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
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
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]);
}
10
20
30
40
50
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);
}
10
20
30
40
50
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 _ to the number of elements - _.
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 LOOP 1
for (int i = 0; i < list.length; i++){
list[i] = rand.nextInt(4);
}
// FOR LOOP 2
for(int element: list){
System.out.println(element);
}
}
}
RandomArray.main(null);
2
3
0
1
1
3
Class Discussion-Take Notes, these will count for points in your hacks!
- What do the for loops accomplish? Let you traverse through the list and accessing each element.
- What is the difference between how elements of the array list are accessed? No difference, because it goes through every element in the array list
- BONUS: When the array list of ints was first created, what was each int in the list initialized to? 0
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?
Change the index ++ to index+=2 - What about odd?
Set starting index to one and once again increment index by 2
// 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 variable i is out of scope
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? Make it list.length - 1 or just say i < list.length;
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 ++)
// Have to remove the equal sign and make it i < list.length - 1 ;
// This won't access the first element in the list
int index = 1;
while (index <= list.length)
// Index should start at zero
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++) {
// for (int i = 0; i <= numbers.length - 1; i++) {
sum += numbers[i];
}
average = (double) sum / numbers.length;
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);
}
1
2
3
4
5
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 i = 0;
for (String quote : quotes) {
if (i < emotions.size()) {
String emotion = emotions.get(i);
System.out.println("Quote: \"" + quote + "\"");
System.out.println("Emotion: " + emotion);
System.out.println("---------------------------");
}
i++;
}
}
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();
}
}
What are some of the benefits of using an enhanced for loop in this case versus a regular for loop?
It makes the code simpler and easier to read.
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
Enhanced because it would make the code simpler.
import java.util.ArrayList;
public class SearchArrayList {
public static void main(String[] args) {
ArrayList<String> names = new ArrayList<>();
String searchName = "Vivian";
// Add some names to the list (this is just an example)
names.add("Alice");
names.add("Bob");
names.add("Charlie");
// Check if the ArrayList contains the searchName
if (names.contains(searchName)) {
System.out.println(searchName + " is found in the list.");
} else {
System.out.println(searchName + " is not found in the list.");
}
}
}
- Removing Even Numbers from an ArrayList
Regular, because you can’t modify an element with an enhanced for loop.
import java.util.ArrayList;
import java.util.Iterator;
public class RemoveEvenNumbers {
public static void main(String[] args) {
ArrayList<Integer> numbers = new ArrayList<>();
// Add some numbers to the list (this is just an example)
numbers.add(1);
numbers.add(2);
numbers.add(3);
numbers.add(4);
numbers.add(5);
// Use an iterator to safely remove even numbers
Iterator<Integer> iterator = numbers.iterator();
while (iterator.hasNext()) {
Integer number = iterator.next();
if (number % 2 == 0) {
iterator.remove(); // Removes even numbers
}
}
// Print the updated ArrayList
System.out.println("ArrayList after removing even numbers: " + numbers);
}
}
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 findMaxIndex(int[] values) {
int max = values[0];
int maxIndex = 0;
for (int index = 1; index < values.length; index++) {
if (values[index] > max) {
max = values[index];
maxIndex = index;
}
}
System.out.println(maxIndex); // Print the index of the maximum value
return maxIndex; // Return the index of the maximum value
}
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 no matter how you iterate through the list, the sum is still gonna be the same.
Can you declare the variable inside the loop?
No, because then you wouldn’t be able to access it outside the loop
Shfiting 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’t access certain elements.
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 + " ");
}
3
4
5
1
2
Why are we using the % operator?
It would make sure that the shifted index is still in bounds.
Popcorn hack #2
How would we code a left shift? Write a left shift using the variables below
String [] words = {"alpha", "beta", "gamma", "delta"};
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);
}
}
Why should the array index be wrapped in a call to Math.abs?
Math.abs is used to obtain the absolute (non-negative) value of a number by ignoring its sign. It’s commonly employed when you need to calculate the magnitude, absolute difference, or ensure non-negativity in various programming scenario.
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.
import java.util.Arrays;
public class WeatherSorter {
public static void main(String[] args) {
double[] temperatures = {72.5, 80.0, 88.3, 76.7, 85.1, 95.2, 78.9};
Arrays.sort(temperatures);
for (int i = 0; i < temperatures.length / 2; i++) {
double temp = temperatures[i];
temperatures[i] = temperatures[temperatures.length - 1 - i];
temperatures[temperatures.length - 1 - i] = temp;
}
System.out.println("Weather from hottest to coldest:");
for (double temp : temperatures) {
System.out.println(temp);
}
}
}
WeatherSorter.main(null)
Weather from hottest to coldest:
95.2
88.3
85.1
80.0
78.9
76.7
72.5
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 method
public class PrimeNumberFinder {
public static void main(String[] args) {
int[] numbers = {2, 3, 5, 10, 13, 17, 20, 23, 29};
System.out.println("Prime numbers in the array:");
for (int num : numbers) {
if (isPrime(num)) {
System.out.println(num);
}
}
}
public static boolean isPrime(int number) {
if (number <= 1) {
return false;
}
for (int i = 2; i * i <= number; i++) {
if (number % i == 0) {
return false;
}
}
return true;
}
}
PrimeNumberFinder.main(null)
Prime numbers in the array:
2
3
5
13
17
23
29
// while loop method
public class PrimeNumberFinder2 {
public static void main(String[] args) {
int[] numbers = {2, 3, 5, 10, 13, 17, 20, 23, 29};
System.out.println("Prime numbers in the array:");
int i = 0;
while (i < numbers.length) {
if (isPrime(numbers[i])) {
System.out.println(numbers[i]);
}
i++;
}
}
public static boolean isPrime(int number) {
if (number <= 1) {
return false;
}
int i = 2;
while (i * i <= number) {
if (number % i == 0) {
return false;
}
i++;
}
return true;
}
}
PrimeNumberFinder2.main(null)
Prime numbers in the array:
2
3
5
13
17
23
29
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
C
Write why you chose that answer! It would go through each element of the list and print the length of each element separated by a hyphen.
- 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(/*missing code*/)
{
System.out.println(numbers[i]);
}
- 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
E
Write why you chose that answer!
This for loop starts at index 0 and increments i by 2 in each iteration, which effectively selects every other number in the array for printing.
- (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
B
In case you are having trouble with question 3 the answer is B. Write about why!
By changing the loop condition to i < arr.length/2, you ensure that you iterate only up to half of the array’s length. This is because you’re swapping elements symmetrically from the beginning and end of the array. Using i < arr.length/2 is the correct way to reverse the array elements.
6.3 HACK
- Just finish the popcorn hacks throughout the lesson!
6.4 HACK
- Just finish the 2 popcorn hacks in the lesson!