js - Declare multidimensional array + push key and value

Question

Where exactly is wrong here? I get a "Cannot read property 'push' of undefined" error, and I believe that's because the array can't be set to multidimensional by simply declaring it in the loop.

var single = [];
for (var i = 0; i < all.length; i++) {
    name = all[i].name;
    single[name].push(all[i]);
}

What I'm trying to achieve is this structure:

Array (
([name1] => [node1],[node2],[node3]),
([name2] => [node1],[node2],[node3],[node4],[node5],[node6]),
([name2] => [node1],[node2])
etc...
)

I've tried searching here on SO, but so far only got two options:

Option 1: Shorthand in declaring the variable, aka [[]], which doesn't work.

var single = [[]];

Option 2: link Add another loop to work out the array before filling it.

var matrix = []
cols = 3;
//init the grid matrix
for ( var i = 0; i < cols; i++ ) {
 matrix[i] = []; 
}

I'd rather find a shorter solution, also because my array elements MUST have the key, while on the solution above they are numbered.

EDIT: since keys in JS array are not an option, would an object do the trick in this case? something like:

var obj = {
key1: value1,
key2: value2
};
obj.key3 = "value3";

The structure you want is invalid, you can't have named keys in arrays — adeneo
– adeneo, Commented Dec 26, 2015 at 13:25

madox2 · Accepted Answer · 2015-12-26 13:34:28Z

9

You can use object or Map (ES6) data structure to achieve this, e.g.:

var single = {};
for (var i = 0; i < all.length; i++) {
    name = all[i].name;
    if (!single[name]) {
        single[name] = [];
    }
    single[name].push(all[i]);
}

the result (single object) looks like:

{
    name1: [node1, node2, node3],
    name2: [node1, node2, node3, node4, node5, node6],
    name2: [node1, node2]
}

edited Dec 26, 2015 at 13:34

answered Dec 26, 2015 at 13:30

madox2

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Comments

Norbert · Accepted Answer · 2015-12-26 13:40:05Z

Javascript uses objects and arrays. There is no multidimensional-array.

The structure you want would look like this in javascript:

{
    name1: [a,b,c],
    name2: [d,e,f]
}

In fact in JS, you can just define a variable with such a object like this (object literal):

var myObject = {
    name1: [a,b,c],
    name2: [d,e,f]
}

You do not need to iterate to construct an object in JS, object-literals are easy to understand and fast.

If for some reason you need to map some data to this new format you want, I personally would use methods such as the Array.prototype.reduce.

myMap = all.reduce(function (result, item) {
    result[item.name] = (result[item.name] || []).push(item);
    return result;
},{});

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