7.4 - Developing Algorithms Using ArrayLists

• 7.4 Developing Algorithms Using ArrayLists
  • Now, how can we modify this to use an ArrayList?

7.4 Developing Algorithms Using ArrayLists

Common Arraylist Methods:

• size(): Returns the size of the arraylist as an Integer
• add(object): Adds an object to the end of your ArrayList
• void add(index, object): Addes an object to an index of your choice. Shifts the index of everything to the right by one and increases size by 1
• get(index): Retrieves the object at the index specified
• set(index, obj): Like void add, but instead of adding, it replaces the object that’s already in that index
• remove(index): Removes the object at specified index

//size() & add(object)

ArrayList<Double> numbers = new ArrayList<>();
    numbers.add(1.0);
    numbers.add(2.0);
    numbers.add(3.0);

int size = numbers.size();

System.out.println(size);

3

//void add(index, object)
//get(index)
ArrayList<Double> numbers = new ArrayList<>();
    numbers.add(1.0);
    numbers.add(2.0);
    numbers.add(3.0);

System.out.println(numbers.get(2));

    numbers.add(2,4.0);

System.out.println(numbers.get(2));
System.out.println(numbers.get(3));

3.0
4.0
3.0

// set(index, obj)


ArrayList<Double> numbers = new ArrayList<>();
    numbers.add(1.0);
    numbers.add(2.0);
    numbers.add(3.0);

System.out.println(numbers.get(2));

    numbers.set(2,4.0);

System.out.println(numbers.get(2));

3.0
4.0

// remove(index)


ArrayList<Double> numbers = new ArrayList<>();
    numbers.add(1.0);
    numbers.add(2.0);
    numbers.add(3.0);
System.out.println(numbers.get(2));
    numbers.remove(2);

System.out.println(numbers.get(0));
System.out.println(numbers.get(1));
System.out.println(numbers.get(2));

//anybody know why we get an IndexOutofBoundsException eror?

3.0
1.0
2.0



---------------------------------------------------------------------------

java.lang.IndexOutOfBoundsException: Index 2 out of bounds for length 2

	at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)

	at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)

	at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:266)

	at java.base/java.util.Objects.checkIndex(Objects.java:361)

	at java.base/java.util.ArrayList.get(ArrayList.java:427)

	at .(#38:1)

Here’s an example of a program using Arrays that finds the maximum value:

public class Main {
    public static void main(String[] args) {
        double[] values = {1, 2, 3, 4, 5};

        double maxValue = findMax(values);
        System.out.println("The maximum value is: " + maxValue);
    }

    private static double findMax(double[] values) {
        double max = values[0];
        for (int index = 1; index < values.length; index++) {
            if (values[index] > max) {
                max = values[index];
            }
        }
        return max;
    }
}
Main.main(null);

The maximum value is: 5.0

Now, how can we modify this to use an ArrayList?

public class Main {
    public static void main(String[] args) {
        ArrayList<Double> values = new ArrayList<>();
        values.add(1.2);
        values.add(3.4);
        values.add(2.6);
        values.add(4.9);
        values.add(0.8);

        double maxValue = findMax(values);
        System.out.println("The maximum value is: " + maxValue);
    }

    private static double findMax(ArrayList<Double> values) {
        double max = values.get(0);

        for (int index = 1; index < values.size(); index++) {
            if (values.get(index) > max) {
                max = values.get(index);
            }
        }
        return max; 
    }
}
Main.main(null);

The maximum value is: 4.9

Homework:

(Paragraph Answer)

1. What is the difference between the two examples above. Which one is better and why? Using an array is better because we are not changing/do not need to change the list of numbers. Arraylists are more flexible but take more time, while Arrays are more rigid but less time.

(Code Answer)

1. Make your own algorithm using ArrayLists that finds the sum of the elements in the ArrayList

import java.util.ArrayList;

int sum = 0;

ArrayList<Integer> myList = new ArrayList<>();
myList.add(50);
myList.add(30);
myList.add(20);


for (int i = 0; i < myList.size(); i++) {
    sum += myList.get(i); 
}


System.out.println(sum);

100