COMP-202: Foundations
of Programming
Lecture 3: Boolean, Mathematical Expressions, and
Flow Control
Sandeep Manjanna, Summer 2015
Announcements
• Slides will be posted before the class. There might be few
minor changes later.
• Extra class on the 20th of May (13:05-15:25) at TR0100
• Assignment 1 extended due date.
• New Due Date : 17th of May (@ 23:30)
• Try all the exercises on the class slides. See me during
office hours for any doubts and troubles solving them.
• Thanks for the responses on the survey.
Survey Results
Can be a bit faster
Good, slower is
better
Prefer more
details
Very helpful
Review
• Variables
• Data Types
• Strings
• User inputs
Review
import java.util.Scanner;
public class SimpleInterest
{
// A program to compute Simple Interest
public static void main(String[] args)
{
double principal, rateInterest;
int yrs;
Scanner inputReader = new Scanner(System.in);
System.out.println(“Enter the Principal Amount”);
principal = inputReader.nextDouble();
System.out.println(“Enter the Annual Rate of Interest”);
rateInterest = inputReader.nextDouble();
System.out.println(“Enter the number of years since loan”);
yrs = inputReader.nextInt();
double simpleInterest = (principal * rateInterest * yrs) / 100;
System.out.println(“Your simple interest after ” + yrs + “ years = ” + simpleInterest);
}
}
Data Types Review
• What data type and value do the following expressions
evaluate to?
(int) 5.67 + (3 / 2)
(7 / 4) < (7.0 / 4.0)
25 + “Hello”
((int) 3.1 + 5) / 3.0
This Lecture
Boolean Expressions
Mathematical Expressions
Flow Control
One step at a
time….
Expressions
• An expression is a construct made up of variables, operators,
and method invocations, which are constructed according to the
syntax of the language, that evaluates to a single value.
• An Expression has Operator acting on the Operands to generate
a single valued result.
int x = 5 + 10;
String s = “Hello ” + “COMP202”;
• The data type of the result depends both on the operator and the
operands.
• An Expression is always evaluated from left to right, unless it is
explicitly specified.
• There are exceptions depending on the precedence of the operator. We
will see operator precedence later in the class.
Boolean Expressions
Boolean Expressions
• Boolean expressions evaluate to either
true or false.
myNumber > 0 // can be either true or false
• You can assign the result of a boolean
expression to a variable of type boolean:
boolean positive;
positive = (myNumber > 0);
Boolean Expressions in Java
• A boolean expression is one of the
following:
•
•
•
•
Comparison of two values using a comparison
operator like ‘<’. ( x < 5)
true or false (Java's boolean literals – these are
keywords).
A variable which has type boolean. (boolean x;)
The result of a logical operator over other
boolean variables.
Comparison Operators
• The result of a comparison is always true or
false. Used to compare numeric or character
values (remember ASCII?)
==
!=
<
>
<=
>=
: equal to
: not equal to
: less than
: greater than
: less than or equal to
: greater than or equal to
Examples
int x = 5;
x<6
x <= 5
x>5
x >= 3
x == 5
x != 5
true
true
false
true
true
false
Careful!
• Terrible things will happen if you mistype
== as =
int x = 5;
int y = 3;
(x = y);
x is overwritten with the value of y, i.e. x=3 after this
statement. This expression will return the value
assigned to x, that will be int in this case not boolean
(because, “=” is not a comparison operator).
(x = = y);
This sets check to false because x is not equal to y. This is
the right way to compare. This expression will return a
boolean (true or false).
Logical Operators
• These take boolean expressions as input, and
produce a result of type boolean.
!
||
&&
Logical NOT
Logical OR
Logical AND
• They can have either one operand (unary), as
in NOT, or two operands (binary), as in OR
and AND.
! Operator
• The NOT operator flips the truth value of the boolean
expression that follows.
• For an input boolean expression X:
X
!X
true
false
false
true
! Operator Examples
!(2 < 3)
!(3 < 2)
!true
!false
!(!(3>2))
!(September comes after August)
!(Earth is the 6th planet from the sun)
&& Operator
• The AND operator is true if both of the input boolean
expressions evaluate to true.
• For input boolean expressions X and Y:
X
Y
X&&Y
true
true
true
true
false
false
false
true
false
false
false
false
&& Operator Examples
(1 < 2) && (2 < 3)
(1 < 1) && (2 < 3)
(!(1 < 1)) && (2 < 3)
(Television starts with a T) && (Java can also be
coffee)
(true && false)
|| Operator
• The OR operator is true if at least one of the input
boolean expressions evaluate to true.
