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id	title	challengeType	forumTopicId	dashedName
594810f028c0303b75339ad3	Vector dot product	5	302343	vector-dot-product

--description--

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

--instructions--

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 such as vectors of different lengths.

--hints--

dotProduct should be a function.

assert.equal(typeof dotProduct, 'function');

dotProduct() should return null.

assert.equal(dotProduct(), null);

dotProduct(1], [1) should return 1.

assert.equal(dotProduct([1], [1]), 1);

dotProduct(1], [1, 2) should return null.

assert.equal(dotProduct([1], [1, 2]), null);

dotProduct([1, 3, -5], [4, -2, -1]) should return 3.

assert.equal(dotProduct([1, 3, -5], [4, -2, -1]), 3);

dotProduct(...nVectors) should return 156000.

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
);

--seed--

--seed-contents--

function dotProduct(...vectors) {

}

--solutions--

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;
}

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