Questions

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Concept - Declaring an array

• An array is a sequence of values that can accessed by an integer index
• Index numbers start at 0
• A data structure similar to an ArrayList
• When you declare an array, you specify how many elements it has and the type
  • the size can never change
• Example

  int[] elevations = new int[5];
  

• Contents of the array [0, 0, 0, 0, 0]
  • array elements are initialized for you like instance variables
• If you know what elements you want in the array, create it this way

  String[] professors = {"Horstmann", "Heller", "O'Brien"} ;

• Contents of the array: [ "Horstmann", "Heller", "O'Brien"]
• You do not need to import anything to create an array

 

Concept - Accessing array elements

• You do not use methods to get and set array elements
• You use brackets [ ] with an integer index inside to indicate which element
• To set an element

  elevations[2] = 5270;
  professors[1] = "Wesley";
  things[i] = 10; //changes the ith element to 10

• Contents of the first two arrays

  [0,0,5270,0,0]
  ["Horstmann", "Wesley", "O'Brien]

• To get an element

  int e = elevations[2];
  String p = professors[0];
  int value = things[i]; // value contains whatever is at element i

• Contents of variables

  e => 5270
  p => "Horstmann"

• The number of elements in an array is in length (no parentheses)

  elevations.length => 5
  professors.length => 3

Activity - Arrays

Record your participation in Piazza clicker question Lesson17 Q1

Assume the array primes has been initialized like this

int[] primes = { 2, 3, 5, 7, 11 };

What does it contain after executing the following loop?

for (int i = 0; i < primes.length; i++)
{
   primes[i]++;
}

Note: primes[i] accesess the ith element of the primes array and primes[i]++  increments the ith element of the array

1. 2, 3, 5, 7, 11
2. 3, 4, 6, 8, 12
3. 7, 8, 10, 12, 16
4. Nothing. There is a syntax error

Concept: Enhanced for loop

• Let's go back to out professors array: Contents: ["Horstmann", "Wesley", "O'Brien"]
• Processing an array with a traditional for loop

  for (int i = 0; i < professors.length; i++)
  {
      System.out.println(professors[i]);
  }
  

• Processing with the enhanced for loop

  for (String p: professors)
  {
      System.out.println(p);
  }

• Output for both:

  Horstmann
  Wesley
  O'Brien

• Can not use an enhanced for loop if we want to change the contents of the array

Activity - Enhanced for loop

Record your participation in Piazza clicker question Lesson17 Q2

Assume the array primes has been initialized like this

int[] primes = { 2, 3, 5, 7, 11 };

What does it contain after executing the following loop?

for (int p: primes)
{
     p++;
}

1. 2, 3, 5, 7, 11
2. 3, 4, 6, 8, 12
3. 7, 8, 10, 12, 16
4. Nothing. There is a syntax error

Activity - Creating and filling an array

Record your participation in Piazza clicker question Lesson17 Q3

Complete the method body for the getCombination method in the class Lottery. It returns an array of n random numbers less than 100 that might then be used in buying a lottery ticket. Create a Bluej project and copy in this code. You will need to instantiate a Random object. Test the method in the workbench

import java.util.Random;
public class Lottery
{
   public int[] getCombination(int n) 
   { 
       //create random object
       //create an array that can hold n ints
       //for every cell in the array, fill it with a random number
   }
   //possibly other methods
}

1. Got it
2. Got it, but it was hard
3. Tried but did not get it in the time available
4. Did not know where to start

Activity - Array Algorithms: finding the max

Record your participation in Piazza clicker question Lesson17 Q4

Here is some starter code (arrays.zip). It contains a class IntArrayProcessor and an IntArrayProcessorTester. You can download it, unzip it, and open the Bluej project. We are going to practice working with array algorithms

• Look at the code to see what is there
• Methods are coded as stubs
• Complete the method getMax to return the largest element in IntArrayProcessor
  • In the textbook: Section 7.3.3 page 319
    

Activity - Array Algorithms: counting matches

Record your participation in Piazza clicker question Lesson17 Q5

• Complete the method evenCount to the IntArrayProcessor class. evenCount finds and returns the number of even integers in the array
  • In the textbook: Section 6.7.2 page 268 counts the matches in a string. It can be adapted to count mathces in an array
• Uncomment the code in the tester that tests this method
  

Activity - Array Algorithms: Separating elements

Record your participation in Piazza clicker question Lesson17 Q6

• Complete the method commaSeparatedList in IntArrayProcessor which returns a String listing the integers in the array. The integers are listed separated with a ", " (That is
  * a comma followed by a space) Notice that there is no comma after the last element
  Example: 5, 3, 7, 1, 4, 7, 6
  • In the textbook: For separators: Section 7.3.4 page 319.
• Uncomment the code in the tester that tests this method
  

Activity - Swapping Elements

Record your participation in Piazza clicker question Lesson17 Q7

• Add a method to IntArrayProcessor to swap the elements at index1 and index2

  public void swap(int index1, int index2)

  • In the textbook: Section7.3.8 page 322
• Add code to IntArrayProcessorTester to test the method.

Activity - Partially filled Arrays

Record your participation in Piazza clicker question Lesson17 Q8

Here is a starter project for working with partially filled arrays

• Write the code to remove the element at index 2 of an ordered array.
  • In the textbook: Section 7.3.6 page 320
• Print the size of the filled portion after the removal
• Print just the elements in the filled porton after the removal