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(Find the link in ‘coding’ on SLACK)

Topic 6.1 - Array Creation and Access (Sreeja)

Vocabulary

Array: a data strucutre used to implement a collection of object referance data
Element: a single value within an array
Index of an element: position of an element in the array (In java, the first element of an array is at index 0)
Length of an array: number of elements in the array

Declaring an Array

Defines the array variable, specifying its data type and name.

// Syntax: dataType[] arrayName;
int[] numbers; // Declare an integer array
String[] names; // Declare a string array

Creating an Array

Gives memory for the array and specifies its size.

// Syntax: arrayName = new dataType[size];
numbers = new int[5]; // Create an integer array with 5 elements
names = new String[3]; // Create a string array with 3 elements

Initializing an Array

Populates the array with initial values.

// Syntax: arrayName = new dataType[size];
numbers = new int[5]; // Create an integer array with 5 elements
names = new String[3]; // Create a string array with 3 elements

Accessing Array Elements

Retrieves a specific element’s value from the array using its index.

int[] numbers = {10, 20, 30, 40, 50};
int element = numbers[2]; // Access the third element (30) using index 2
System.out.println(element); // Output: 30

Array Length

Obtains and displays the number of elements in the array.

int[] numbers = {10, 20, 30, 40, 50};
int length = numbers.length; // Get the length of the array
System.out.println("Array length: " + length); // Output: Array length: 5

Array length: 5

Modifying Array Elements

Updates the value of a specific element in the array.

int[] numbers = {10, 20, 30, 40, 50};
numbers[2] = 35; // Change the third element to 35

Iterating Through an Array

Loops through the array, printing each element.

int[] numbers = {10, 20, 30, 40, 50};
for (int i = 0; i < numbers.length; i++) {
    System.out.println(numbers[i]);
}

Enhanced For Loop (For-each)

Iterates through the array using a simplified loop structure, printing each element.

int[] numbers = {10, 20, 30, 40, 50};
for (int number : numbers) {
    System.out.println(number);
}

Topic 6.2 - Traversing Array (1D) (Tanisha)

Using iteration statements (standard for loops and while loops) to access each element in an array.

Standard For Loop

An array in java is indexed from 0 to the number of elements - n.

Review on For Loops

init: The init expression is used for initializing a variable, and it is executed only once.
condition: It executes the condition statement for every iteration
incr/decr: It is the increment or decrement statement applied to the variable, updates the initial expression.

import java.util.Random;

public class RandomArray {
    public static void main(String[] args){
        int [] list = new int[6];
        Random rand = new Random();

        for (int i = 0; i < list.length; i++){
            list[i] = rand.nextInt(4);
        }

        for(int element: list){
            System.out.println(element);
        }
    }
}

RandomArray.main(null);

Class Discussion-Take Notes, these will count for points in your hacks!

What do the for loops accomplish?
- They iterate through each element in a list
What is the difference between how elements of the array list are accessed?
- The difference between how elements of the array are accessed is that the int element:list has the element value equal to the element in the array
BONUS: When the array list of ints was first created, what was each int in the list initialized to?
- Each int in the list is initialized to a certain value starting from 0.

download

For loop : Accessing Some Elements of a List

Class Discussion-Take Notes, these will count for points in your hacks!

If I only wanted to access the elements at even indices of the list (0, 2, 4), what could I change in the statement below to accomplish that?
What about odd?

// EVEN
int[] list = {0, 1, 2, 3, 4, 5};
System.out.println("Even Index");
for(int index = 0; index < list.length; index+=2){
    System.out.println(list[index]);
}

// ODD
int[] list = {0, 1, 2, 3, 4, 5};
System.out.println("Odd Index");
for(int index = 1; index < list.length; index+=2){
    System.out.println(list[index]);
}

Even Index
0
2
4
Odd Index
1
3
5

Note: These are NOT traversals, even though these are for loops. This is because not every element in the array is accessed.

