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190 lines
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HTML
190 lines
6.7 KiB
HTML
<!DOCTYPE html>
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<meta name="author" content="Fredrik Danielsson, http://lostkeys.se">
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<article class="article">
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<h2>Counting Loop</h2>
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<p>Looking at the following example code</p>
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<pre><code class="language-java">int i;
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for (i = 0; i < 3; i++) { //begin for
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System.out.println("i = " + i); //body
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} //end for
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System.out.println("After loop, i = " + i);</code></pre>
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<p><code>i = 0</code> is the initializing statement</p>
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<p><code>i < 3</code> is the conditional, that is when the loop ends</p>
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<p><code>i++</code> is the increment/decrement</p>
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<p><code>i++</code> is synonymous with <code>i = i + 1</code></p>
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<p>The initialization statement only occurs once at the beginning of the loop. </p>
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<h3>Execution Example</h3>
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<p>Let us go through this for loop example</p>
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<ul>
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<li>Let us set <code>i = 0</code></li>
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<li>Is <code>i < 3</code>? Yes execute the body
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<ul>
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<li>The body executes an output of <code>"i = 0"</code></li>
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</ul></li>
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<li>Now we increment <code>i ++</code>, i is now 1</li>
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<li>Is <code>i < 3</code>? Yes, 1 is less than 3. Execute body
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<ul>
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<li>The computer prints out <code>"i = 1"</code></li>
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</ul></li>
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<li>Increment <code>i++</code> i is now 2</li>
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<li>Is <code>i < 3</code>? Yes 2 is less than 3. Execute body
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<ul>
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<li>The computer prints out <code>"i = 2"</code></li>
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</ul></li>
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<li>Increment <code>i++</code>, i is now 3</li>
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<li>Is <code>i < 3</code>? No 3 is not less than 3
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<ul>
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<li>Don't execute body of loop</li>
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</ul></li>
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</ul>
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<p>Exit loop. Print <code>"After loop, i = 3"</code></p>
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<h3>Condensing Syntax</h3>
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<p>You can also do the declaration in the initialization statement</p>
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<pre><code class="language-java">for (int i = 0; i < 3; i++) {
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System.out.println("i = " + i);
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}</code></pre>
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<p>This now runs like above without the <code>"After loop, i = 3"</code> print. You cannot access the variable <code>i</code> outside the for loop since in this example, it belongs to the for loop's scope.</p>
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<h2>Logic Expressions</h2>
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<h3>And Statements</h3>
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<p>With the AND operator <code>&&</code> both the left and right side needs to be true for the expression to be true.</p>
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<pre><code class="language-java">true && true // true
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true && false // false
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false && true // false
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false && false // false</code></pre>
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<h3>Or Statements</h3>
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<p>With the OR operator <code>||</code> either the left or right side needs to be true for the expression to be true.</p>
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<pre><code class="language-java">true || true // true
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true || false // true
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false || true // true
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false || false // false</code></pre>
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<h3>Examples</h3>
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<p><strong>Example</strong>: Print out the number <code>n</code> if it is between 10 and 20 (inclusive)</p>
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<pre><code class="language-java">if (n >= 10 && n <= 20) {
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System.out.println(n);
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}</code></pre>
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<p><strong>Example</strong>: Print out the <code>age</code> if it is not of young adult age. Young adult range is from 18 to 39 (inclusive)</p>
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<pre><code class="language-java">if (!(age >= 18 && age <= 39)) {
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System.out.println(age);
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}</code></pre>
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<p>Or you can use De Morgan's Law (for the curious)</p>
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<pre><code class="language-java">if (age < 18 || age > 39) {
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System.out.println(age);
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}</code></pre>
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<h2>For Loops (Cont.)</h2>
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<h3>Backwards counting</h3>
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<p>You can use the loop to count backwards</p>
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<pre><code class="language-java">for (int i = 10; i > -1; i--) {
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System.out.println(i);
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}</code></pre>
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<p>This prints the following</p>
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<pre><code class="language-java">10
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9
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8
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7
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6
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5
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4
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3
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2
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0</code></pre>
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<h3>Rows-Columns</h3>
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<p>You can make rows and columns of asterisks</p>
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<pre><code class="language-java">for (int j = 0; j < someNumber; j++) { // Corresponds to rows
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for (int i = 0; i < someNumber2; i++) { // Corresponds to columns
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System.out.print("*");
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}
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System.out.println(""); // Goes to the next row
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}</code></pre>
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<p>If <code>someNumber</code> equals <code>someNumber2</code>, then we have the same amount of rows as columns.</p>
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<p>Let <code>someNumber</code> equal to 2 and <code>someNumber2</code> equal to 2</p>
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<p>Output:</p>
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<pre><code>**
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**</code></pre>
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<h3>Right Triangles</h3>
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<p>You can make a right triangle of Tilda with the following code</p>
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<pre><code class="language-java">for (int i = 1; i <= num; i++) { // Corresponds to the row
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for (int j = 0; j < i; j++) { // Corresponds to the column and stops at the current row number
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System.out.print("~");
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}
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System.out.println(""); // Moves to next row
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}</code></pre>
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<h5>What are for-loops used for? <em>Reusing code</em></h5>
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</article>
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