📘 PART 2: ARRAYS, STRINGS AND FUNCTIONS

🎯 General Objectives

• Get familiar with using arrays to store and process data collections.
• Understand and work with strings in Java.
• Create and use functions to divide programs into independent logical blocks.

🧑‍🏫 Lesson 1: Arrays in Java

Array concept

• Array is a data structure used to store multiple values of the same data type in a single variable.
• Array characteristics:
  • Fixed size after initialization
  • Elements are stored consecutively in memory
  • Elements can be accessed through index, starting from 0
  • Can be one-dimensional or multi-dimensional arrays

Array declaration and initialization

// Array declaration
int[] numbers;              // Method 1: data_type[] array_name;
int scores[];               // Method 2: data_type array_name[];

// Array initialization
numbers = new int[5];       // Initialize array with 5 elements, default is 0
scores = new int[]{90, 85, 75, 80, 95}; // Initialize and assign values

// Declare and initialize together
int[] grades = {92, 88, 78, 85, 90};  // Shorthand notation

Multi-dimensional arrays

// 2D array
int[][] matrix = new int[3][4]; // 3 rows, 4 columns array

// Initialize with values
int[][] grid = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

Array storage in memory

1. One-dimensional array:

   int[] numbers = {10, 20, 30, 40, 50};

   Variable numbers  →  [ Address ] → [ 10 | 20 | 30 | 40 | 50 ]
   (Reference)          (Heap)        [0]  [1]  [2]  [3]  [4]  (index)

2. Two-dimensional array:

   int[][] matrix = {
       {1, 2, 3},
       {4, 5, 6},
       {7, 8, 9}
   };

   Variable matrix    →  [ Address ] → [ Address row 0 ] → [ 1 | 2 | 3 ]
   (Reference)           (Heap)        [ Address row 1 ] → [ 4 | 5 | 6 ]
                                       [ Address row 2 ] → [ 7 | 8 | 9 ]

Accessing and modifying elements

int[] numbers = {10, 20, 30, 40, 50};

// Access element
System.out.println("3rd element: " + numbers[2]); // Result: 30

// Modify element
numbers[1] = 25; // Change value of 2nd element to 25

// Iterate array
for (int i = 0; i < numbers.length; i++) {
    System.out.println("Element " + i + ": " + numbers[i]);
}

// Iterate array using for-each
for (int num : numbers) {
    System.out.println(num);
}

Real-world example - Calculate average score

public class AverageCalculator {
    public static void main(String[] args) {
        double[] grades = {85.5, 90.0, 78.5, 92.5, 88.0};
        double sum = 0;

        for (double grade : grades) {
            sum += grade;
        }

        double average = sum / grades.length;
        System.out.println("Average score: " + average);
    }
}

🧑‍🏫 Lesson 2: Strings in Java

• Strings are objects in Java, not primitive data types.
• A string is an array of characters (char).
• Strings in Java are immutable, meaning the content of a string cannot be changed after creation.

String declaration and initialization

// String declaration using literal
String greeting = "Hello JAVA";

// String declaration using constructor
String message = new String("Learning JAVA is fun");

Common String methods

String text = "Learn JAVA programming";

// String length
int length = text.length();  // length = 22

// Get character at position
char firstChar = text.charAt(0);  // 'L'

// Substring extraction
String subText = text.substring(6, 10);  // from position 6 to 9 => "JAVA"
 String subText2 = text.substring(6);     // from position 6 to end => "JAVA programming"

// Find position of occurrence
int position = text.indexOf("JAVA");  // 6

// Convert to uppercase/lowercase
String upperCase = text.toUpperCase();  // "LEARN JAVA PROGRAMMING"
String lowerCase = text.toLowerCase();  // "learn java programming"

// Concatenate strings
String newText = text.concat(" basics");  // "Learn JAVA programming basics"

// Replace string
String replaced = text.replace("JAVA", "Python");  // "Learn Python programming"

// Check starts/ends with
boolean startsWith = text.startsWith("Learn");  // true
boolean endsWith = text.endsWith("programming");    // true

// Remove leading/trailing whitespace
String trimmed = "  Hello  ".trim();  // "Hello"

