71 lines
2.8 KiB
Markdown
71 lines
2.8 KiB
Markdown
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---
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id: 5a23c84252665b21eecc7e03
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title: Cumulative standard deviation
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challengeType: 5
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---
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## Description
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<section id='description'>
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Write a function that takes an array of numbers as parameter and returns the <a href="https://en.wikipedia.org/wiki/Standard Deviation">standard deviation</a> of the series.
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</section>
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## Instructions
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<section id='instructions'>
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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: <code>standardDeviation</code> should be a function.
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testString: assert(typeof standardDeviation == 'function', '<code>standardDeviation</code> should be a function.');
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- text: <code>standardDeviation([2, 4, 4, 4, 5, 5, 7, 9])</code> should return a number.
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testString: assert(typeof standardDeviation([2, 4, 4, 4, 5, 5, 7, 9]) == 'number', '<code>standardDeviation([2, 4, 4, 4, 5, 5, 7, 9])</code> should return a number.');
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- text: <code>standardDeviation([2, 4, 4, 4, 5, 5, 7, 9])</code> should return <code>2</code>.
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testString: assert.equal(standardDeviation([2, 4, 4, 4, 5, 5, 7, 9]), 2, '<code>standardDeviation([2, 4, 4, 4, 5, 5, 7, 9])</code> should return <code>2</code>.');
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- text: <code>standardDeviation([600, 470, 170, 430, 300])</code> should return <code>147.323</code>.
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testString: assert.equal(standardDeviation([600, 470, 170, 430, 300]), 147.323, '<code>standardDeviation([600, 470, 170, 430, 300])</code> should return <code>147.323</code>.');
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- text: <code>standardDeviation([75, 83, 96, 100, 121, 125])</code> should return <code>18.239</code>.
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testString: assert.equal(standardDeviation([75, 83, 96, 100, 121, 125]), 18.239, '<code>standardDeviation([75, 83, 96, 100, 121, 125])</code> should return <code>18.239</code>.');
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- text: <code>standardDeviation([23, 37, 45, 49, 56, 63, 63, 70, 72, 82])</code> should return <code>16.87</code>.
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testString: assert.equal(standardDeviation([23, 37, 45, 49, 56, 63, 63, 70, 72, 82]), 16.87, '<code>standardDeviation([23, 37, 45, 49, 56, 63, 63, 70, 72, 82])</code> should return <code>16.87</code>.');
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- text: <code>standardDeviation([271, 354, 296, 301, 333, 326, 285, 298, 327, 316, 287, 314])</code> should return <code>22.631</code>.
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testString: assert.equal(standardDeviation([271, 354, 296, 301, 333, 326, 285, 298, 327, 316, 287, 314]), 22.631, '<code>standardDeviation([271, 354, 296, 301, 333, 326, 285, 298, 327, 316, 287, 314])</code> should return <code>22.631</code>.');
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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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function standardDeviation (arr) {
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// Good luck!
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}
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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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function standardDeviation (arr) {
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var sum = 0,
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sum_sq = 0,
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n = arr.length;
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arr.forEach(function(e) {
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sum += e;
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sum_sq += e * e;
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})
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var std_dev=Math.sqrt((sum_sq / n) - Math.pow(sum / n, 2))
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return Math.round(std_dev*1000)/1000;
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}
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```
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</section>
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