2018-09-30 22:01:58 +00:00
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---
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title: Vector dot product
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id: 594810f028c0303b75339ad3
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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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<p>
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A vector is defined as having three dimensions as being represented by an ordered collection of three numbers: (X, Y, Z).
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</p>
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<p>
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Task:
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Write a function that takes any numbers of vectors (arrays) as input and computes their dot product.
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Your function should return <code>null</code> on
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invalid inputs (ie vectors of different lengths).
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</p>
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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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2018-10-04 13:37:37 +00:00
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tests:
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- text: dotProduct must be a function
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2018-10-20 18:02:47 +00:00
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testString: assert.equal(typeof dotProduct, 'function', 'dotProduct must be a function');
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2018-10-04 13:37:37 +00:00
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- text: dotProduct() must return null
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2018-10-20 18:02:47 +00:00
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testString: assert.equal(dotProduct(), null, 'dotProduct() must return null');
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- text: dotProduct([[1], [1]]) must return 1.
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testString: assert.equal(dotProduct([1], [1]), 1, 'dotProduct([[1], [1]]) must return 1.');
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- text: dotProduct([[1], [1, 2]]) must return null.
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testString: assert.equal(dotProduct([1], [1, 2]), null, 'dotProduct([[1], [1, 2]]) must return null.');
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- text: dotProduct([1, 3, -5], [4, -2, -1]) must return 3.
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testString: assert.equal(dotProduct([1, 3, -5], [4, -2, -1]), 3, 'dotProduct([1, 3, -5], [4, -2, -1]) must return 3.');
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2018-10-04 13:37:37 +00:00
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- text: <code>dotProduct(...nVectors)</code> should return 156000
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2018-10-20 18:02:47 +00:00
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testString: assert.equal(dotProduct([ 0, 1, 2, 3, 4 ], [ 0, 2, 4, 6, 8 ], [ 0, 3, 6, 9, 12 ], [ 0, 4, 8, 12, 16 ], [ 0, 5, 10, 15, 20 ]), 156000, '<code>dotProduct(...nVectors)</code> should return 156000');
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2018-09-30 22:01:58 +00:00
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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 dotProduct() {
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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 dotProduct(...vectors) {
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if (!vectors || !vectors.length) {
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return null;
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}
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if (!vectors[0] || !vectors[0].length) {
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return null;
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}
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const vectorLen = vectors[0].length;
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const numVectors = vectors.length;
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// If all vectors not same length, return null
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for (let i = 0; i < numVectors; i++) {
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if (vectors[i].length !== vectorLen) {
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return null; // return undefined
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}
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}
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let prod = 0;
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let sum = 0;
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let j = vectorLen;
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let i = numVectors;
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// Sum terms
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while (j--) {
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i = numVectors;
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prod = 1;
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while (i--) {
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prod *= vectors[i][j];
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}
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sum += prod;
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}
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return sum;
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}
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```
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</section>
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