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6 KiB
HTML
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<h1>Lecture Notes Oct 2nd</h1>
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<h2>Array</h2>
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<p><code>array</code> is not a reserved word, it's a concept. Arrays are able to hold multiple values under one name of the same type.</p>
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<p>For instance, you can have an array of integers.</p>
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<p>Properties of an array</p>
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<ul>
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<li>Size (n)</li>
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<li>index [0, n - 1]</li>
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</ul>
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<p>You can declare arrays by saying the type '[]' and the name of the array</p>
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<pre><code class="language-java">int[] numbers;
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double[] gpas;
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float[] grades;</code></pre>
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<p>Before you can use the array, you must <code>new</code> it</p>
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<pre><code class="language-java">numbers = new int[10];</code></pre>
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<p>Where 10 is the size of the array.</p>
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<p>You can combine both the declaration and initialization</p>
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<pre><code class="language-java">double[] points = new double[7];</code></pre>
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<p>You can access individual elements of the array by using its index. Indexes start from zero</p>
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<pre><code class="language-java">points[0] = 5.4; // The first element in the point array is 5.4</code></pre>
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<p>The <code>.length</code> property of an array gives the size of the array</p>
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<h2>For-Loops + Arrays</h2>
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<p>You can print out each element in the array using a for loop</p>
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<pre><code class="language-java">for (int i = 0; i < numbers.length; i++) {
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System.out.println(numbers[i]);
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}</code></pre>
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<p>You can ask a user to input a value to each element in the array</p>
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<pre><code class="language-java">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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}</code></pre>
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<h2>While-Loops + Arrays</h2>
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<p>You can use a while loop to search the array</p>
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<pre><code class="language-java">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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}</code></pre>
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<p>If you don't include the <code>i != number.length</code> you will obtain an <code>IndexOutOfBounds</code> error.</p>
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<p>The example above is called a <em>Linear Search</em>. </p>
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<p>Linear searches work on an unsorted and sorted arrays.</p>
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<h2>Methods + Arrays</h2>
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<p>You can pass an array into a method</p>
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<pre><code class="language-java">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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}</code></pre>
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<h2>Do-While Loops</h2>
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<p>For-loops can run 0 or more times. If you want something to execute at least once. Use a do-while loop.</p>
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<pre><code class="language-java">do {
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// Code
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} while (condition);</code></pre>
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<p>For example, to search at least once and asking whether the user wants to search again</p>
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<pre><code class="language-java">// 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');</code></pre>
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<p>You can create any type of loop just by using a while loop.</p>
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<h2>Example: Finding the Max</h2>
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<p>You can find the max of an array using the following method</p>
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<pre><code class="language-java">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);</code></pre>
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<h2>Example: Summing up an array</h2>
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<p>You can sum the array using the following method</p>
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<pre><code class="language-java">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);</code></pre>
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