Standard While Loop

Does the following loop accomplish traversing the array?

int [] list = new int[5];
int index = 0; 

while (index < list.length) 
{
    // Do something
    index ++; 
}

This while loop and the for loop we used earlier accomplish the same task. The main difference is that after the loop is completed, the variable ‘index’ in the while loop will still exist. The variable ‘i’ in the for loop will not. Why?
- the for loop created the temporary variable i, and since it is initialized as a condition of the loop, the variable is removed after the loop.

Bounds Errors

When traversing an array, we need to be careful with the indices to avoid an ArrayIndexOutOfBoundsException being thrown.

ATTENTION: MOST COMMON MISTAKE:

What is wrong with the for loop and while loop below? Why does this produce an ArrayIndexOutOfBoundsException error? Since the first element is at index 0, the length of the list will be 1 more than the index of the last element. If the you try and access an index that is greater than the last index, it will give an ArrayIndexOutOfBoundsException error.

for(int i = 0; i < list.length; i ++)

int index = 0; 
while (index = list.length)

Off by One Error : missing the first or last element of an array when trying to traverse

[0, 1, 2, 3, 4]

// This won't access the last element in the list
for(int i = 0; i < list.length - 1; i ++)

// This won't access the first element in the list
int index = 1; 
while (index <= list.length)

Developing Methods Using Arrays

Reviewing common methods asked on AP Exam FRQs

Average Value

Complete the popcorn hack below in order to return the average value of the elements in the list numbers.

public class ArrayAverage {
    public static void main(String[] args) {
        int[] numbers = {5, 10, 15, 20, 25};
        int sum = 0;
        double average;
        
        for (int i = 0; i < numbers.length;i++) {
            sum += numbers[i]; 
        }
        
       
        average = (double) sum / numbers.length; /* missing code */
        
        System.out.println("The average of the numbers is: " + average);
    }
}

ArrayAverage.main(null);

The average of the numbers is: 15.0

6.3 Enhanced for loop for Arrays (Vivian)

the enhanced for loop is also known as the “for each” loop
provides a simplified way to loop through elements in an array, collection, or other iterable data structures.

//syntax for enhanced for loop
for (dataType element : array) {
    // code to process 'element'
}

the data type in the loop must match the array’s element data type.

//array of int matches element int
int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
    System.out.println(num);
}

Comparing a regular for loop with the enhanced for loop

Popcorn Hack: Rewrite this code to use an enhanced for loop instead. make comments explaining what you added/changed

import java.util.List;

class Quote {
    private List<String> quotes;
    private List<String> emotions;

    public Quote(List<String> quotes, List<String> emotions) {
        this.quotes = quotes;
        this.emotions = emotions;
    }

    public void printQuotesWithEmotions() {
        int j = 0;
        for (String quote : quotes) {
            System.out.println("Quote: \"" + quote + "\""); // Fixed this line
            System.out.println("Emotion: " + emotions.get(j));
            System.out.println("---------------------------");
            j++;
        }
    }

    public static void main(String[] args) {
        List<String> quotes = List.of(
            "Success is not final, failure is not fatal: It is the courage to continue that counts.",
            "The only way to do great work is to love what you do.",
            "The best way to predict the future is to create it."
        );

        List<String> emotions = List.of(
            "Courageous",
            "Passionate",
            "Innovative"
        );

        Quote quotePrinter = new Quote(quotes, emotions);
        quotePrinter.printQuotesWithEmotions();
    }
}

Quote.main(null);

Quote: "Success is not final, failure is not fatal: It is the courage to continue that counts."
Emotion: Courageous
---------------------------
Quote: "The only way to do great work is to love what you do."
Emotion: Passionate
---------------------------
Quote: "The best way to predict the future is to create it."
Emotion: Innovative
---------------------------

What are some of the benefits of using an enhanced for loop in this case versus a regular for loop?

The enhanced for loop makes sure there are no index out of bound errors here, and we can also set it to automatically increment the other variable for the other list so there won’t be index errors for that list either.

