ArrayLists-WE6-P1

Section 13.14 ArrayLists-WE6-P1

Subgoals for Evaluating ArrayLists.

Declaring and initialization of an ArrayList
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1. Set up a one-dimensional dynamic list (initially empty or with a specified initial capacity)
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2. Upon instantiation, an ArrayList contains zero elements initially, but elements can be added dynamically using add(). Elements not yet added do not exist until explicitly inserted.
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Determine access or change of element, or action on entire ArrayList object, and update elements as needed (remembering assignment subgoals)
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Accessing an ArrayList element
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1. Evaluate expression within get(index) which will be the index for the element to be accessed
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2. arrayListName.get(index) returns the value stored at that index
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3. index must be between 0 and arrayListName.size() - 1, inclusive; otherwise IndexOutOfBoundsException occurs
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Changing value of an ArrayList element
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1. Evaluate expression within set(index, value) which will be the index for the element to be replaced
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2. arrayListName.set(index, value) replaces the element at index with the specified value
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3. (remember the assignment subgoals for verifying data types and evaluating expressions)
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4. (remember rules for index values)
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Whole ArrayList actions
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1. Passing as argument - a copy of the reference to the instantiated ArrayList is passed to the method. This means that any changes made to the elements inside the method persist outside the method. The exception is if the argument is assigned to reference a different ArrayList inside the method.
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2. Assignment - changes the reference to point to the ArrayList on the right-hand side of the assignment operator.
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Subsection 13.14.1

Exercises Exercises

1.

Q18: Put the following code in order to create a program that will declare and instantiate an ArrayList of 20 random Integer values between 1 and 100 and then calculate and print the average of the first 10 numbers.

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import java.util.*;
public class main{
   public static void main (String[] args) {
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      Random r = new Random();
      ArrayList<Integer> arr = new ArrayList<Integer>();
      int sum = 0;
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      for (int i = 0; i < 20; i++) {
         arr.add(r.nextInt(100) + 1);
         System.out.println("arr.get("+i+") is " + arr.get(i));
      }
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      for (int i = 0; i < 10; i++)
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         sum += arr.get(i);
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      System.out.println("Avg of first 10 is " + sum/10.0);
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   }
}

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2.

Q19: Put the following code in order to create a program that will declare and instantiate an ArrayList of 20 random Integer values between 1 and 100 and then calculate and print the average of the last n numbers.

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import java.util.*;
public class main{
   public static void main (String[] args) {
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      Random r = new Random();
      int size = 20;
      ArrayList<Integer> arr = new ArrayList<Integer>();
      int sum = 0;
      int n = r.nextInt(size);
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      for (int i = 0; i < 20; i++) {
         arr.add(r.nextInt(100) + 1);
         System.out.println("arr.get("+i+") is " + arr.get(i));
      }
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      if (n <= 0)
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         System.out.println("No numbers to average.");
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      else {
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         for (int i = 20-n; i < 20; i++)
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             sum += arr.get(i);
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         System.out.println("Avg of last n is " + (sum * 1.0)/n);
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      }
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   }
}

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