// Check if string is empty
boolean empty = "".isEmpty();  // true

String comparison

String str1 = "Hello";
String str2 = "Hello";
String str3 = new String("Hello");

// Compare reference (memory location)
System.out.println(str1 == str2);  // true (same reference in String pool)
System.out.println(str1 == str3);  // false (different reference)

// Compare value (content)
System.out.println(str1.equals(str2));  // true
System.out.println(str1.equals(str3));  // true

// Compare ignoring case
System.out.println("hello".equalsIgnoreCase("HELLO"));  // true

Real-world example - Count words in string

public class WordCounter {
    public static void main(String[] args) {
        String sentence = "JAVA is a popular object-oriented programming language";
        String[] words = sentence.split(" ");

        System.out.println("Number of words in sentence: " + words.length);

        // Print words
        for (String word : words) {
            System.out.println(word);
        }
    }
}

🧑‍🏫 Lesson 3: Functions in Java

• Functions (or methods) are code blocks that can be called and executed multiple times in a program. They help organize code, enable reusability, and make programs more readable.

Function declaration syntax

[modifier] [return_type] [method_name]([parameter_list]) {
    // Method body
    [return statement]
}

Example:

public static int sum(int a, int b) {
    return a + b;
}

Types of functions

// Function with no return value (void)
public static void sayHello() {
    System.out.println("Hello!");
}

// Function returning primitive type
public static int square(int number) {
    return number * number;
}

// Function returning object
public static String formatName(String firstName, String lastName) {
    return lastName + " " + firstName;
}

// Function with multiple parameters
public static double average(double a, double b, double c) {
    return (a + b + c) / 3;
}

Parameters and arguments

public class ParameterExample {
    public static void main(String[] args) {
        // value1, value2 are arguments
        int result = add(5, 3);  // 5, 3 are arguments
        System.out.println("Sum: " + result);
    }

    // a, b are parameters
    public static int add(int a, int b) {  // a, b are parameters
        return a + b;
    }
}

Variable scope

public class ScopeExample {
    // Global variable (class/instance variable)
    static int globalVar = 10;

    public static void main(String[] args) {
        // Local variable of main
        int localVar = 5;

        System.out.println(globalVar);  // Can access
        System.out.println(localVar);   // Can access

        // Call function and pass parameter
        testScope(20);
    }

    public static void testScope(int paramVar) {
        // paramVar is a parameter
        int anotherLocal = 15;

        System.out.println(globalVar);     // Can access
        System.out.println(paramVar);      // Can access
        System.out.println(anotherLocal);  // Can access

        // System.out.println(localVar);  // Error! Cannot access localVar from main
    }
}

Real-world example - Calculate bank interest

public class BankInterestCalculator {
    public static void main(String[] args) {
        double principal = 10000000;  // 10 million VND
        double rate = 0.06;          // 6% annual interest rate
        int years = 5;               // 5 years

        double result = calculateInterest(principal, rate, years);
        System.out.printf("After %d years, the amount is: %.2f VND\n", years, result);
    }

    // Function to calculate compound interest
    public static double calculateInterest(double principal, double rate, int years) {
        // Compound interest formula: A = P(1 + r)^t
        return principal * Math.pow(1 + rate, years);
    }
}

🧑‍🏫 Lesson 4: Using arrays and strings together

Declaring and initializing string arrays

// Declare string array
String[] names;

// Initialize array
names = new String[5];

// Assign values
names[0] = "Nguyen Van A";
names[1] = "Tran Thi B";
names[2] = "Le Van C";
names[3] = "Pham Thi D";
names[4] = "Hoang Van E";

// Declare and initialize together
String[] fruits = {"Apple", "Banana", "Orange", "Mango", "Watermelon"};

Searching in string arrays

public class StringArraySearch {
    public static void main(String[] args) {
        String[] fruits = {"Apple", "Banana", "Orange", "Mango", "Watermelon"};
        String searchFor = "Orange";

        // Linear search
        boolean found = false;
        for (int i = 0; i < fruits.length; i++) {
            if (fruits[i].equals(searchFor)) {
                System.out.println("Found '" + searchFor + "' at position " + i);
                found = true;
                break;
            }
        }

        if (!found) {
            System.out.println("'" + searchFor + "' not found in array");
        }
    }
}

