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160 lines
3.1 KiB
Markdown
160 lines
3.1 KiB
Markdown
# Lecture Notes Oct 2nd
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## Array
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`array` is not a reserved word, it's a concept. Arrays are able to hold multiple values under one name of the same type.
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For instance, you can have an array of integers.
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Properties of an array
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- Size (n)
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- index [0, n - 1]
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You can declare arrays by saying the type '[]' and the name of the array
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```java
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int[] numbers;
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double[] gpas;
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float[] grades;
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```
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Before you can use the array, you must `new` it
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```java
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numbers = new int[10];
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```
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Where 10 is the size of the array.
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You can combine both the declaration and initialization
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```java
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double[] points = new double[7];
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```
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You can access individual elements of the array by using its index. Indexes start from zero
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```java
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points[0] = 5.4; // The first element in the point array is 5.4
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```
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The `.length` property of an array gives the size of the array
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## For-Loops + Arrays
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You can print out each element in the array using a for loop
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```java
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for (int i = 0; i < numbers.length; i++) {
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System.out.println(numbers[i]);
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}
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```
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You can ask a user to input a value to each element in the array
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```java
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for (int i = 0; i < points.length; i++) {
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System.out.print("Enter a number: ");
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points[i] = scnr.nextInt();
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}
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```
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## While-Loops + Arrays
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You can use a while loop to search the array
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```java
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int i = 0;
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int number = 5;
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// While the index is within the array size and the number isn't found
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while (i != number.length && number != numbers[i]) {
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i++
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}
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if (i == numbers.length) {
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System.out.println(number + " was not found.")
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} else {
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System.out.println(number + " was found at index " + i)
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}
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```
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If you don't include the `i != number.length` you will obtain an `IndexOutOfBounds` error.
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The example above is called a *Linear Search*.
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Linear searches work on an unsorted and sorted arrays.
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## Methods + Arrays
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You can pass an array into a method
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```java
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public static void exampleMethod(int[] sample) {
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// Do something
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}
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public static void main(String[] args) {
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int[] s = new int[30];
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exampleMethod(s);
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}
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```
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## Do-While Loops
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For-loops can run 0 or more times. If you want something to execute at least once. Use a do-while loop.
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```java
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do {
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// Code
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} while (condition);
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```
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For example, to search at least once and asking whether the user wants to search again
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```java
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// Assume linearSearch and array are defined
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char answer;
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Scanner input = new Scanner(System.in);
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do {
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linearSearch(array, input);
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System.out.print("Do you want to search again? (Y/N) ");
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input.nextLine();
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answer = input.next().charAt(0);
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} while( answer != 'N');
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```
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You can create any type of loop just by using a while loop.
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## Example: Finding the Max
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You can find the max of an array using the following method
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```java
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double max = arrayName[0];
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for (int i = 1; i < arrayName.length; i++) {
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if (max < arrayName[i]) {
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max = arrayName[i];
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}
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}
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System.out.println("The max is " + max);
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```
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## Example: Summing up an array
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You can sum the array using the following method
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```java
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double sum = 0;
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for (int i = 0; i < arrayName.length; i++) {
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sum += arrayName[i];
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}
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System.out.println("The sum is " + sum);
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```
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