2 Arrays, different sizes, reuse one array within loop once max index is reached

Question

I have 2 arrays structured like so and want to loop through the larger array and assign a property to it, using random ids from a smaller array

//Actual array length is 140
const users = [
  {
    name: "John Roberts",
    uid: "49ikds_dm3idmssmmi9sz"
  },
  {
    name: "Peter Jones",
    uid: "fmi33_sm39imsz9z9nb"
  }
]

//Actual array lenth is 424
const cars = [
  {
    manufacturer: "BMW",
    model: "320d",
    year: "2010",
    user: null
  },
  {
    manufacturer: "BMW",
    model: "530d",
    year: "2018",
    user: null
  },
  {
    manufacturer: "AUDI",
    model: "RS6",
    year: "2014",
    user: null
  }
]

for(let i = 0; i < cars.length; i++){
  //if index is 2 or greater in this example. users[2] will be undefined
  cars[i].user = users[i].uid;
}

I basically want to re use the small users array. In the example above, once the variable i is 2 or greater, then users[2] will be undefined.

Can anyone recommend an elegant solution that will help me solve this problem.?

Orelsanpls · Accepted Answer · 2018-09-28 13:14:26Z

You can use the % to always have a correct indice and reuse users.

const users = [
  {
    name: 'John Roberts',
    uid: '49ikds_dm3idmssmmi9sz',
  },
  {
    name: 'Peter Jones',
    uid: 'fmi33_sm39imsz9z9nb',
  }
];

const cars = [
  {
    manufacturer: 'BMW',
    model: '320d',
    year: '2010',
    user: null,
  },
  {
    manufacturer: 'BMW',
    model: '530d',
    year: '2018',
    user: null,
  },
  {
    manufacturer: 'AUDI',
    model: 'RS6',
    year: '2014',
    user: null,
  },
];

for (let i = 0; i < cars.length; i += 1){
  cars[i].user = users[i % users.length].uid;
}

console.log(cars);

An ES6 soluce you won't believe! (click bait)

const users = [
  {
    name: 'John Roberts',
    uid: '49ikds_dm3idmssmmi9sz',
  },
  {
    name: 'Peter Jones',
    uid: 'fmi33_sm39imsz9z9nb',
  }
];

const cars = [
  {
    manufacturer: 'BMW',
    model: '320d',
    year: '2010',
    user: null,
  },
  {
    manufacturer: 'BMW',
    model: '530d',
    year: '2018',
    user: null,
  },
  {
    manufacturer: 'AUDI',
    model: 'RS6',
    year: '2014',
    user: null,
  },
];

const ret = cars.map((x, xi) => ({
  ...x,
  
  user: users[xi % users.length].uid,
}));

console.log(ret);

Arthur · Accepted Answer · 2018-09-28 13:11:23Z

1

You just have to test you user[i] var and put a default value:

for(let i = 0; i < cars.length; i++){
  //if index is 2 or greater in this example. users[2] will be undefined
  cars[i].user = users[i] && users[i].uid || /*Default value:*/ null;
}

You can also do it with simple if:

for(let i = 0; i < cars.length; i++){
  //if index is 2 or greater in this example. users[2] will be undefined 
  if (users[i]) {
    cars[i].user = users[i].uid;
  } else {
    cars[i].user = null; // Or another default
  }
}

------------------ Update to take account of comment -----------------

If you want to loop on user and start again the array when you finish you can use modulo %

let user_idx = null
for(let i = 0; i < cars.length; i++){

  user_idx = i % users.length

  //if index is 2 or greater in this example. users[2] will be undefined
  cars[i].user = users[user_idx].uid || null;
}

edited Sep 28, 2018 at 13:11

answered Sep 28, 2018 at 13:02

Arthur

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3 Comments

juicy89 Over a year ago

Thanks for replying Arthur, My problem lies in that I need a user to be assigned to the cars array, it can't be a null value. I need someone way of cycling over my users array once again whilst continuing through my cars loop as normal.

Arthur Over a year ago

@juicy89 I did an update an use module (rest of divide). In you case we have 3 cars and 2users: 0 with 0, 1 with 1, and 2 with 2.. But user 2 didn't exist, and 2 % 2 (index % user length) = 0.. so car 2 is link to user 0

juicy89 Over a year ago

That's excellent. Thanks Arthur! Never thought to use modulus

saumil_ · Accepted Answer · 2018-09-28 13:17:51Z

0

It's petty simple. Just write following logic.

$.grep(users, function(user, i ) {  cars[i].user = user.uid;});

answered Sep 28, 2018 at 13:17

saumil_

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Comments

Yves Kipondo · Accepted Answer · 2018-09-28 13:20:17Z

0

var users = [
	{
		name: "John Roberts",
		uid: "49ikds_dm3idmssmmi9sz"
	},
	{
		name: "Peter Jones",
		uid: "fmi33_sm39imsz9z9nb"
	}
]

//Actual array lenth is 424
var cars = [
	{
		manufacturer: "BMW",
		model: "320d",
		year: "2010",
		user: null
	},
	{
		manufacturer: "BMW",
		model: "530d",
		year: "2018",
		user: null
	},
	{
		manufacturer: "AUDI",
		model: "RS6",
		year: "2014",
		user: null
	}
]

cars.map(function(car){
	var randomIndex = Math.floor(Math.random() * Math.floor(users.length))
	car.user = users[randomIndex].uid;
})
console.log(cars)

answered Sep 28, 2018 at 13:20

Yves Kipondo

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Comments

aleksandr_hramcov · Accepted Answer · 2018-09-28 13:21:35Z

0

option 1:

for(let i = 0; i < cars.length; i++){
   cars[i].user = users[i % users.length].uid;
}

option 2:

for(let i = 0; i < cars.length; i++){
   cars[i].user = users[Math.round(Math.random() * (users.length - 1))].uid;
}

edited Sep 28, 2018 at 13:21

answered Sep 28, 2018 at 13:13

aleksandr_hramcov

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