116 lines
3.7 KiB
Markdown
116 lines
3.7 KiB
Markdown
---
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id: 5a2efd662fb457916e1fe604
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title: Iterate with JavaScript Do...While Loops
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challengeType: 1
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videoUrl: 'https://scrimba.com/c/cDqWGcp'
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forumTopicId: 301172
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---
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## Description
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<section id='description'>
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The next type of loop you will learn is called a <code>do...while</code> loop. It is called a <code>do...while</code> loop because it will first <code>do</code> one pass of the code inside the loop no matter what, and then continue to run the loop <code>while</code> the specified condition evaluates to <code>true</code>.
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```js
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var ourArray = [];
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var i = 0;
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do {
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ourArray.push(i);
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i++;
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} while (i < 5);
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```
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The example above behaves similar to other types of loops, and the resulting array will look like <code>[0, 1, 2, 3, 4]</code>. However, what makes the <code>do...while</code> different from other loops is how it behaves when the condition fails on the first check. Let's see this in action:
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Here is a regular <code>while</code> loop that will run the code in the loop as long as <code>i < 5</code>:
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```js
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var ourArray = [];
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var i = 5;
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while (i < 5) {
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ourArray.push(i);
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i++;
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}
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```
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In this example, we initialize the value of <code>myArray</code> to an empty array and the value of <code>i</code> to 5. When we execute the <code>while</code> loop, the condition evaluates to <code>false</code> because <code>i</code> is not less than 5, so we do not execute the code inside the loop. The result is that <code>ourArray</code> will end up with no values added to it, and it will still look like <code>[]</code> when all of the code in the example above has completed running.
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Now, take a look at a <code>do...while</code> loop:
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```js
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var ourArray = [];
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var i = 5;
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do {
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ourArray.push(i);
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i++;
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} while (i < 5);
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```
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In this case, we initialize the value of <code>i</code> to 5, just like we did with the <code>while</code> loop. When we get to the next line, there is no condition to evaluate, so we go to the code inside the curly braces and execute it. We will add a single element to the array and then increment <code>i</code> before we get to the condition check. When we finally evaluate the condition <code>i < 5</code> on the last line, we see that <code>i</code> is now 6, which fails the conditional check, so we exit the loop and are done. At the end of the above example, the value of <code>ourArray</code> is <code>[5]</code>.
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Essentially, a <code>do...while</code> loop ensures that the code inside the loop will run at least once.
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Let's try getting a <code>do...while</code> loop to work by pushing values to an array.
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</section>
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## Instructions
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<section id='instructions'>
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Change the <code>while</code> loop in the code to a <code>do...while</code> loop so the loop will push only the number <code>10</code> to <code>myArray</code>, and <code>i</code> will be equal to <code>11</code> when your code has finished running.
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: You should be using a <code>do...while</code> loop for this exercise.
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testString: assert(code.match(/do/g));
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- text: <code>myArray</code> should equal <code>[10]</code>.
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testString: assert.deepEqual(myArray, [10]);
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- text: <code>i</code> should equal <code>11</code>
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testString: assert.equal(i, 11);
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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```js
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// Setup
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var myArray = [];
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var i = 10;
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// Only change code below this line
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while (i < 5) {
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myArray.push(i);
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i++;
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}
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```
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</div>
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### After Test
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<div id='js-teardown'>
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```js
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if(typeof myArray !== "undefined"){(function(){return myArray;})();}
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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var myArray = [];
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var i = 10;
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do {
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myArray.push(i);
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i++;
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} while (i < 5)
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```
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</section>
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