Use float as array key

Question

I want to use an array to look up a value given a key. AFAIK, it should be possible when converting the float to a string, like the second example below: (jsfiddle)

arr = [];
arr[1.3] = "One point three";
console.log(arr.length);

arr = [];
arr["1.3"] = "One point three";
console.log(arr.length);

But both result in a zero-length array. What am I doing wrong? I.e. how can I look up an object/string/whatever given a float value?

It would be awesome to have a reference guide on common operations when using having to look up values, such as:

get an element, given a float key
get total number of elements
test if float key exists
put new float key / value pair
and maybe others, such as loop through all keys/values

index should be integer for an array..... use object instead and set property arr = {}; arr['1.3'] = "One point three"; console.log(Object.keys(arr).length); — Pranav C Balan
– Pranav C Balan, Commented Aug 6, 2016 at 7:46
length doesn't include non-integer keys, otherwise it would have to count itself. — melpomene
– melpomene, Commented Aug 6, 2016 at 7:48

Suren Srapyan · Accepted Answer · 2016-08-06 07:51:40Z

1

Array's index must be an integer for adding it as an array item. If you will pass to the [] not an integer, it will be added as a property.

arr = [];
arr[1.3] = "One point three";

console.log(arr.length);

console.log(arr.hasOwnProperty('1.3'));

console.log(Object.keys(arr).length);

answered Aug 6, 2016 at 7:51

Suren Srapyan

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1 Comment

T.J. Crowder Over a year ago

Array entries are also properties.

james emanon · Accepted Answer · 2016-08-06 07:54:02Z

1

I could be wrong, but it seems you are trying to create a dict. I would just use an object like others have suggested.

var arr = {
    1.2: 'One Point Three'
}

arr[1.2] or arr["1.2"] // One Point Three

or you can use es6 Map

var arr = new Map()
    arr.set(1.3, "One Point Three")
    arr.get(1.3) // One Point Three 
    arr.size // 1

answered Aug 6, 2016 at 7:54

james emanon

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Comments

Flying Gambit · Accepted Answer · 2016-08-06 07:55:30Z

1

An array takes an integer value as index.

You should be using an Object for this kind of mapping.

var arr = {};
arr[1.3] = "One point three";
console.log(Object.keys(arr).length);

answered Aug 6, 2016 at 7:55

Flying Gambit

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Comments

Jayy · Accepted Answer · 2016-08-06 08:24:05Z

0

Suren's answer was the first, and gave me what I needed to establish a reference of operations:

Insert an item:

var x = 1.3;

// Not actually an array! arr.length will be undefined. Although
// an array could still be used, it seems
var arr = {}; 

arr[x] = "One point three";

Count items:

Object.keys(arr).length

Test if key exists:

arr.hasOwnProperty(x)

Get an element:

arr[x]

Loop through key/value pairs

for (key in arr)
{
    var value = arr[key];
}

Interesting quirks

+1 to James Emanon for several points: Firstly, showing that a string is not necessary for lookup:

arr[1.3] // works

Secondly, the array can be defined as an object like so:

arr = {
    1.3: "One point three"
};

Thirdly, ES6 provides a Map object. Will be useful.

edited Aug 6, 2016 at 8:24

answered Aug 6, 2016 at 8:18

Jayy

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