Sorting string arrays

import java.util.Arrays;

public class StringArraySort {
    public static void main(String[] args) {
        String[] names = {"Nam", "An", "Hoa", "Binh", "Mai"};

        System.out.println("Original array:");
        for (String name : names) {
            System.out.print(name + " ");
        }

        // Sort array (alphabetically)
        Arrays.sort(names);

        System.out.println("\nSorted array:");
        for (String name : names) {
            System.out.print(name + " ");
        }
    }
}

Processing strings in arrays

public class StringArrayProcessing {
    public static void main(String[] args) {
        String[] sentences = {
            "JAVA is a programming language",
            "Python is very popular nowadays",
            "JavaScript is used for web",
            "JAVA can build many applications"
        };

        // Count sentences containing "JAVA"
        int javaCount = 0;
        for (String sentence : sentences) {
            if (sentence.contains("JAVA")) {
                javaCount++;
            }
        }
        System.out.println("Number of sentences containing 'JAVA': " + javaCount);

        // Convert all sentences to uppercase
        System.out.println("\nSentences converted to uppercase:");
        for (int i = 0; i < sentences.length; i++) {
            sentences[i] = sentences[i].toUpperCase();
            System.out.println(sentences[i]);
        }
    }
}

Real-world example - Student list analysis

public class StudentAnalysis {
    public static void main(String[] args) {
        // Student list with format: "Name:Score"
        String[] students = {
            "Nguyen Van An:8.5",
            "Tran Thi Binh:9.0",
            "Le Van Cuong:7.5",
            "Pham Thi Diep:6.5",
            "Hoang Van Em:5.0"
        };

        // Separate name and score information
        String[] names = new String[students.length];
        double[] scores = new double[students.length];

        for (int i = 0; i < students.length; i++) {
            String[] parts = students[i].split(":");
            names[i] = parts[0];
            scores[i] = Double.parseDouble(parts[1]);
        }

        // Calculate average score
        double sum = 0;
        for (double score : scores) {
            sum += score;
        }
        double average = sum / scores.length;

        // Find student with highest score
        double maxScore = scores[0];
        int maxIndex = 0;

        for (int i = 1; i < scores.length; i++) {
            if (scores[i] > maxScore) {
                maxScore = scores[i];
                maxIndex = i;
            }
        }

        // Display results
        System.out.printf("Class average score: %.2f\n", average);
        System.out.println("Student with highest score: " + names[maxIndex] +
                           " with score " + scores[maxIndex]);
    }
}

🧑‍🏫 Lesson 5: Functions and arrays

Passing arrays to functions

public class ArrayAsParameter {
    public static void main(String[] args) {
        int[] numbers = {5, 10, 15, 20, 25};

        // Call function and pass array as parameter
        printArray(numbers);

        // Arrays are references, so changes in function will affect original array
        modifyArray(numbers);

        System.out.println("\nArray after modification:");
        printArray(numbers);
    }

    // Function to print array
    public static void printArray(int[] arr) {
        for (int num : arr) {
            System.out.print(num + " ");
        }
        System.out.println();
    }

    // Function to modify array
    public static void modifyArray(int[] arr) {
        for (int i = 0; i < arr.length; i++) {
            arr[i] *= 2; // Double each element
        }
    }
}

Common array processing functions

public class ArrayHelperFunctions {
    // Function to calculate array sum
    public static int sum(int[] arr) {
        int total = 0;
        for (int num : arr) {
            total += num;
        }
        return total;
    }

    // Function to find maximum value
    public static int findMax(int[] arr) {
        if (arr.length == 0) {
            throw new IllegalArgumentException("Empty array");
        }

        int max = arr[0];
        for (int i = 1; i < arr.length; i++) {
            if (arr[i] > max) {
                max = arr[i];
            }
        }
        return max;
    }

    // Function to find minimum value
    public static int findMin(int[] arr) {
        if (arr.length == 0) {
            throw new IllegalArgumentException("Empty array");
        }

        int min = arr[0];
        for (int i = 1; i < arr.length; i++) {
            if (arr[i] < min) {
                min = arr[i];
            }
        }
        return min;
    }

