Javascript group by multiple and count in new field

Question

I am so ridiculously lost with JS...I've been trying literally for hours to do stuff that takes 5 seconds to do in R or Python. Trying to learn just for this one homework.

Lets say I have an object like this:

myObject = [ {'location': 'california', 'day': 'wednesday', 'company': 'Tesla'},
             {'location': 'washington', 'day': 'tuesday', 'company': 'Microsoft'},
             {'location': 'california', 'day': 'wednesday', 'company': 'Tesla'},
             {'location': 'california', 'day': 'monday', 'company': 'Apple'},
             {'location': 'california', 'day': 'monday', 'company': 'SalesForce'}
             {'location': 'washington', 'day': 'tuesday', 'company': 'Microsoft'},
             {'location': 'california', 'day': 'wednesday', 'company': 'Apple'}
            ]

I want to group by location and day, and count the number of different companies and get an output like this:

myOutputObject = [ {'location': 'california', 'day': 'wednesday', 'count':2},
                   {'location': 'washington', 'day': 'tuesday', 'count':1},
                   {'location': 'california', 'day': 'monday', 'count':2}
                 ]

Coming from a python background JS is a nightmare in a box to me.

I tried just creating a 'count' key in every element and setting it 0 to start with like this:

var dataset = []
dataset = data.forEach(function(d){
     d['count'] = 0;
     return d
})
console.log(dataset);

Nothing...zilch...undefined...

Can someone please be kind enough to explain to me how to get my head around this nonsense?

hgb123 · Accepted Answer · 2020-10-02 04:30:29Z

You could use reduce

const myObject = [
  { location: 'california', day: 'wednesday', company: 'Tesla' },
  { location: 'washington', day: 'tuesday', company: 'Microsoft' },
  { location: 'california', day: 'wednesday', company: 'Tesla' },
  { location: 'california', day: 'monday', company: 'Apple' },
  { location: 'california', day: 'monday', company: 'SalesForce' },
  { location: 'washington', day: 'tuesday', company: 'Microsoft' },
  { location: 'california', day: 'wednesday', company: 'Apple' }
]

const res = myObject.reduce((acc, obj) => {
  const existingIndex = acc.findIndex(
    el => el.location === obj.location && el.day === obj.day
  )
  if (existingIndex > -1) {
    acc[existingIndex].count += 1
  } else {
    acc.push({
      location: obj.location,
      day: obj.day,
      count: 1
    })
  }
  return acc
}, [])

console.log(res)

gurisko · Accepted Answer · 2020-10-02 07:03:00Z

Your start wasn't bad. However, Array.forEach [docs] returns undefined (+ you were actually updating the original object not adding into the new one). So to fix your start, one has to do:

var myOutputObject = [];
myObject.forEach(function(d) {
  myOutputObject.push({...d, count: 0});
})
console.log(myOutputObject);

(Docs for ... operator)

To extend answer provided by hgb123, let's make a solution that doesn't have quadratic time complexity:

const myObject = [
  {location: 'california', day: 'wednesday', company: 'Tesla'},
  {location: 'washington', day: 'tuesday', company: 'Microsoft'},
  {location: 'california', day: 'wednesday', company: 'Tesla'},
  {location: 'california', day: 'monday', company: 'Apple'},
  {location: 'california', day: 'monday', company: 'SalesForce'},
  {location: 'washington', day: 'tuesday', company: 'Microsoft'},
  {location: 'california', day: 'wednesday', company: 'Apple'},
];

const m = new Map();
myObject.forEach(({day, location}) => {
  // Create a key with values that we want to group by
  // A list of key-value pairs is chosen to make use of `Object.fromEntries` later
  const hash = JSON.stringify([['day', day], ['location', location]]);
  m.set(hash, (m.get(hash) || 0) + 1);
});
const myOutputObject = [...m].map(([rec, count]) => ({
  ...Object.fromEntries(JSON.parse(rec)),
  count,
}))
console.log(JSON.stringify(myOutputObject));

Collectives™ on Stack Overflow

Javascript group by multiple and count in new field

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related