: Lottery is 32
Your program: Match all digits: you win $300
Your program: Enter your lottery pick (two digits):
User: 50
Your program: Lottery is 25
Your program: Match one digit: you win $100
Recall that, Java library has a method Math.random() that
generates a random double value in the range [0.0,1.0) (that is
greater than or equal to 0.0 and less than 1.0). To generate a
random integer value in the range [a,b] (that is, greater than or
equal to the value of a or less than or equal to the value of b), you
should type-cast the double value to an int value as follows:
randomNum = a + (int)(Math.random() * ((b – a) + 1));
