freeCodeCamp/curriculum/challenges/en/08-coding-interview-prep/rosetta-code/vector-dot-product.en.md

title, id, challengeType

title	id	challengeType
Vector dot product	594810f028c0303b75339ad3	5

Description

A vector is defined as having three dimensions as being represented by an ordered collection of three numbers:   (X, Y, Z).

Task:

Write a function that takes any numbers of vectors (arrays) as input and computes their dot product.

Your function should return null on invalid inputs (ie vectors of different lengths).

Instructions

Tests

- text: dotProduct must be a function
  testString: 'assert.equal(typeof dotProduct, ''function'', ''dotProduct must be a function'');'
- text: dotProduct() must return null
  testString: 'assert.equal(dotProduct(), null, ''dotProduct() must return null'');'
- text: 'dotProduct([[1], [1]]) must return 1.'
  testString: 'assert.equal(dotProduct([1], [1]), 1, ''dotProduct([[1], [1]]) must return 1.'');'
- text: 'dotProduct([[1], [1, 2]]) must return null.'
  testString: 'assert.equal(dotProduct([1], [1, 2]), null, ''dotProduct([[1], [1, 2]]) must return null.'');'
- text: 'dotProduct([1, 3, -5], [4, -2, -1]) must return 3.'
  testString: 'assert.equal(dotProduct([1, 3, -5], [4, -2, -1]), 3, ''dotProduct([1, 3, -5], [4, -2, -1]) must return 3.'');'
- text: <code>dotProduct(...nVectors)</code> should return 156000
  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'');'

Challenge Seed

function dotProduct() {
    // Good luck!
}

Solution

function dotProduct(...vectors) {
  if (!vectors || !vectors.length) {
    return null;
  }
  if (!vectors[0] || !vectors[0].length) {
    return null;
  }
  const vectorLen = vectors[0].length;
  const numVectors = vectors.length;

  // If all vectors not same length, return null
  for (let i = 0; i < numVectors; i++) {
    if (vectors[i].length !== vectorLen) {
      return null;  // return undefined
    }
  }

  let prod = 0;
  let sum = 0;
  let j = vectorLen;
  let i = numVectors;
  // Sum terms
  while (j--) {
    i = numVectors;
    prod = 1;

    while (i--) {
      prod *= vectors[i][j];
    }
    sum += prod;
  }
  return sum;
}