1. Lesson 16 Practice Exam Questions

Q1: Write a proper Javadoc comment for the below method from last class
Name: displayPrice
takes in one double parameter for a price
Prints the price to the console with a dollar sign and to two decimal places
returns nothing
Q2: Write the below method:
Name: displayPrice
takes in one double parameter for a price
Prints the price to the console with a dollar sign and to two decimal places
returns nothing

Q3: Now call the method using the starter code below:

public static void main(String[] args) {
Scanner input = new Scanner(System.in);
double number;
System.out.print("Enter the price: ");
number = input.nextDouble();
System.out.print("The formatted price is: ");
//call the method here!
}

2. Arrays

Using Lists to Store Data in Computer Science

• We are all familiar with lists.
• We use lists all the time for things like grocery shopping, e.g.:
  
• Bread
  
• Kale
• Chocolate
• Or, keeping track of our assignments:
• Physics exam Wednesday.
• Do Calc homework tonight.
• Read short story for English.

Defining Arrays

• An array is a collection of data items all of the same type
• You declare an array like this:

  dataType[] variableName = new dataType[length];
  

• You can also declare an array like this:

  dataType variableName[] = new dataType[length];
  

• Where:
  • dataType: the data type of all the array items
  • variableName: the name you make up for the array
  • length: the number of data items the array can hold
• For example, the following is the declaration of an array named scores that holds 10 values of type int:

  int[] scores = new int[10];

• When a program executes this statement, it creates 10 contiguous slots in memory like this:

  scores =

• Each of the memory slots can hold one data value
• We can have arrays of any type:

double[] anArrayOfDoubles;
boolean anArrayOfBooleans[];
char[] anArrayOfChars;
String anArrayOfStrings[];

• Once you define the length of an array, it cannot change length later in the program.
  
• When defining an array, you must guess the maximum number of elements you need to store:

  int[] scores = new int[100];
  

• If you need to know the length of an array, you can always use the length property (more on this below):
  

          System.out.println(scores.length) //Will print out 100

Initializing Array Elements

• We specify which slot of an array to access with the [] operator:

  scores[4] = 98;

• The indexes of arrays are numbered starting at 0
  
• We can assign a value to an array element any time after it is declared:

  scores[0] = 90;
  scores[1] = 95;
  scores[2] = 87;
  scores[3] = 89;
  scores[4] = 98;
  

• We can also initialize array elements in the declaration statement:
  • Called static initialization
  • We use a comma-separated list inside curly-braces
• For example:

  int[] scores = { 90, 95, 87, 89, 98 };
  

• This produces the same array as in the previous example
• The compiler computes the size automatically by counting the items in the list

Default Array Values

• When an array is declared, Java automatically assigns each element a default value:
  
• false for booleans
  
• 0 for ints
  
• 0.0 for doubles
• null for Strings
  

Accessing Array Elements

• To access the slots in an array, we must specify which slot to use with the [] operator
• For instance:

  scores[4] = 98;

• The number inside the brackets is called an index or subscript
• In Java, the slots of an array are numbered starting at 0, as shown below:

  scores =

  98

  [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]

• Thus, assignment to the slot with an index of 4 is put into the fifth slot

Using Slots

• We declared our example array with a data type of int:

  int[] scores = new int[10];
  

• Because scores is an array containing int values, we can use a slot, such as scores[4], just like any variable of type int:

  scores[4]++;
  System.out.println(scores[4]);
  

• This includes using a slot as an argument to a method with a parameter of the same type:

  public static void myMethod(int singleScore){//method body}
  ...
  myMethod(scores[4]); 
  

Array Length

• We can always access the length of an array using .length

System.out.println(scores.length);

• Note that the length of the array is always one more than its last index

Image source.

Using Arrays to Collect Data Items

• Note that the index of an array can be any integer value
• Thus, we can use an integer variable for the index
• We can use an integer variable with a loop to read data into the array
  
• Also, we can display the contents of an array using a loop
• The following program shows an example of collecting and displaying data items

Example Program Using Arrays to Collect and Display Data Items

/** * Array Example * @author Jennifer Parrish */ import java.util.Scanner; public class Arrays { public static void main(String[] args) { Scanner input = new Scanner(System.in); int[] scores = new int[5]; System.out.println("Array elements initialized to 0:"); for (int i =0; i <scores.length; i++){ System.out.println("Index " + i + ": " + scores[i]); } System.out.println("\nEnter " + scores.length + " scores:"); for (int i =0; i <scores.length; i++){ scores[i] = input.nextInt(); } System.out.println("\nYou entered:"); for (int i =0; i <scores.length; i++){ System.out.println("Score " + i + ": " + scores[i]); } } }

Activity 17.1: My First Array (10pts)

• Open a new Java project in Eclipse and save it as Arrays
• Inside main(), add a statement to define an array named names that is suitable for holding a list of six (6) names.
• Compile your code to make sure it has correct syntax.
• If you have problems, please post on Piazza.
• Assign values to the first 3 array elements by inventing some names, like so:

  names[0] = "George Washington";
  

• Next, place a print statement below these 3 assignment statements:

System.out.println("The first 3 names:");

• Then, add a for-loop to display all three array values.
• Next, we are going to ask the user to enter the final 3 names to store in the array.
• Declare a Scanner variable at the top of your program.
• Add a second for loop to read in the next 3 names from the user input into the array:

System.out.println();

for (int i = ???; i < array.????; i++) {

    System.out.print("Enter name " + (i + 1) + ": ");

    array[i] = ???;

}

• Write a last for loop to display all the names in the array.

System.out.println("\nThe 6 names are: ");

//write your for loop here

• When you are finished, run your code and make sure that you are getting output similar to the following (except the names might differ):

The first 3 names:

George Washington

John Adams

Thomas Jefferson

Enter name 4: Grace Hopper

Enter name 5: Ada Lovelace

Enter name 6: Hedy Lamarr

The 6 names are:

George Washington

John Adams

Thomas Jefferson

Grace Hopper

Ada Lovelace

Hedy Lamarr

• When your program is working properly, upload your source code to Canvas.

Wrap Up

• Declare an array called pets, and assign it the values "dog", "cat", "rabbit", "bird" in TWO ways (static and not static)

Static:

Non-static:

• Write a for loop to display the pets array to the console

Upcoming Assignments

• Activity 17.1 due Thursday at 11:59pm
  
• Assignment 16 due Friday at 11:59pm
• Assignment 17 due next Tuesday at 11:59pm
  
• Quiz 8 due Friday at 11:59pm

~Have a Great Weekend!~