Brandon Rozek

Lecture Notes Oct 2nd

Array

array is not a reserved word, it’s a concept. Arrays are able to hold multiple values under one name of the same type.

For instance, you can have an array of integers.

Properties of an array

You can declare arrays by saying the type ‘[]’ and the name of the array

int[] numbers;
double[] gpas;
float[] grades;

Before you can use the array, you must new it

numbers = new int[10];

Where 10 is the size of the array.

You can combine both the declaration and initialization

double[] points = new double[7];

You can access individual elements of the array by using its index. Indexes start from zero

points[0] = 5.4; // The first element in the point array is 5.4

The .length property of an array gives the size of the array

For-Loops + Arrays

You can print out each element in the array using a for loop

for (int i = 0; i < numbers.length; i++) {
    System.out.println(numbers[i]);
}

You can ask a user to input a value to each element in the array

for (int i = 0; i < points.length; i++) {
    System.out.print("Enter a number: ");
    points[i] = scnr.nextInt();
}

While-Loops + Arrays

You can use a while loop to search the array

int i = 0;
int number = 5;
// While the index is within the array size and the number isn't found
while (i != number.length && number != numbers[i]) {
    i++
}
if (i == numbers.length) {
    System.out.println(number + " was not found.")
} else {
    System.out.println(number + " was found at index " + i)
}

If you don’t include the i != number.length you will obtain an IndexOutOfBounds error.

The example above is called a Linear Search.

Linear searches work on an unsorted and sorted arrays.

Methods + Arrays

You can pass an array into a method

public static void exampleMethod(int[] sample) {
    // Do something
}
public static void main(String[] args) {
    int[] s = new int[30];
    exampleMethod(s);
}

Do-While Loops

For-loops can run 0 or more times. If you want something to execute at least once. Use a do-while loop.

do {
    // Code
} while (condition);

For example, to search at least once and asking whether the user wants to search again

// Assume linearSearch and array are defined
char answer;
Scanner input = new Scanner(System.in);
do { 
  linearSearch(array, input);
  System.out.print("Do you want to search again? (Y/N) ");
  input.nextLine();
  answer = input.next().charAt(0);
} while( answer != 'N');

You can create any type of loop just by using a while loop.

Example: Finding the Max

You can find the max of an array using the following method

double max = arrayName[0];
for (int i = 1; i < arrayName.length; i++) {
    if (max < arrayName[i]) {
        max = arrayName[i];
    }
}
System.out.println("The max is " + max);

Example: Summing up an array

You can sum the array using the following method

double sum = 0;
for (int i = 0; i < arrayName.length; i++) {
    sum += arrayName[i];
}
System.out.println("The sum is " + sum);