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

Size (n)
index [0, n - 1]

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);

~/

Brandon Rozek