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title	id	challengeType	forumTopicId
Vector dot product	594810f028c0303b75339ad3	5	302343

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.

Tests

tests:
  - text: dotProduct must be a function
    testString: assert.equal(typeof dotProduct, 'function');
  - text: dotProduct() must return null
    testString: assert.equal(dotProduct(), null);
  - text: dotProduct([[1], [1]]) must return 1.
    testString: assert.equal(dotProduct([1], [1]), 1);
  - text: dotProduct([[1], [1, 2]]) must return null.
    testString: assert.equal(dotProduct([1], [1, 2]), null);
  - text: dotProduct([1, 3, -5], [4, -2, -1]) must return 3.
    testString: assert.equal(dotProduct([1, 3, -5], [4, -2, -1]), 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);

Challenge Seed

function dotProduct(...vectors) {
    // 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;
}

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