Append values to javascript dictionary

Question

I am trying to create the following data structure in javascript:

d = {"foo": [3, 77, 100], "bar": [10], "baz": [99], "biff": [10]}

My starting data structure is a a list of dictionaries:

input = [{"key": "foo", "val": 3}, {"key": "bar", "val": 10}, {"key": "foo", "val": 100}, {"key": "baz", "val": 99}, {"key": "biff", "val": 10}, {"key": "foo", "val": 77]

How can I generate my desired data structure? The following code doesn't seem to append values to the value array.

var d = {}

for (var i in input) {
    var datum = input[i];
    d[datum.key] = datum.val
}

What's hops? Don't you mean var datum = input[i];? P.S. Don't use for..in for arrays: stackoverflow.com/a/500531 — gen_Eric
– gen_Eric, Commented Oct 25, 2013 at 21:03
Iterate the input, see if d has a property for each value of "key", and if not, give d a property using that value, and assign an array. Then push the value of "val" into the Array. — Blue Skies
– Blue Skies, Commented Oct 25, 2013 at 21:05

Barmar · Accepted Answer · 2013-10-25 21:07:02Z

28

for (var i = 0; i < input.length; i++) {
    var datum = input[i];
    if (!d[datum.key]) {
        d[datum.key] = [];
    }
    d[datum.key].push(datum.val);
}

FYI, you shouldn't use for (var i in input) to iterate over an array.

answered Oct 25, 2013 at 21:07

Barmar

789k57 gold badges555 silver badges669 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Akshaya Shanbhogue Over a year ago

You were a tad bit faster than me. Same exact answer :)

l30.4l3x · Accepted Answer · 2020-07-10 05:39:11Z

5

Another alternative:

const input = [{ "key": "foo", "val": 3 }, { "key": "bar", "val": 10 }, { "key": "foo", "val": 100 }, { "key": "baz", "val": 99 }, { "key": "biff", "val": 10 }, { "key": "foo", "val": 77 }]
const dict = {}

input.forEach(({ key, val }) =>
    key in dict ? dict[key].push(val) : dict[key] = [val] )

console.log(dict);

And a one-liner, with immutability

input.reduce((dict, { key, val }) => ({ ...dict, [key]: [...dict[key] || [], val] }), {})

edited Jul 10, 2020 at 5:39

answered Jul 10, 2020 at 4:26

l30.4l3x

6031 gold badge8 silver badges9 bronze badges

2 Comments

TomeeNS Over a year ago

It is unreadable. Please do not use arrow operators in maintainable code. Thank you.

l30.4l3x Over a year ago

Not all javascript/typescript programmers adopt as their primary language, some imperative one. Someone can look at an imperative code and consider it bloated and having unnecessary mutations. Where did you get this about not using the arrow operator in maintainable code? I can see lots of them in lodash, react and axios source code. In fp-ts, I can only see arrow functions.

plalx · Accepted Answer · 2013-10-25 21:09:48Z

1

Another way, with reduce.

var d = input.reduce(function (res, item) {
    var key = item.key;

    if (!res[key]) res[key] = [item.val];
    else res[key].push(item.val);

    return res;

}, {});

answered Oct 25, 2013 at 21:09

plalx

43.8k7 gold badges77 silver badges94 bronze badges

2 Comments

techie.brandon Over a year ago

Probably not the clearest for newbie but I like it :)

plalx Over a year ago

@user1523882, Yeah it might not be, but I really like to make use of the initialValue of the reduce functions to build maps like this. Once you get used to it it's nice because you can do it in a single external statement.

techie.brandon · Accepted Answer · 2013-10-25 21:15:21Z

1

var result = {}
input.forEach(function(keyObject){
  //Make array for key if doesn't exist
  result[keyObject.key] = result[keyObject.key] ? result[keyObject.key] : [];
  //Add value to array
  result[keyObject.key].push(keyObject.val);
});
console.log(result);

answered Oct 25, 2013 at 21:15

techie.brandon

1,6882 gold badges18 silver badges27 bronze badges

Comments

gartox · Accepted Answer · 2013-10-25 21:20:41Z

1

You should be do the next:

for (var i in input){
    var datuml = input[i];    
    if(!d[datuml.key]){
        d[datuml.key]=[];
    }
    d[datuml.key].push(datuml.val);
}

answered Oct 25, 2013 at 21:20

gartox

89110 silver badges6 bronze badges

Comments

Community · Accepted Answer · 2017-05-23 12:08:57Z

0

you will have more then 1 key? Well, I think you want something like convert JSON to ArrayString.

Check this Convert JSON To Array Javascript and this How to convert JSON object to JavaScript array

etc

edited May 23, 2017 at 12:08

CommunityBot

11 silver badge

answered Oct 25, 2013 at 21:07

X-Pippes

1,1707 silver badges25 bronze badges

Comments

Wagner DosAnjos · Accepted Answer · 2013-10-25 21:08:49Z

0

Please try the following:

        var input = [{ "key": "foo", "val": 3 }, { "key": "bar", "val": 10 }, { "key": "foo", "val": 100 }, { "key": "baz", "val": 99 }, { "key": "biff", "val": 10 }, { "key": "foo", "val": 77 }];
        var d = {};

        for (var i = 0; i < input.length; i++) {
            var entry = input[i];
            if (d[entry.key] === undefined) d[entry.key] = [];
            d[entry.key].push(entry.val);
        }

        alert(JSON.stringify(d));

answered Oct 25, 2013 at 21:08

Wagner DosAnjos

6,3841 gold badge18 silver badges30 bronze badges

Comments

Pedro Santos · Accepted Answer · 2023-12-28 23:10:17Z

0

PRO-TIP:

You can define it as a expression so you can reuse it multiple times and keep your code readable.

Examples:

const pushToDict = (dictionary:any, key:any, value:any) => {
    (dictionary[key] = dictionary[key] || []).push(value);
};

typescript version

const pushToDict = (dictionary, key, value) => {
    (dictionary[key] = dictionary[key] || []).push(value);
};

javascript version

And you call it like this...

Example of usage:

pushToDict(dict, key, value);

answered Dec 28, 2023 at 23:10

Pedro Santos

3251 gold badge2 silver badges11 bronze badges

Collectives™ on Stack Overflow

Append values to javascript dictionary

8 Answers 8

1 Comment

2 Comments

2 Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

8 Answers 8

1 Comment

2 Comments

2 Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related