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CS121/IS223 Week 2, Slide 1
CS121: Computer Programming I
Conditionals in Java
Dr Olly Gotel
ogotel@pace.edu
http://csis.pace.edu/~ogotel
Having problems?
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CS121/IS223 Week 2, Slide 2
This Week’s Agenda
- Make sure you can do I/O
- Control structures in Java:
- Program flow & flowcharting/pseudocode
- Booleans – values & expressions
- Comparison (relational) operators
- Conditionals:
- if-else statements
- switch statements
- Nested conditionals
CS121/IS223 Week 2, Slide 3
Back to Basics
Input Output Do something
- Programs manipulate values
- We have to get some values in
- We have to get the resulting values out
Just focusing on keyboard
IP & screen OP
CS121/IS223 Week 2, Slide 4
Scanner
import java.util.Scanner;
Scanner scan = new Scanner(System.in);
String name;
int age;
name = scan.nextLine();
age = scan.nextInt();
System.out.println(name + age);
Reminder for input
Pick the method to
read in your type
CS121/IS223 Week 2, Slide 5
From last week
- Area of a circle
- Characters etc. in a name
- Vending machine
- How about converting GBP to USD?
CS121/IS223 Week 2, Slide 6
Writing a Small Program
- Write a Java program that:
- asks for an integer & stores it in a variable (x)
- prints out the integer (x)
- prints out the square of the integer (x
2 )
- prints out the cube of the integer (x
3 )
- is commented (i.e. says what the program is
called, who wrote it, when, what it does &
anything else you consider useful)
Practice at home
CS121/IS223 Week 2, Slide 7
Write Another
- Write a Java program that:
- asks for a number & stores it in a variable
- asks for another number & stores it in another
variable
- prints out the sum of the two numbers
- prints out the average of the two numbers
- is commented (i.e. says what the program is
called, who wrote it, when, what it does &
anything else you consider useful)
Practice at home
CS121/IS223 Week 2, Slide 8
Write Another
- Write a Java program that:
- asks for a user’s name & stores it it a variable
- asks for the user’s year of birth & stores it in a
variable
- prints out the user’s name & displays how old they
will be by the end of 2008
- is commented (i.e. says what the program is
called, who wrote it, when, what it does &
anything else you consider useful)
ALWAYS
DO THIS!
Practice at home
CS121/IS223 Week 2, Slide 9
Control Structures
- Flow of control is the order in which program
statements are executed
- Most programming languages have 2 kinds of
statements to regulate flow of control:
- conditionals – branching/selection statements that
choose one set of statements over one or more
other sets
- loops – repeat a set of statements again & again
until a stopping condition is reached
- They often rely on boolean expressions
The theme for two weeks
CS121/IS223 Week 2, Slide 10
Control Structures
- Java:
- 2 conditional statements
- 3 loop statements
Conditionals enable
selection/choice
Loops enable
repetition
CS121/IS223 Week 2, Slide 11
To Understand Conditionals…
- Program flow & how to control it
- Booleans
- Comparison operators
- Boolean operators
If desired, serve with Maple Mustard Sauce
CS121/IS223 Week 2, Slide 12
Why? Let’s Play a Game to See
“You are standing on the balcony of a castle turret. There is a
cannon & a match. To the north you see a hoard of fire demons
approaching. The rail on the east side of the balcony is broken.”
What do you do?
- Choose from:
- Go (North, East, South or West)
- Take (match or cannon)
- Strike match
- Fire cannon
- It is the logic for a very simple game – source [White 2002]
Hamish McTavish & the Haggis on the Moors
After a few more
classes, you could
write a program
to play this game!
CS121/IS223 Week 2, Slide 19
Being Choosy
- Computers are great at handling choices
- Choice is generally made after testing a condition
- This condition can be the value of a variable or the
value of an expression:
- wantMilk equals yes -> addMilk
- sugarLumps less than 4 -> addSugarLump
- Note, these evaluate to true or false
“to be or not to be”
CS121/IS223 Week 2, Slide 20
Booleans
- Used to control the flow of program execution
- Two boolean values:
- true (I.e. 1 or yes)
- false (I.e. 0 or no)
- A boolean variable or expression is either true or false
Java has a type
called boolean
Qubit?
CS121/IS223 Week 2, Slide 21
What is the Boolean Value of…
This class is a programming class
Pace University is in New York
New York is the capital of New York
There are 20 days until Olly’s birthday
It is raining today
The student next to you has $10 in their pocket
The Brooklyn Bridge is in Queens
Leeds Castle is in Leeds, UK
$1 has the same value as 100 cents
There are 5 cm in 1 inch
Don’t necessarily evaluate to the same truth value!
