  # 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;
``````

Before you can use the array, you must `new` it

``````numbers = new int;
``````

Where 10 is the size of the array.

You can combine both the declaration and initialization

``````double[] points = new double;
``````

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

``````points = 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) {
} 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;
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
Scanner input = new Scanner(System.in);
do {
linearSearch(array, input);
System.out.print("Do you want to search again? (Y/N) ");
input.nextLine();
``````

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;
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);
``````