Assessment: Write ArrayLists

Section 14.6 Assessment: Write ArrayLists

Subgoals for Writing ArrayLists.

Importing the ArrayList class
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1. Before using ArrayList, import it from the java.util package:
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  import java.util.ArrayList;
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Declaring an ArrayList variable
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1. Determine the type of objects to be stored (use wrapper classes for primitives, e.g., Integer instead of int)
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2. Determine the name of the ArrayList variable
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3. Use syntax: ArrayList<DataType> name;
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Instantiating an ArrayList object
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1. Use the new keyword with the constructor to create a new ArrayList object (When ArrayLists are instantiated, they are empty and have a size of 0.)
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Adding elements to an ArrayList
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1. To add to the end of an ArrayList, use: listName.add(valueToBeAdded)
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2. To add an element at a specific location in an ArrayList, use: listName.add(index, valueToBeAdded) where index is within the bounds 0 to listName.size()
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Accessing an element in an ArrayList
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1. Determine the index of the element to be accessed
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2. Use: listName.get(index) to retrieve the element
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3. Ensure the index is within bounds: 0 to listName.size() - 1, otherwise an IndexOutOfBoundsException occurs
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Changing a value in an ArrayList
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1. Determine the index of the element to be changed
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2. Determine the new value or expression to assign
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3. Use: listName.set(index, newValue) to update the value
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4. Ensure the index is within bounds: 0 to listName.size() - 1, otherwise an IndexOutOfBoundsException occurs
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Traversing an ArrayList
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1. Decide whether accessing all elements, updating, or accessing a subset
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2. If only accessing elements, use an enhanced for (for-each) loop:
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  1. for (DataType item : listName) - iterates from first to last, storing a copy of each element in item
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3. If updating or using indices, use a traditional for loop:
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  1. Initialize loop control variable to 0 (or listName.size() - 1 for reverse)
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  2. Set condition: i < listName.size() (or i >= 0 for reverse)
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  3. Increment or decrement loop control variable appropriately
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4. Use Subgoals 5 or 6 to access or change values as appropriate
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Whole list actions
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1. Passing an ArrayList as an argument
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  1. Check if the method expects an ArrayList argument (check documentation or method signature)
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  2. When calling a method, pass a reference to an ArrayList (usually variable name) as an argument in the method call.
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  3. Note: changes to elements in the ArrayList that are done inside the method will persist
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Exercises Exercises

1.

Q1: Put the following code statements in the correct order for a method that takes an array list of doubles and subtracts the value of the second parameter from all the values in the list.

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import java.util.ArrayList;
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public void subtractValue(ArrayList<Double> numList, double value) {
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   for (int i = 0; i < numList.size(); i++) {
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      numList.set(i, numList.get(i) - value);
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   }
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}

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2.

Q2: Put the following code statements in the correct order for a method that takes all strings that have a length longer than minLength and puts them in a new ArrayList that is returned.

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import java.util.ArrayList;
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public ArrayList<String> returnStringsOverXLength (ArrayList<String> candidates, int minLength) {
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   for(String s: candidates)
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      if (s.length() > minLength)
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         result.add(s);
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   return result;
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}

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3.

Q3: 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 num: arr)
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         sum += num;
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      System.out.println("Avg of first 10 is " + sum/10.0);
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   }
}

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4.

Q4: Put the following code in order to create a method that will count the number of occurrences of a target value and return the count.

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public static int count (ArrayList<Integer> arr, int target) {
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   int num = 0;
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   for (int i = 0; i < arr.size(); i++) {
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      if (arr.get(i) == target)
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         num++;
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   }
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   return num;
}

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