CS121/IS223 Week 2, Slide 22
Comparison Operators
- Less than <
- Greater than >
- Less than or equal to <=
- Greater than or equal to >=
- Equal to ==
- Not equal to !=
There are still more
operators to meet…
Also called
relational
operators
Higher precedence
Lower precedence
They help us to
form boolean
expressions
CS121/IS223 Week 2, Slide 23
VERY IMPORTANT!!!
== is the equivalence operator:
- if we use the expression x==y
- checks whether x & y are equal in value
- returns true (1) if they are equal
- returns false (0) if they are not equal
= is the assignment operator:
- if we use the expression x=y
- we make the value of x equal to the current value
of y
- we override any current value of x in so doing
These are VERY DIFFERENT
get used to this!
CS121/IS223 Week 2, Slide 24
Comparison Operators Return Booleans
- Comparison operators compare 2 values & return:
- How comparison is done is dependent on types
- Numbers are compared arithmetically
- Strings are compared by the numerical equivalent of the individual
characters (see http://www.unicode.org/ ) - BUT YOU CANNOT USE ==
IN JAVA!!!! Remember this for later!
- They have lower precedence than arithmetic operators (e.g. / % * + -)
Boolean values
See slide 132 from week 1
CS121/IS223 Week 2, Slide 25
What is the Boolean Value of…
“apples” == “oranges”
type (6.2) != type (6)
“first” < “second”
We cannot compare
strings this way in
Java - try and see
what happens!
CS121/IS223 Week 2, Slide 26
Conditionals
- Conditionals enable you to carve different paths
through a program
- Conditional execution:
- part of a program executes if a
condition is true, otherwise the
program ignores it
- e.g. if it is raining take brolly,
otherwise don’t bother
- You only take your umbrella if the condition “it is
raining” is true
When may you want to use conditionals?
CS121/IS223 Week 2, Slide 27
Why Use Conditionals?
- To allow choices:
- options & alternatives
- decision making
- To deal with bad or unexpected input:
- attempted division by zero
- numbers outside a permitted range
We also test conditions when we check
whether we should do something again
CS121/IS223 Week 2, Slide 28
Conditionals in Java
- if-else statement:
- 2-way alternatives
- many alternatives that depend on different test
conditions
- can nest
- switch statement:
- multiple alternatives with one test condition
- messy to nest
CS121/IS223 Week 2, Slide 29
if-else statements
- Take care with syntax
- Example – if your bank balance is in credit you get
some interest, but if it is in debit you pay an
overdrawn charge:
if (balance >= 0)
balance = balance + (RATE * balance);
else
balance = balance – penalty;
boolean expression to check – true or false?
MUST have the brackets
Executed if expression is true
Executed if expression is false
The else part
is optional
if statements are found
in MANY languages
Reserved words
CS121/IS223 Week 2, Slide 30
Flowchart of Logic
balance >= 0?
balance = balance
balance = balance
YES
NO
CS121/IS223 Week 2, Slide 37
Case Sensitive?
myString.equalsIgnoreCase(anotherString)
Any guesses?
CS121/IS223 Week 2, Slide 38
Write Some if Statements To…
a) Print an error message if the input value is zero
b) Print a greeting if the input value is your name
c) Print a message if the input value is a negative
number
d) Print a message if the input value is greater than an
allowed number
e) Print an error message if the input value is not “Fire
Cannon!”
Practice at home
CS121/IS223 Week 2, Slide 39
Extend the Previous Examples…
- Add an else statement to each of your previous if
statement examples, to (in turn):
a) Print the input value
b) Print a “Hello stranger!” message
c) Print a message that says the number is not negative
d) Print a message that says the input value is valid
e) Print “Firing Cannon Now!”
Practice at home
CS121/IS223 Week 2, Slide 40
Conditionals: The Rules
- The conditions that are tested can be anything that returns a Boolean
value (i.e. true or false value)
- There can be many else if statements associated with a single if
statement
- There can be only 1 else statement associated with a single if
statement
- An else statement cannot be used without an associated if statement
- if statements can be used on their own (i.e. else statements are
optional)
CS121/IS223 Week 2, Slide 41
Turning the Game Flowchart into Java
- Write a Java program to play this sub-part:
Prompt user for command
Is command “Go”?
Is command “Take”?
Is command “Strike
match”?