• For input boolean expressions X and Y:
X
Y
X||Y
true
true
true
true
false
true
false
true
true
false
false
false
|| Operator Examples
(1 < 1) || (2 < 3)
!((3 < 4) || (100 > 1000))
((Vegetables are healthy) || (Fruit is healthy))
(true || false)
Short-Circuit Evaluation
• The evaluation of && and || stops as soon as
you know the end result:
• If the left operand of && is false, the whole thing
is false, so the second is never looked at.
(p1 && p2) – if p1 is false, p2 is never looked at.
• If the left operand of || is true, the whole thing is
true, so the second is never looked at.
(p1 || p2) – if p1 is true, p2 is never looked at.
• This seems like a minor detail, but is important.
How This Is Useful
• Use the first part to check to make sure the
second part can actually be run
((x != 0) && ((1 / x) < 5))
• This can be used to check if the program is
trying to divide by zero.
Example Question
• Which of the following best describes the set of all pairs of values
for boolean variables a and b, such that
(!a && b) == !(a || b)
evaluates to true?
a) Empty set
b) Only one pair: a == true, b == false
c) Two pairs in which a == true
d) Two pairs in which a != b
e) All four possible combinations of values
ANSWER: ( C )
Mathematical
Expressions
Mathematical Operators
• Some operators:
Addition (+)
Subtraction (-)
Multiplication (*)
Division (/)
Remainder (a.k.a., modulo) (%)
• How these operators actually function depends
on the data type of the operands.
+ Operator
As seen already,
int + int
3 + 4 7 (int)
int + double (or double + int)
3 + 4.0 7.0 (double)
• Also defined for:
String + String
"3" + "4" "34" (String)
String + primitive data type (or p.d.t. + String)
3 + "4" "34" (String)
-, * Operators
• Subtraction (e.g., 3 - 4) and multiplication work
as you would expect.
• Like for +, if you mix ints and doubles, you get a
double.
• But not defined for as many data types
• String * String ERROR
Integer Division
• If both operands to the division operator are int,
the result is also an int (with the fractional part
discarded)
11 / 3 3
-11 / 3 -3
• Division by 0 with integers causes a runtime
error. Not detected at compile time!
1 / 0 CRASH
% Modulo Operators
• Modulo operator gives the remainder of a
division.
• Example:
5%32
4%20
1234 % 100 34
Operator Precedence
1.
2.
3.
4.
Parenthesis
Casting
*, /, % from left to right
+, -, from left to right
• Assignment happens after the evaluation of the
expression.
• To reduce ambiguity and to make your life easy,
always use parenthesis.
Operator Precedence
• Few Examples:
1.0 / 2
4 / 3 + 4.0 / 3
(double) 1 / 2
(double) (1 / 2)
(1+2)*3
1+2*3
You can learn more on operator precedence at:
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/oper
ators.html
+=, ++
(shorthand operators)
• Programmers got lazy about writing
x = x + 5;
• So, as a shortcut, you can write!!
x += 5;
• Then they got even lazier about writing
x += 1;
• So, as a short, you can write
x++;
-=, *=, /=, -• Similarly:
x -= 5;
is the same as
x = x - 5;
• And likewise for *=, /=.
• Also,
x--;
is the same as
x -= 1;
or x = x - 1;
++, -(Increment Operator)
• You can put ++ or -- before or after a variable
name, and even as part of a complex expression.
• After (post increment) x++: use and then increment.
• Before (pre increment) ++x: increment and then use.
int x = 5, y = 5;
System.out.println(x++);
System.out.println(++y);
Recommendation
• Only use ++ or -- by themselves, and do not put
them inside other expressions.
int x = 5;
int y = 2 * x++ + 4; // legal, but confusing
• Other shorthand operators are not very useful and
are not seen in general programming practices.
Example Question
1) What is the output?
int i = 10;
int n = i++;
System.out.println(“Value i = ” + i + “ and n = ” + n);
Answer: Value i = 11 and n = 10
2) What is the output?
int i = 10;
i++;
int n = i;
System.out.println(“Value i = ” + i + “ and n = ” + n);
Answer: Value i = 11 and n = 11
Try it out
• Write a program that takes a 3-digit number
and checks if the digits are in ascending order.
• For example: If the input is 321 , your program should
output true.