Limitations to enhanced for loop

it does not provide access to the index of the current element.
- This means you cannot easily determine the position of the element in the array or collection.
- But when you want to search for a specific element in a collection and you don’t necessarily need to access the index
- If you need to work with indices, you should use a traditional for loop instead.
read-only access to elements.
- You cannot modify the elements within the loop
- Thus, when you need to modify a collection based on a condition. You should use a regular for loop

For the next two code blocks, decide whether or not its better to use a regular for loop or an enhanced one, explain why. write the code for them

Searching for an Element in an ArrayList

ArrayList<String> names = new ArrayList<>();
String searchName = "Vivian";

for (String searchName : names) {
    System.out.println(searchName);
}

Removing Even Numbers from an ArrayList

ArrayList<Integer> numbers = new ArrayList<>();

for (i=1; i < numbers.length; i+=2) {
    
}

6.4: Developing Algorithms Using Arrays (Isabelle)

How to identify the maximum or minimum value in an array

It is a common task to determine what the largest or smallest value stored is inside an array. In order to do this, we need a method that can take a parameter of an array of primitve values (int or double) and return the item that is at the appropriate extreme.

Inside the method a local variable is needed to store the current max or min value that will be compared against all the values in the array. You can assign the current value to be either the opposite extreme or the first item you would be looking at.

You can use either a standard for loop or an enhanced for loop to determine the max or min. Assign the temporary variable a starting value based on what extreme you are searching for.

Inside the for loop, compare the current value against the local variable; if the current value is better, assign it to the temporary variable. When the loop is over, the local variable will contain the appropriate value and is still available and within scope and can be returned from the method.

Find max in an array of `double` values

private 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;
}

Find min in an array of `int` values

private int findMin(int [] values) {
    int min = Integer.MAX_VALUE;

    for (int currentValue: values) {
        if (currentValue < min) {
            min = currentValue;
        }
    }
    return min;
}

Let’s Practice!

Popcorn hack #1

// What needs to be changed to find the index of the max value? (write correct code in cell below)
private int findMax(double [] values) {
    int max = (int)values[0];

    for (int index = 1; index < values.length; index++) {
        if (values[index] > max){
            max = (int)values[index];
        }
    }
    return max;
}

How to calculate the average value from objects in an array

It is a common task to determine what is the average value returned from items stored inside an array. In order to do this, we need a method that can take a parameter of an array of Objects (DebugDuck) and calculate and return the average value that each instance of DebugDuck returns from the method.

Inside the method; a local double variable is needed to store the accumulated values. Then we use a for loop to traverse the array and add the current total to the variable. After accumulating all the values we need to divide the total by the number of items stored in the array.

Using a standard `for` loop

private double calculateAverage(DebugDuck [] ducks) {
    double average = 0.0;

    for (int index = 0; index < ducks.length; index++) {
        average += ducks[index].getQuestionCount();
    }
    average = average / ducks.length;

    return average;
}

Using a standard `enhanced` loop

private double calculateAverage(DebugDuck [] ducks) {
    double average = 0.0;

    for (DebugDuck currentDuck: ducks) {
        average += currentDuck.getQuestionCount();
    }
    average = average / ducks.length;

    return average;
}

Does the order of accumulation matter? No because you are just adding which does not require an order.

Can you declare the variable inside the loop? No, because then you wouldn’t be able to access it outside the loop

Shifting Array contents to the right

The contents of an array often need to be shifted as part of a solution to using the data inside.

We need to know how much to shift the array by. This will need to be an int obviously.

In order to move the contents we next need to make an empty array of the same size and then iterate over the original array and properly copy the values to the adjusted index in the new array.

We then need to assign the new array back into the original variable.

What kind of for loop should we use? Why?

Regular for loop because an enhanced for loop can only read the elements, not access them.

int [] numbers = {1,2,3,4,5};
int [] shifted = new int [numbers.length];
int shift = 8;
for (int index = 0; index < numbers.length; index++) {
    shifted [Math.abs((index + shift) % numbers.length)] = numbers[index];
}
numbers = shifted;
for (int num : numbers) {
    System.out.println(num + " ");
}

Why are we using the % operator?

it is modulus and it makes sure that the shifted index is in bounds.