Print “Go”
Print “Take”
Print “Strike match”
Print “Fire cannon”
Call your file
Game.java
Yes
Yes
Yes
No
No
No
CS121/IS223 Week 2, Slide 42
PROJECT 1!!!
- An interactive text-based computer game:
- something you design, code & test (on your own)
- something you take as far as you are able
- Project description will be posted on my website later
in the week…so get thinking!
Download separate
handout on my website
CS121/IS223 Week 2, Slide 43
Game Design Process
- If it isn’t smart or funny,
you’ll get really bored
writing it, & think of the
poor users!
creative!
- I want to see your ideas by
02/17/09– note, this is not
the code – this is your game
idea and logic (your
responsibility to show me if
you want guidance)
© According to http://www.looneylabs.com/ CS121/IS223 Week 2, Slide 44
Where Does the else Belong?
if ( a < 0)
if ( b < 0)
b = b + 1;
else
a = a + 1;
if ( a < 0)
if ( b < 0)
b = b + 1;
else
a = a + 1;
Use braces or you will get confused !!!
CS121/IS223 Week 2, Slide 45
Compound if-else Statements
if (balance >= 0){
balance = balance + (RATE * balance);
System.out.println(“In credit”);
else{
balance = balance – penalty;
System.out.println(“In debt”);
Braces enclose a sequence of
statements to execute
A compound statement is a list
of statements within braces
Also called a block statement
CS121/IS223 Week 2, Slide 46
General Structure of if-else Statements
if (condition1){
statement 1;
else if (condition2){
statement 2;
else
default statement X;
Boolean expression
These parts of the if
statement are optional
statement with no
else ; you can have an
if – else if
statement with no
else , etc
Only 1 if; only 1 else; multiple else if
CS121/IS223 Week 2, Slide 47
Booleans
- Boolean expressions are fundamental to control flow
- they evaluate to true or false
- Java equality operators test whether or not 2 values
are equal:
- Java relational operators test relationships between
values:
Lower precedence than
arithmetic operators
Think: can we use these with doubles?
CS121/IS223 Week 2, Slide 48
Booleans – A Simplification
- Boolean expressions can be rewritten as boolean variables &
used in test situations
boolean isPositive = (number >= 0);
boolean isNegative = (number < 0);
boolean okTemp = ((temp > 0) && (temp < 100));
if (okTemp)
doSomething;
Used sensibly, can be easier to read
(boolean variable)
CS121/IS223 Week 2, Slide 55
An Aside - A Bit of Terminology
- A Boolean variable (a variable that holds a Boolean
value) is often called a flag
isHot
keepPlaying
gameOver
gotMoney
if (gotMilk):
print “I’m drinking it…”
CS121/IS223 Week 2, Slide 56
Boolean (Logical) Operators in Java
- Logical AND in Java is &&:
if ((time > min) && (time < max))
System.out.println(“Ok to sunbathe”);
else
System.out.println(“Come inside!”);
- Logical OR in Java is ||:
if ((temp < min) || (temp > max))
System.out.println(“Not healthy”);
else
System.out.println(“Healthy!”);
Produce boolean results
CS121/IS223 Week 2, Slide 57
Boolean Operators Cont…
if (!complete)
System.out.println(“Finish up”);
else
System.out.println(“Well done!”);
Logical complement
CS121/IS223 Week 2, Slide 58
Using Boolean Variables in Java
boolean ok = ((time > min) && (time < max));
if (ok)
System.out.println(“Ok to sunbathe”);
else
System.out.println(“Come inside!”);
Use if you like this
(boolean variable)
CS121/IS223 Week 2, Slide 59
Multi-Branch if-else Statements
if (balance > 0)
System.out.println(“In credit”);
else if (balance < 0)
System.out.println(“In debt”);
else if (balance == 0)
System.out.println(“No money”);
if (balance > 0)
System.out.println(“In credit”);
else if (balance < 0)
System.out.println(“In debt”);
else
System.out.println(“No money”);
Better
practice to
provide a
default too
What if you’ve
missed all
options?
CS121/IS223 Week 2, Slide 60
Exercise
- Write a Java program that assigns a grade to a mark
in a student test, according to the following chart:
A = 90 – 100 (inclusive)
B = 80 – 89 (inclusive)
C = 70 – 79 (inclusive)
D = 60 – 69 (inclusive)
E = 50 – 59 (inclusive)
F = 0 – 49 (inclusive)
- Your program should prompt the user to enter a mark
Call your file Grade.java
Eliminate options – no redundant
comparisons in the checking!