    // Function to calculate average
    public static double average(int[] arr) {
        if (arr.length == 0) {
            throw new IllegalArgumentException("Empty array");
        }

        return (double) sum(arr) / arr.length;
    }

    // Function to sort array (using Bubble Sort algorithm)
    public static void bubbleSort(int[] arr) {
        int n = arr.length;
        boolean swapped;

        for (int i = 0; i < n - 1; i++) {
            swapped = false;

            for (int j = 0; j < n - i - 1; j++) {
                if (arr[j] > arr[j + 1]) {
                    // Swap arr[j] and arr[j+1]
                    int temp = arr[j];
                    arr[j] = arr[j + 1];
                    arr[j + 1] = temp;
                    swapped = true;
                }
            }

            // If no elements were swapped, array is sorted
            if (!swapped) {
                break;
            }
        }
    }
}

Real-world example - Sales data analysis

public class SalesAnalysis {
    public static void main(String[] args) {
        // Monthly sales data (in millions)
        double[] monthlySales = {120.5, 115.2, 130.8, 140.3, 175.2, 168.7,
                                 155.4, 160.1, 190.3, 185.6, 178.4, 220.5};

        System.out.printf("Total annual sales: %.2f million\n", sumSales(monthlySales));
        System.out.printf("Average monthly sales: %.2f million\n", averageSales(monthlySales));
        System.out.printf("Highest sales month: %d with %.2f million\n",
                        findHighestMonth(monthlySales) + 1, monthlySales[findHighestMonth(monthlySales)]);
        System.out.printf("Lowest sales month: %d with %.2f million\n",
                        findLowestMonth(monthlySales) + 1, monthlySales[findLowestMonth(monthlySales)]);

        // Trend analysis
        analyzeTrend(monthlySales);
    }

    // Calculate total sales
    public static double sumSales(double[] sales) {
        double total = 0;
        for (double sale : sales) {
            total += sale;
        }
        return total;
    }

    // Calculate average sales
    public static double averageSales(double[] sales) {
        return sumSales(sales) / sales.length;
    }

    // Find highest sales month
    public static int findHighestMonth(double[] sales) {
        int highestMonth = 0;
        for (int i = 1; i < sales.length; i++) {
            if (sales[i] > sales[highestMonth]) {
                highestMonth = i;
            }
        }
        return highestMonth;
    }

    // Find lowest sales month
    public static int findLowestMonth(double[] sales) {
        int lowestMonth = 0;
        for (int i = 1; i < sales.length; i++) {
            if (sales[i] < sales[lowestMonth]) {
                lowestMonth = i;
            }
        }
        return lowestMonth;
    }

    // Analyze trend
    public static void analyzeTrend(double[] sales) {
        // Calculate change between months
        System.out.println("\nSales trend analysis:");

        for (int i = 1; i < sales.length; i++) {
            double change = sales[i] - sales[i-1];
            double percentChange = (change / sales[i-1]) * 100;

            System.out.printf("Month %d to month %d: %.2f%% ", i, i+1, percentChange);

            if (change > 0) {
                System.out.println("(Increase)");
            } else if (change < 0) {
                System.out.println("(Decrease)");
            } else {
                System.out.println("(No change)");
            }
        }
    }
}

🧪 FINAL PROJECT: Student grade management with arrays

Problem Description

Write a program that:

• Declares an array containing student score information (3 subjects: Math, Physics, Chemistry).
• Calculates each student's average score and classifies academic performance.
• Displays the student list and their average scores.

Required functions:

• Function to input scores for students and store in array.
• Function to calculate student's average score.
• Function to classify academic performance based on average score.
• Function to display results for all students.

Program Output (Example)

Enter number of students: 3
Enter student 1 name: Nguyen Van A
Enter Math score: 8.5
Enter Physics score: 7.5
Enter Chemistry score: 9.0

Enter student 2 name: Tran Thi B
Enter Math score: 6.5
Enter Physics score: 7.0
Enter Chemistry score: 8.0

Enter student 3 name: Le Van C
Enter Math score: 5.0
Enter Physics score: 6.0
Enter Chemistry score: 7.0

-----------------------------------
Student list and average scores:
Nguyen Van A - Average score: 8.67 - Classification: Excellent
Tran Thi B - Average score: 7.17 - Classification: Very Good
Le Van C - Average score: 6.00 - Classification: Average