Popcorn hack #2

How would we code a left shift? Write a left shift using the variables below

public class Main {
    static String[] words = {"one", "two", "three", "four"};
    static int shiftWord = 2;

    public static String[] leftShift(String[] words, int shiftWord) {
        int length = words.length;
        String[] shiftedWords = new String[length];

        for (int i = 0; i < length; i++) {
            int newIndex = (i + shiftWord) % length;   
            shiftedWords[i] = words[newIndex];
        }

        return shiftedWords;
    }

    public static void main(String[] args) {
        String[] shiftedWords = leftShift(words, shiftWord);
        for (String word : shiftedWords) {
            System.out.println(word);
        }
    }
}

Main.main(null);

three
four
one
two

Why should the array index be wrapped in a call to Math.abs?

Math.abs is a tool in programming that helps us find the absolute value of a number. It’s useful when we want to know how far one number is from another, or when we want to ensure that we’re working with positive numbers.

Hacks

Scoring Guidelines:

0.2 for completeing each of the sub-unit hacks mentioned below.
- FRQ/PopCorn hacks will be graded AP Style
0.1 for having organized notebook with note taking when appropriate.
Extra 0.1 for going above expectations for the hacks (being creative!)

6.1 HACK 1 FRQ (<5 min)

Follow the steps in the lesson to just make an array that has some relation to your project. Feel free to use the code examples we provided in your hack if you would like.

public class PeriodSorter {
    public static void main(String[] args) {
        String[] periods = {"Period 3", "Period 5", "Period 1", "Period 4", "Period 2"};
        Arrays.sort(periods);

        System.out.println("Periods sorted:");
        for (String period : periods) {
            System.out.println(period);
        }
    }
}

PeriodSorter.main(null);

Periods sorted:
Period 1
Period 2
Period 3
Period 4
Period 5

6.2 HACK 1 FRQ (<10 min)

Prime Numbers in an Array (5-10 min)

Create a loop to identify and print the prime numbers from an array of integers. Your loop MUST traverse through the given list. Some things to consider:

BONUS: Do this with a for loop AND a while loop

Understand prime numbers and how to check for primality.
Implement a loop and conditional statements to iterate through the array.
Consider data storage (either displaying prime numbers immediately or storing them for later display)

// For Loop

public class PrimeNumberFinder {
    public static void main(String[] args) {
        int[] numbers = {7, 10, 23, 15, 2, 19, 8, 17, 31, 6};
        
        System.out.println("Prime numbers in the array:");
        for (int number : numbers) {
            if (isPrime(number)) {
                System.out.println(number);
            }
        }
    }

    public static boolean isPrime(int num) {
        if (num < 2) {
            return false;
        }
        for (int i = 2; i <= Math.sqrt(num); i++) {
            if (num % i == 0) {
                return false;
            }
        }
        return true;
    }
}

PrimeNumberFinder.main(null);

Prime numbers in the array:
7
23
2
19
17
31

// While Loop

public class PrimeNumberFinder {
    public static void main(String[] args) {
        int[] numbers = {7, 10, 23, 15, 2, 19, 8, 17, 31, 6};
        int i = 0;
        
        System.out.println("Prime numbers in the array:");
        while (i < numbers.length) {
            if (isPrime(numbers[i])) {
                System.out.println(numbers[i]);
            }
            i++;
        }
    }

    public static boolean isPrime(int num) {
        if (num < 2) {
            return false;
        }
        int i = 2;
        while (i <= Math.sqrt(num)) {
            if (num % i == 0) {
                return false;
            }
            i++;
        }
        return true;
    }
}

PrimeNumberFinder.main(null);

Prime numbers in the array:
7
23
2
19
17
31

6.2 HACK 2 MCQ (<5 min)

Multiple Choice Questions

Do NOT Run the code cells. Try to do this on your own.

What will be displayed as the output?

