How to get an array of counts of specific objects in an existing array?

Question

I have an array of objects like this:

[
  {
    importantKey: 'x',
    foo: 'bar',
    ...
  },
  {
    importantKey: 'y',
    foo: 'bar',
    ...
  },
  {
    importantKey: 'z',
    foo: 'bar',
    ...
  },
  {
    importantKey: 'x',
    foo: 'bar',
    ...
  },
  {
    importantKey: 'y',
    foo: 'bar',
    ...
  },
  {
    importantKey: 'z',
    foo: 'bar',
    ...
  },
  ...
]

And another array that has the values of importantKey:

keysArray = [x, y, z]

How would I get an array with values that are counts of all the objects that have each importantKey in the same order as keysArray? So the final result would be:

[ numberOfObjectsWithKeyX, numberOfObjectsWithKeyY, numberOfObjectsWithKeyZ ]

For this example, the result would be:

[2, 2, 2]

Also keysArray is dynamically generated, so x, y, and z cannot be hard-coded.

please add the wanted result of the example.

Nina Scholz
– Nina Scholz

2017-04-28 18:47:54 +00:00
Commented Apr 28, 2017 at 18:47 — Nina Scholz
– Nina Scholz, Commented Apr 28, 2017 at 18:47
Could the result be an object not an array?

ibrahim mahrir
– ibrahim mahrir

2017-04-28 18:50:43 +00:00
Commented Apr 28, 2017 at 18:50 — ibrahim mahrir
– ibrahim mahrir, Commented Apr 28, 2017 at 18:50
@Matt, kindly have a look at my easy solution using map

Devendra Lattu
– Devendra Lattu

2017-04-28 19:17:35 +00:00
Commented Apr 28, 2017 at 19:17 — Devendra Lattu
– Devendra Lattu, Commented Apr 28, 2017 at 19:17

samanime · Accepted Answer · 2017-04-28 18:48:51Z

2

You could do something like this:

keysArray.map(key => values.filter(v => v.importantKey === key).length);

Basically, you call map() on the keysArray which will create a parallel array to it with whatever values map() spits out.

For that value, you just call values.filter() and filter out only the things that have the appropriate key, then just check the length to get the count.

const values = [
  {
    importantKey: 'x',
    foo: 'bar'
  },
  {
    importantKey: 'y',
    foo: 'bar'
  },
  {
    importantKey: 'y',
    foo: 'bar'
  },
  {
    importantKey: 'x',
    foo: 'bar'
  },
  {
    importantKey: 'y',
    foo: 'bar'
  },
  {
    importantKey: 'z',
    foo: 'bar'
  }
];

const keys = ['x', 'y', 'z'];

const result = keys.map(key => values.filter(v => v.importantKey === key).length);

console.log(result);

answered Apr 28, 2017 at 18:48

samanime

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1 Comment

Matt Gween Over a year ago

Thanks. Unfortunately this is the exact implementation I already have and I'm getting and array with the correct number of elements, but they are all undefined instead of the number. The problem must have something to do with the fact that keys and values are both being dynamically created by rxjs observables. Thanks anyway. At least I know that this isn't the problem.

Abhinav Galodha · Accepted Answer · 2017-04-28 18:50:33Z

Using the Array map function and then filter method.

map function transforms the array into another array. filter function allows to filter the array with a specified predicate. In this case, you need to compare the importantKey value.

var arr1 = [
  {
    importantKey: 'x',
    foo: 'bar',
    
  },
  {
    importantKey: 'y',
    foo: 'bar',
    
  },
  {
    importantKey: 'z',
    foo: 'bar',
    
  },
  {
    importantKey: 'x',
    foo: 'bar',
    
  },
  {
    importantKey: 'y',
    foo: 'bar',
    
  },
  {
    importantKey: 'z',
    foo: 'bar',
    
  },
  {
    importantKey: 'x',
    foo: 'bar',
    
  }
 
];

var keysArray = ['x', 'y', 'z']
console.log(keysArray.map(a => arr1.filter(b=>b.importantKey == a).length));

Nina Scholz · Accepted Answer · 2017-04-28 18:56:15Z

1

You could use a hash table as reference to the indices for the result array.

This proposal uses a slingle loop for iterating the data.

var data = [{ importantKey: 'x', foo: 'bar' }, { importantKey: 'y', foo: 'bar' }, { importantKey: 'z', foo: 'bar' }, { importantKey: 'x', foo: 'bar' }, { importantKey: 'y', foo: 'bar' }, { importantKey: 'z', foo: 'bar' }],
    keysArray = ['x', 'y', 'z'],
    hash = Object.create(null),
    result = keysArray.map(function (k, i) { hash[k] = i; return 0; });

data.forEach(function (o) {
    result[hash[o.importantKey]]++;
});

console.log(result);

answered Apr 28, 2017 at 18:56

Nina Scholz

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Comments

Devendra Lattu · Accepted Answer · 2017-04-28 19:16:30Z

Simple to understand solution.

Read the data
Insert into map
Iterate through map
Insert into array k in order of [x,y,z]

var data = 
[
  {importantKey: 'z', foo: 'bar' },
  {importantKey: 'y', foo: 'bar' },
  {importantKey: 'z', foo: 'bar' },
  {importantKey: 'x', foo: 'bar' },
  {importantKey: 'y', foo: 'bar' },
  { importantKey: 'z', foo: 'bar' },
]

document.write(JSON.stringify(data) + "<br>");

const map = {};
for (i in data)
{
  const key = data[i].importantKey;
  map[key] = (map[key] || 0) + 1;
  document.write(key); // zyzxyz
}
document.write("<br>map = " + JSON.stringify(map)); // {"z":3,"y":2,"x":1}

const k = [];
for(i in map) {
  //k.push(map[i]);
  k.splice(map[i] - 'x', 0, map[i]);
}
document.write("<br>k = " + JSON.stringify(k)); // [1,2,3]

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