JavaScript: create a sequence for each unique value in array of objects

Question

I have an array of objects and I want to create a sequence based on each value. For example take the array:

let starting_data = [
  {str: "a"},
  {str: "a"},
  {str: "b"},
  {str: "c"},
  {str: "a"},
  {str: "c"},
  {str: "a"},
  {str: "b"}
]

I know I can use a map and reduce to find the unique number of each str value

let uniques = starting_data.map(function(value) {return value.str;}).reduce((acc, val) => {
      acc[val] = acc[val] === undefined ? 1 : acc[val] += 1;
      return acc;
}, {})

But I'm not sure how to leverage this to eventually get what I want as my desired output:

let desired_output = [
  {str: "a", count: 1},
  {str: "a", count: 2},
  {str: "b", count: 1},
  {str: "c", count: 1},
  {str: "a", count: 3},
  {str: "c", count: 2},
  {str: "a", count: 4},
  {str: "b", count: 2}
]

Where the counts start over for each unique value, but the as go to 4, and b and c go to 2. I tried messing with arr.sort if I need to do that first but I wasn't sure how to sort based on a value inside an object... Any help appreciated!!

dimiguel · Accepted Answer · 2022-03-15 05:40:05Z

You mentioned it you want it to be purely functional, so here it is.

let starting_data = [
  { str: "a" },
  { str: "a" },
  { str: "b" },
  { str: "c" },
  { str: "a" },
  { str: "c" },
  { str: "a" },
  { str: "b" },
];

const uniqueCount = starting_data
  .reduce(
    (arr, { str }) => {
      const hash = arr[0];
      hash[str] ||= 0;
      arr.push({ str, count: ++hash[str] });
      return arr;
    },
    [{}]
  )
  .slice(1);

console.log(uniqueCount);

This prints:

[
  { str: 'a', count: 1 },
  { str: 'a', count: 2 },
  { str: 'b', count: 1 },
  { str: 'c', count: 1 },
  { str: 'a', count: 3 },
  { str: 'c', count: 2 },
  { str: 'a', count: 4 },
  { str: 'b', count: 2 }
]

How it works

This solution keeps a hash as the first element of the array being built by reduce.

The array starts as [{}].
Populate the hash key with a 0 if the key doesn't already exist.
Push a new element to the array with str and a count.
hash[str] is incremented by 1 and then used as the value.

The array now looks like:

[{ a: 1 }, { str: "a", count: 1 }]

We continue this until we've iterated through every item.

[ { a: 2 }, { str: 'a', count: 1 }, { str: 'a', count: 2 } ]
[ { a: 2, b: 1 }, { str: 'a', count: 1 }, { str: 'a', count: 2 }, { str: 'b', count: 1 } ]
// Continues until it finishes

At the end, we remove this initial hash by slicing the array onwards from the first element.

Hope this helps.

cmgchess · Accepted Answer · 2022-03-15 05:08:51Z

1

can do something like keeping a seperate object to keep track of the counts

let starting_data = [
  {str: "a"},
  {str: "a"},
  {str: "b"},
  {str: "c"},
  {str: "a"},
  {str: "c"},
  {str: "a"},
  {str: "b"}
]


let counts = {};
let res = starting_data.map(({str}) => {
    if(!counts[str])counts[str]=0;
  counts[str]+=1
  return {str,count:counts[str]}
})

console.log(res)

answered Mar 15, 2022 at 5:08

cmgchess

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