String [] list = {"red", "yellow", "blue"}; 
for (int i = 0; i < list.length; i++)
{
    System.out.print(list[i].length()+ "-" )
}

A. red-yellow-blue
B. 3-3-3-
C. 3-6-4-
D. 3-6-
E. 3-6-4

Write why you chose that answer!

the length of “red” is 3, the length of “yellow” is 6 and the length of “blue” is 4

The code below is meant to display every other number in the list numbers. Which of the following should replace the missing code in order to do this?

int [] numbers = {3, -4, 6, -7, 2}; 
for(int i = 0; i < numbers.length; i+=2)
{
    System.out.println(numbers[i]);
}

3
6
2

A. int i = 0; i < numbers.length/2; i++
B. int i = 1; i < numbers.length; i++
C. int i = 1; i < numbers.length; i+=2
D. int i = 0; i < numbers.length; i++
E. int i = 0; i < numbers.length; i+=2

Write why you chose that answer!

starting index is 0, index cannot be more than the length of the array, you want every other number

(This one is a little hard) Which of the following would fix the code so that the elements in arr are reversed. Hint: try creating a list in your head and trace the code to see if the code accomplishes its goal.

public static void reverseArray(double [] arr)
{
    for(int = 0; i < arr.length; i++)
    {
        double temp = arr[i];
        arr[i] = arr[arr.length-1-i];
        arr[arr.length-1-i] = temp; 
    }
}

A. Change loop condition to: i < arr.length - 1
B. Change loop condition to: i < arr.length/2
C. Change loop condition to: i < arr.length/2 - 1

In case you are having trouble with question 3 the answer is B. Write about why!

It makes sure the loop iterates through the first half of the array which allows the the swapping process to correctly reverse the elements.

6.3 HACK

Just finish the popcorn hacks throughout the lesson!

6.4 HACK

Just finish the 2 popcorn hacks in the lesson!

Unit 6 Lesson

Lesson for APCSA Unit 6. Authors are Soham Kamat

Unit 6 Lesson • 55 min read

Hello! wget this notebook `RIGHT NOW`

Topic 6.1 - Array Creation and Access (Sreeja)

Vocabulary

Declaring an Array

Creating an Array

Initializing an Array

Accessing Array Elements

Array Length

Modifying Array Elements

Iterating Through an Array

Enhanced For Loop (For-each)

Topic 6.2 - Traversing Array (1D) (Tanisha)

Standard For Loop

For loop : Accessing Some Elements of a List

Standard While Loop

Bounds Errors

Developing Methods Using Arrays

Average Value

6.3 Enhanced for loop for Arrays (Vivian)

Comparing a regular for loop with the enhanced for loop

Limitations to enhanced for loop

6.4: Developing Algorithms Using Arrays (Isabelle)

How to identify the maximum or minimum value in an array

Find max in an array of `double` values

Find min in an array of `int` values

Let’s Practice!

How to calculate the average value from objects in an array

Using a standard `for` loop

Using a standard `enhanced` loop

Shifting Array contents to the right

Hacks

6.1 HACK 1 FRQ (<5 min)

6.2 HACK 1 FRQ (<10 min)

6.2 HACK 2 MCQ (<5 min)

6.3 HACK

6.4 HACK

Hello! wget this notebook RIGHT NOW

Topic 6.1 - Array Creation and Access (Sreeja)

Vocabulary

Declaring an Array

Creating an Array

Initializing an Array

Accessing Array Elements

Array Length

Modifying Array Elements

Iterating Through an Array

Enhanced For Loop (For-each)

Topic 6.2 - Traversing Array (1D) (Tanisha)

Standard For Loop

For loop : Accessing Some Elements of a List

Standard While Loop

Bounds Errors

Developing Methods Using Arrays

Average Value

6.3 Enhanced for loop for Arrays (Vivian)

Comparing a regular for loop with the enhanced for loop

Limitations to enhanced for loop

6.4: Developing Algorithms Using Arrays (Isabelle)

How to identify the maximum or minimum value in an array

Find max in an array of double values

Find min in an array of int values

Let’s Practice!

How to calculate the average value from objects in an array

Using a standard for loop

Using a standard enhanced loop

Shifting Array contents to the right

Hacks

6.1 HACK 1 FRQ (<5 min)

6.2 HACK 1 FRQ (<10 min)

6.2 HACK 2 MCQ (<5 min)

6.3 HACK

6.4 HACK

Hello! wget this notebook `RIGHT NOW`

Find max in an array of `double` values

Find min in an array of `int` values

Using a standard `for` loop

Using a standard `enhanced` loop