If the input is 213, your program should
output false.
• Hint: break it down into the following steps:
1. Set up main method in a class
2. Get the Scanner working
3. Separate every digit of the number (as in unit
place, ten’s place, hundred’ place)
4. Use Boolean expressions to generate the required
result.
Conditionals
(Testing)
..........
Control Flow
• Up to this point, our programs proceed in a
"straight" fashion.
• Just follow each line of code in the main method
• There are commands that can decide if a particular
block of code needs to be executed or not.
• These commands refer to the control flow because
you control where the program execution goes next.
Control Flow
1.
2.
3.
4.
5.
if statements
if-else statements
if-else if-else statements
while loops
for loops
if and if-else
if (<condition>)
{
<body statements 1>
}
if (<condition>)
{
<body statements 1>
}
else
{
<body statements 2>
}
( if ) and (if-else)
Example
• Example: Print different things depending on what the
String we are given is.
Scanner scanner = new Scanner(System.in);
String languageChoice = scanner.nextLine();
if (languageChoice.equals("English"))
{
System.out.println("You chose English.");
}
else
{
System.out.println("You chose non-English.");
}
Structure of a Conditional
if (languageChoice.equals("English"))
{
System.out.println("You chose English.");
}
else
{
System.out.println("You chose non-English.");
}
The condition to check (make sure it is surrounded by
parentheses)
Structure of a Conditional
if (languageChoice.equals("English"))
{
System.out.println("You chose English.");
}
else
{
System.out.println("You chose non-English.");
}
The block of code executes if the condition evaluates to
true.
Structure of a Conditional
if (languageChoice.equals("English"))
{
System.out.println("You chose English.");
}
else
{
System.out.println("You chose non-English.");
}
The second block of code gets evaluated if the condition
evaluates to false. If there is no else block, no extra code
block is run.
if-if vs. if-else
• What is the difference?
if (condition) {
//some commands here
}
if (not condition) {
//more commands here
}
if (condition) {
//some commands here
}
else {
//more commands here
}
Both blocks on the left could execute, if at the end of
the first block, (not condition) somehow becomes
true.
Going Into Two if Statements
int x = 1;
if (x > 0)
{
System.out.println("Positive. Altering the value.");
x--;
}
if (x <= 0) {
System.out.println("Not positive");
}
Only One Branch Executes
int x = 1;
if (x > 0)
{
System.out.println("Positive. Resetting value.");
x--;
}
else
{
System.out.println("Not positive");
}
if - else if - else
• For when you have a more complicated set of
conditions with more than two options.
• Option 1: Nested if-else statements
if (condition 1) { … }
else {
if (condition 2) { … }
else {
if (condition 3) { … }
}
}
Better Way (else if)
• Option 2: All at the same level
if (condition 1) {
}
else if (condition 2) {
}
else if (condition 3) {
}
else {
}
if - else if - else
Order Matters
• Much like else, the "else if" is only entered if
the prior conditions were false.
• So, there can be a big difference if the order of
the options are changed.
What Is Wrong Here?
if (money > 0.0) {
System.out.println("Positive balance");
}
else if (money > 1000.0) {
System.out.println("You're rich!");
}
else {
System.out.println("Uh-oh.");
}
What happens if the variable money = 1200?
Omitting Curly Braces
• A dangerous game.
if (x < 0)
System.out.println("x is less than zero.");
System.out.println("x is definitely negative.");
• Remember that Java ignores the indentation, so
this is very misleading!
Example Question
1) What will be the value of x after the following section of code executes:
int x = 5;
if (x > 3)
x = x – 2;
else
Answer: B
x = x + 2;
A. 1
B.
3
C.
5
D.
7
2) What will be the value of b after the following section of code executes:
int a = 4, b = 0;
if (a < 3)
b = 4;
else if ( (a < 10) && (b>0) )
b = 3;
else if ( (a > 5) || (b==0) )
b = 2;
else
b = 1;
A.
1
B.
0
C. 2
Answer: C
D.
4
Try it out!!
• Extend your previous program to convert
temperatures.
1. Now, it first asks if the user wants to convert
from Fahrenheit to Celsius.
2. If the user replies "true", it works as before.
3. Otherwise, it converts from Celsius to
Fahrenheit.
Try it out!!
• Read three positive integers x, y, and z from the
user and report the maximum and the minimum
integers.
Summary
Boolean Expressions
Mathematical Expressions
Conditional Statements
if
if – else
if – else if