Transcript Week 5 Topics - Computing Sciences

Week 9
Introduction to Computer Science
and Object-Oriented Programming
COMP 111
George Basham
Week 9 Topics
9.1.1 Nested Loops
9.1.2 Processing Sentinel Values
9.1.3 Random Numbers and
Simulations
9.2.1 Arrays
9.2.2 Array Lists
9.1.1 Nested Loops
• Sometimes the body of a loop is again a
loop. We say that the inner loop is nested
inside an outer loop.
• As an example, assume that we want to
generate the following output:
[]
[][]
[][][]
[][][][]
9.1.1 Nested Loops Cont.
String r = "";
// Triangle is 4 rows high
for (int i = 1; i <= 4; ++i)
{
// Number of blocks per row
for (int j = 1; j <= i; j++)
r = r + "[]";
r = r + "\n“;
}
System.out.println(r);
9.1.2 Processing Sentinel Values
• Sometimes, the termination of a loop can
only be evaluated in the middle of a loop.
• You can introduce a Boolean variable to
control such a loop.
• The value that you evaluate to determine
whether to terminate such a loop construct
is called a sentinel value.
9.1.2 Processing Sentinel Values Cont.
Example program run:
Enter value, Q to quit:
Enter value, Q to quit:
Enter value, Q to quit:
Enter value, Q to quit:
Enter value, Q to quit:
Average = 2.5
1
2
3
4
Q
Scanner in = new Scanner(System.in);
double total = 0;
double i = 0;
boolean done = false;
while (!done)
{
System.out.print(“Enter value, Q to quit:”);
String input = in.next();
if (input.equalsIgnoreCase(“Q”))
done = true;
else
{
double x = Double.parseDouble(input);
total = total + x;
i++;
}
}
if (i > 0)
System.out.println("Average = " + total/i);
9.1.3 Random Numbers and
Simulations
• In a simulation, you repeatedly generate random
numbers and use them to simulate an activity.
• The basic approach of a simulation is to have a
loop, generate a large number of sample values,
and record these values. When completed,
averages or other statistics are calculated.
• See the Buffon needle experiment for an
example.
9.1.3 Random Numbers and Simulations
Cont.
•
•
•
•
Here is how to simulate the cast of a die:
Random generator = new Random();
int d = 1 + generator.nextInt(6);
The call generator.nextInt(6) gives you a random
number between 0 and 5 (inclusive). Add 1 to
obtain a number between 1 and 6.
• import java.util.Random;
• nextInt(n) – a random integer between 0
(inclusive) and n (exclusive)
• nextDouble() – a random floating-point number
between 0 (inclusive) and 1 (exclusive)
9.2.1 Arrays
• An array is a sequence of values of the
same type. Arrays can be very useful, but
suffer from the limitation that their length is
fixed.
• double[] data = new double[10];
• for (i = 0; i < data.length; ++i)
data[i] = i * 10;
data[0] value is 0, data[1] is 10, data[2] is
20, data[3] is 30 … data[9] is 90
9.2.1 Arrays Cont.
• Representation of the array data from
the previous slide:
Value
Subscript
0.0
0
10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0
1
2
3
4
5
6
7
8
9
9.2.2 Array Lists
• ArrayList is a collection class.
• Array lists can grow and shrink as needed.
• The ArrayList class provides methods for
many common tasks.
• ArrayList<BankAccount> accounts =
new ArrayList<BankAccounts>();
accounts.add(new BankAccount(1001));
9.2.2 Array Lists Cont.
• BankAccount anAccount =
accounts.get(2);
• BankAccount anAccount = new
BankAccount(1729);
• accounts.set(2, anAccount);
• accounts.remove(0);
• System.out.println(accounts.size());
9.2.2 Array Lists Cont.
• Note, don’t try to access an element of an array
or an array list that does not exist, or you will get
an out-of-bounds exception and the program will
be terminated!
• For example, this will generate an out-of-bounds
exception:
• BankAccount anAccount =
accounts.get(accounts.size());
• Error since accounts.size() – 1 is the last valid
element of the array list
Reference: Big Java 2nd Edition by Cay
Horstmann
9.1.1 Nested Loops (section 7.3 in Big
Java)
9.1.2 Processing Sentinel Values
(section 7.4 in Big Java)
9.1.3 Random Numbers and
Simulations (section 7.5 in Big Java)
9.2.1 Arrays (section 8.1 in Big Java)
9.2.2 Array Lists (section 8.2 in Big
Java)