how to merge two arrays to key value pairs with repeated values javascript-CodePudding

if I have two arrays, array A and array B both of equal size and length and with possibly repeated values.

how can I map them into an key-value pair object such that when a key is repetitive the value from the B gets pushed into an array of key of A perhaps a small code illustration could help clarify my point.

A = [2,3,1,3,2,0]
B = [1,4,5,6,3,2]

// desired object 

obj = {'2': [1,3], '3': [4,6], '1': [5], '0': [2]}

I tried reducing it like the following but it seems that i'm doing something wrong.

A = [2,3,1,3,2,0];
B = [1,4,5,6,3,2];
var result = A.reduce(function (obj, id, index) {
  obj[id] = obj[id] ? obj[id].push(B[index]) : (obj[id] = [B[index]]);
  return obj;
}, {});
console.log('result', result);

CodePudding user response：

Here's one way to do it using the logical nullish assignment operator (??=):

function transform (a, b) {
  if (a.length !== b.length) throw new Error('Length mismatch');
  const result = {};

  for (let i = 0; i < a.length; i  = 1) {
    // Object keys must be strings (or symbols):
    const key = JSON.stringify(a[i]);
    const value = b[i];
    // Set the value at the key in the result object
    // to a new array if it doesn't already exist:
    const array = result[key] ??= [];
    array.push(value);
  }

  return result;
}

const a = [2, 3, 1, 3, 2, 0];
const b = [1, 4, 5, 6, 3, 2];

const expected = {2: [1,3], 3: [4,6], 1: [5], 0: [2]};
const actual = transform(a, b);

console.log(actual);
const equal = JSON.stringify(actual) === JSON.stringify(expected);
console.log({equal});

CodePudding user response：

The straight forward approach without correct order using a basic JavaScript object:

A = [2,3,1,3,2,0];
B = [1,4,5,6,3,2];
const result = A.reduce((obj, id, index) => {
  if (id in obj) obj[id].push(B[index]);
  else obj[id] = [B[index]];
  return obj;
}, {});
console.log('result', result);

The same approach using Map to keep the order:

A = [2,3,1,3,2,0];
B = [1,4,5,6,3,2];
const result = A.reduce((obj, id, index) => {
  if (obj.has(id)) obj.get(id).push(B[index]);
  else obj.set(id, [B[index]]);
  return obj;
}, new Map());
let resultStr = 'result {\n';
for (let el of result) resultStr  = `${el[0]}: ${el[1]}\n`;
resultStr  = '}';
console.log(resultStr);