CS121/IS223 Week 2, Slide 61
An Answer
Change your program to
throw out bogus marks
above 100 or below 0
CS121/IS223 Week 2, Slide 62
Nested Conditionals
if (cash >= 0){
if (shopping > cash)
System.out.println (“Return items”);
else
cash = cash – shopping;
else
System.out.println (“Can’t even shop!”);
Braces NOT strictly needed, but add clarity
CS121/IS223 Week 2, Slide 63
Which Do You Prefer?
if (bonusOne ==‘Y‘ && bonusTwo ==‘Y' && bonusThree ==‘Y')
score = score + 12;
if (bonusOne == ‘Y')
if (bonusTwo == ‘Y’)
if (bonusThree == ‘Y')
score = score + 12;
CS121/IS223 Week 2, Slide 64
Exercise
- Write a Java program to prompt the user to enter 3
numbers. Use a nested conditional to work out & print
the maximum of the 3 numbers entered:
- you can assume the numbers are all different
- work out the logic first (i.e. the algorithm)
Call your file MaxOfThree.java
CS121/IS223 Week 2, Slide 65
An Answer
How would you change this to
detect equal max values?
CS121/IS223 Week 2, Slide 66
Conditional (Ternary) Operator
-? with : provide a short cut for if-else statements
if (x > y)
max = x;
else
max = y
(x > y)? x : y
Can be hard to
read, so avoid!
CS121/IS223 Week 2, Slide 73
switch Statement in Java
- switch statement is a multi-way branch that makes its decision on the
way to branch based on the value of an integer or character expression
switch (value){
case ‘g’:
case ‘G’:
film = “the good”;
break;
case ‘b’:
case ‘B”: film = “the bad”;
break;
case ‘u’:
case ‘U’:
film = “the ugly”;
break;
default:
film = “the good, the bad & the ugly”; break;
}
NEW!
Often used with switch
Sometimes, a
better way of
writing nested ifs
value must be
int or char type
CS121/IS223 Week 2, Slide 74
Example
switch (numberOfQuarts){
case 1:
System.out.println(“25 cents”);
break;
case 2:
System.out.println(“50 cents”);
break;
case 3:
System.out.println(“75 cents”);
break;
case 4:
System.out.println(“1 dollar”);
break;
default:
System.out.println(“Lots of money”);
break;
}
Controlling expression
(in brackets)
Case label
break statement
Optional, but good practice
The break statement –
allows you to exit a
conditional or a loop
CS121/IS223 Week 2, Slide 75
General Structure of switch Statements
switch (controllingExpression){
case caseLabel:
Statement;
break;
case caseLabel:
Statement;
break;
…
default:
Statement;
break;
}
Controlling expression
must evaluate to integer
type (byte, short, int, long)
or char type
Each case label must be a constant of
same type as controllingExpression
Each case must have a
different case label
CS121/IS223 Week 2, Slide 76
Exercise
- Write a Java program that asks a user what class they
want to fly (1, 2 or 3), then prints out the cost of an
airline ticket for that class of travel (NYC-LHR) – use a
switch statement & suitable cases – think about it:
- 1 is Upper Class - $4,
- 2 is Premium Economy - $2,
- 3 is Economy - $
- If the user entered u, p or e… how would you read in
a character using the Scanner class? Do some
research!
Call your file Flights.java
CS121/IS223 Week 2, Slide 77
Key Points
- Java has 2 conditional statements:
- if-else statements (suitable when many alternatives that
depend on different test conditions)
- switch statements (suitable when multiple alternatives with
one test condition)
- the choice of condition to execute depends on evaluating the
truth value of boolean expressions (you need to know your
operators)
- conditional statements can be nested to an arbitrary level
BUT performance suffers if messy!
CS121/IS223 Week 2, Slide 78
Coming Up Next
- Loops in Java – Java has 3 distinct flavours:
- We will be practicing lots with loops and conditionals
CS121/IS223 Week 2, Slide 79
Before Next Time…
th or 5th or 6th edition – read Chapter 5.1-5.4 of the Java
book
- think about it, make brief notes, ask questions & follow up
things that confuse you
- make sure you run the example code & understand the
answers to the self-review questions
- you MAY need your reading assignments to do exercises/
tests … and there may be one coming up soon!
- Be sure to have tried all the samples and exercises given in
these slides or you will fall behind … come see me or the tutors
if stuck!
- Continue with Java Exercise Sheet 1 for more practice -- the better the foundations, the easier you will build skills…
Java project… check the website for details
