Sum array of object value by key and append result to existing object

Question

Hello I'm currently struggling with this and I don't really know what to do.

I have array of objects like this:

[
  {
    rating: "good",
    count: 4,
  },
  {
    rating: "bad",
    count: 2,
  },
  {
    rating: "bad",
    count: 4,
  }
]

And what I'm trying to get is:

[
  {
    rating: "good",
    count: 4,
    total: 4
  },
  {
    rating: "bad",
    count: 2,
    total: 6
  },
  {
    rating: "bad",
    count: 4,
    total: 6
  }
]

I know that I need to use array reduce but I don't know how to write code to make a structure of array like this.

Please add the code you've written to the question. SO is here to help you debug code, not to write it for you. — Rory McCrossan
– Rory McCrossan, Commented Oct 4, 2022 at 7:51
"I know that I need to use array reduce..." No, you don't. reduce is fine if you're doing functional programming with predefined, reusable, tested reducer functions. If you aren't, it's just an overcomplicated loop -- hard to read, easy to get wrong. You need a loop (probably two -- one to do the totals, probably storing them in a Map, and one to put the totals on all the objects once you have them). — T.J. Crowder
– T.J. Crowder, Commented Oct 4, 2022 at 7:53

Terry Lennox · Accepted Answer · 2022-10-04 08:06:47Z

1

You could use Array.reduce() to group the items by rating creating a totals map, with a total for each rating.

We can then use Array.map() to get the final result.

const input = [ { rating: "good", count: 4, }, { rating: "bad", count: 2, }, { rating: "bad", count: 4, } ];

// Get the total for each rating...
const totals = input.reduce((acc, { rating, count }) => { 
    acc[rating] = (acc[rating] || 0) + count;
    return acc;
}, {})

// Map to get final result...
const result = input.map(({ rating, count }) => { 
    return { rating, count, total: totals[rating] };
});

console.log('Result:', result);

.as-console-wrapper { max-height: 100% !important; }

answered Oct 4, 2022 at 8:06

Terry Lennox

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Comments

Valery Kulakou · Accepted Answer · 2022-10-04 08:21:02Z

First you need to create a hash table (object) with values:

{
    good: 4,
    bad: 6
}

This can be done like this:

const initArray = [
  {
    rating: "good",
    count: 4,
  },
  {
    rating: "bad",
    count: 2,
  },
  {
    rating: "bad",
    count: 4,
  }
];
const table = {};
for (let i = 0; i < initArray.length; i++) {
    if (!table[initArray[i].rating]) {
        table[initArray[i].rating] = 0;
    }
    table[initArray[i].rating] += initArray[i].count; //filling the table
}

And finally add the values from the table back to the array as a total field:

for (let i = 0; i < initArray.length; i++) {
    initArray[i].total = table[initArray[i].rating];

}

const table = {};

const initArray = [
  {
    rating: "good",
    count: 4,
  },
  {
    rating: "bad",
    count: 2,
  },
  {
    rating: "bad",
    count: 4,
  }
];

for (let i = 0; i < initArray.length; i++) {
    if (!table[initArray[i].rating]) {
        table[initArray[i].rating] = 0;
    }
    table[initArray[i].rating] += initArray[i].count;
}

for (let i = 0; i < initArray.length; i++) {
    initArray[i].total = table[initArray[i].rating];
}

console.log(table, initArray);

flyingfox · Accepted Answer · 2022-10-04 08:00:08Z

0

A simple reference using reduce for you.

It would be better if you have post some code to show us what you have tried to solve it as the comments said.

const data = [
  {
    rating: "good",
    count: 4,
  },
  {
    rating: "bad",
    count: 2,
  },
  {
    rating: "bad",
    count: 4,
  }
]

let goodTotal = 0
let badTotal = 0
const result = data.reduce((a,v) =>{
  if(v.rating == 'good'){
    goodTotal += v.count
    v['total'] = goodTotal
   }else{
    badTotal += v.count
    v['total'] = badTotal
   }
  a.push(v)
  return a
},[])
console.log(result)

answered Oct 4, 2022 at 8:00

flyingfox

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