Sometimes a program's decision making needs to be a little bit
more complex. For example, you might have a switch statement
that determines the maximum number of days in a month, but for
February, you'll also have to check for leap years. The case
for February would there for need a conditional or if/else to return
either 28 or 29 days, depending on the year.
You can nest any kind of
selection structure into the body of any other selection structure:
The else block of an if-else could contain a multi-sided if, or the
if-block of a multi-sided if could contain a switch statement, for
example. In this lesson we'll do some examples of nested
selection structures.
Imagine a situation where you ask
the user for a log-in name. If they enter a valid name, ask
them for a password. If the password is valid, display a message
on the screen. If either the password or login name is not valid,
display a specific message to the user. For this we might
want to have selection structures nested inside other selection
structures.
To perform this task, you will first need an if statement to
check the log-in name and possibly display a message if it's invalid:
import java.util.Scanner;
public class Nested {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = in.nextLine();
if (name.equals("Artemesia")) {
// check password
} else {
System.out.println("Invalid Login Name");
}
}
}
Next, we can add another if statement inside the if-block
to test the password:
import java.util.Scanner;
public class Nested {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = in.nextLine();
if (name.equals("Artemesia")) {
// check password
System.out.print("Enter your password: ");
String pass = in.next();
if (pass.equals("chicken")) {
System.out.println("Welcome to Arti's Cat House!");
} else {
System.out.println("Invalid password.");
}
} else {
System.out.println("Invalid Login Name");
}
}
}
Be careful when working with nested if's that have no braces.
To see why, examine the following code:
public class IfThing {
public static void main(String[] args) {
int x = 5;
int y = 10;
if (x < 2)
if (y > 5)
System.out.println("Meow");
else
System.out.println("hi");
System.out.println("bye");
}
}
What do you think the output will be? Run the program - what is
the output you actually get? Why?
In this example, the else-block appears to belong to the top if
statement. However, without the braces, an else-block will always
belong to the if-statement directly above it. In this case, the
else-block actually belongs to the if (y > 5) statement. When
working with nested Ifs, it's best to use braces to avoid such
confusion.
Nesting vs. Stacking
Don't confuse nesting if-statements with stacking if-statements.
Nesting occurs when an if-statement appears inside
the block or body of another if:
What is the output of each of the following code segments?
After determining the output, fix the indentation:
int x = 5;
int y = 10;
int z = 2;
if (x > y) {
System.out.println("twice x is " + (x * 2));
if (x * z == y) {
System.out.println("xz equals y");
}
} else {
System.out.println(y /= z);
}
int x = 5;
int y = 10;
int z = 2;
if (x > y) {
System.out.println("twice x is " + (x * 2));
}
if (x * z == y) {
System.out.println("xz equals y");
} else {
System.out.println(y /= z);
}
A program at a video store calculates the cost of renting a
certain number of DVD's. Some members have purchased a special Rewards
Program membership that allows them to rent DVDs for only $3.59 per DVD.
For all other customers, DVD rentals are $4.99 per DVD unless they rent
more than 4 DVDs at one time, in which case the cost per DVD rental is
$3.99. Write the part of the program that calculates the sub total before
taxes.
With this kind of problem, a decision table might be helpful (if you haven't
done decision tables in logic class, then don't worry about this):
Charge per Rental
1
2
3
Number of Rentals
--
<=4
>4
Rewards Program?
Y
N
N
$4.99 per Rental
X
$3.99 per Rental
X
$3.59 per Rental
X
What if the psuedocode for this problem was written as:
Print "Enter number of DVDs:"
Get numDvds
Print "Is customer in Rewards Program?"
Get rewardsMember
If rewardsMember = "Y" Then
subTotal = numDVDs * 3.59;
End If
If numDvds <= 4 Then
subTotal = numDvds * 4.99;
Else
subTotal = numDvds * 3.99;
End If
Print "Sub Total: ", subTotal
What is the output when the user enters 5 DVDs and "Y" for the Rewards Program?
The problem with this code is that even though we calculate the correct
sub total when we evaluate that rewardsMember = "Y" is true, we later
encounter a true condition for "numDvds > 4", which resets the sub
total to numDvds * 3.99. This would output 19.95 instead of the correct
result 17.95!
Write this program correctly!
The decision table below shows the Residence Assignment
policy for a local
university:
Residence Assignment
Rules
1
2
3
Student Age
<21
<21
>=21
Residence Type
Co-Ed
Non Co-Ed
--
Residence Assigned
Trudeau
MacDonald
Laurier
Write a program that prompts the user to enter the student's age
and whether or not the student wants a co-ed dorm room. Then
display the student's residence hall on the screen using the
criteria in the table. Write the most efficient code possible.
The decision table below shows the Medical Insurance Policy that
defines how much of a reimbursement patients receive:
Medical Insurance Policy
Rules
1
2
3
4
Deductible Status
Met
Met
Met
Not Met
Type of visit
Doctor
Hospital
Lab
--
Amt. of Reimbursement
50%
80%
70%
0%
Write a program that prompts the user to enter the
total amount due for the medical visit, then find out
if the client has paid the deductible. If they have, then ask the user
which type of visit the client had. Display the appropriate reimbursement
amount percentage based on the table. Write the most efficient code possible
Write a program that validates a date entered by the user. The user will enter a month number and then a
day number. Your program should display a message, "Invalid date!" if the date is not valid. e.g. if the
inputs are
2 and 30 (30th of February) or the inputs are 6 and 31 (31st of June) then message should appear. If a date
is valid,
display the message "This is a valid date." For now, assume Feb 29 is invalid. BONUS: in addition, as for
the year
and include leap years.
1. What is the output of each of the following code segments:
a)
int x = 5;
int y = 10;
int z = 2;
if (x > y) {
System.out.println("twice x is " + (x * 2));
if (x * z == y) {
System.out.println("xz equals y");
}
} else {
System.out.println(y /= z);
}
5
(the first condition x > z is false, therefore it jumps to that
if's matching else block and prints y /= z)
b)
int x = 5;
int y = 10;
int z = 2;
if (x > y) {
System.out.println("twice x is " + (x * 2));
}
if (x * z == y) {
System.out.println("xz equals y");
} else {
System.out.println(y /= z);
}
xz equals y
(the braces have moved around in this question: the if (x * z == y) is no
longer inside the if (x > y) block, so it's evaluated regardless of
the truth value of x > y)
Question 2:
ERROR NOTE: DISC_COST should be initialized to 4.99 and REG_COST
should be initialized to 3.99.
