Convert array to object by concatenating values under same key

Question

I tried searching through previous questions but couldn't find the answer so here's a very generic question.

I have a multidimensional array like so:

array = [
    [ "1", "2013-14" , 1234],
    [ "2", "2013-14", 2345],
    [ "1", "2014-15" , 5234],
    [ "2", "2014-15", 7345],
]

I am trying to change it to a nested object based on the year:

obj = {
  "2013-14": { 1: "1234", 2: "2345" },
  "2014-15": { 2: "5234", 2: "7345" }
}

I tried this, which gave a similar but wrong solution:

let obj = {}
for (let i = 0; i < array.length; i++) {
  obj[array[i][1]] = {
    [array[i][0]]: array[i][2]
  }
}

the solution I got:

obj = {
      "2013-14": { 2: "2345" },
      "2014-15": { 2: "7345" }
}

so instead of concatenating values under the same key, which in this case is the year, it is overriding it and saving the last one

Nina Scholz · Accepted Answer · 2021-08-24 11:37:25Z

You need to respect the object for ech date.

const
    array = [["1", "2013-14", 1234], ["2", "2013-14", 2345], ["1", "2014-15", 5234], ["2", "2014-15", 7345]],
    obj = {};
    
for (let i = 0; i < array.length; i++) {
    obj[array[i][1]] ??= {};
    obj[array[i][1]][array[i][0]] = array[i][2];
}

console.log(obj);

A shorter approach with destructuring

const
    array = [["1", "2013-14", 1234], ["2", "2013-14", 2345], ["1", "2014-15", 5234], ["2", "2014-15", 7345]],
    obj = {};
    
for (const [inner, outer, value] of array) {
    obj[outer] ??= {};
    obj[outer][inner] = value;
}

console.log(obj);

Jane · Accepted Answer · 2021-08-24 11:42:23Z

1

you can use spread operator {...} to concat it.

Here is the code:

var array = [
  ["1", "2013-14", 1234],
  ["2", "2013-14", 2345],
  ["1", "2014-15", 5234],
  ["2", "2014-15", 7345],
];

let obj = {};
for (let i = 0; i < array.length; i++) {
  obj[array[i][1]] = { ...obj[array[i][1]], [array[i][0]]: array[i][2] };
}

console.log(obj);

answered Aug 24, 2021 at 11:42

Jane

3341 silver badge4 bronze badges

Comments

Vibhansh · Accepted Answer · 2021-08-24 11:37:21Z

The problem I think is that you're trying to access objects within the key you just initialised. The code is writing the data you wanted but since you're accessing the same key multiple times, the data is being overwritten.

I tried the following approach in Python and it worked :

data = [
[ "1", "2013-14" , 1234],
[ "2", "2013-14", 2345],
[ "1", "2014-15" , 5234],
[ "2", "2014-15", 7345],
]

# EXTRACT KEYS TO INITIALIZE OBJECT AND INSERT VALUES LATER
keys=[]
for i in data :
    if i[1] not in keys:
        keys.append(i[1]) 
parsed_obj = {k:{} for k in keys}

for i in data :
    parsed_obj[i[1]][i[0]] = i[2]
print(parsed_obj)

Output :

{'2013-14': {'1': 1234, '2': 2345}, '2014-15': {'1': 5234, '2': 7345}}

I know it's not ideal but maybe someone can help you translate this into JS.

Rinshan Kolayil · Accepted Answer · 2021-08-24 11:52:54Z

1

const array = [["1", "2013-14", 1234], ["2", "2013-14", 2345], ["1", "2014-15", 5234], ["2", "2014-15", 7345]];
var result = {}
array.forEach(([i,date,num]) => {
    result[date] ??= {};
    result[date][i] = num;
});
console.log(result);

answered Aug 24, 2021 at 11:52

Rinshan Kolayil

1,1491 gold badge10 silver badges16 bronze badges

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