define associate array without value

Question

I have a very large hard coded index array that I would like to easily convert to an associative array so lookups are much quicker.

var arr = ["a", "b", "c"];

right now I am looping through arr and comparing it's values to a value to see if there is a match. Now that I have hundreds of elements it's getting rather slow and it would be faster to have an associative array.

It seems I can't just do

var arr = {"a", "b", "c"};

I can't really add a value since it is too slow.

Sure I could copy the elements to an associate array or sort the array and do a binary search but it would be much easier to just able to assign a default value to the array elements and use the syntax above.

I guess this is not possible though?

what does adding a value to an object property have to do with speed? — the system
– the system, Commented Feb 17, 2013 at 5:03
@thesystem If it's using a hash lookup, it can hash the key to find the bucket it's in, which should reduce the lookup space. — alex
– alex, Commented Feb 17, 2013 at 5:04
@alex: no, I was referring to this: "I can't really add a value since it is too slow." — the system
– the system, Commented Feb 17, 2013 at 5:06
I said they are hard coded, manually, and already exist, I'd have to manually add all values by hand(I could use a search and replace but still slow). I'd have to add the values by hand to all the keys I've added. I am not using the values. Just the keys for O(1) lookup. — jsmdnq
– jsmdnq, Commented Feb 17, 2013 at 5:09

alex · Accepted Answer · 2013-02-17 05:03:28Z

1

var mapLookup = arr.reduce(function (accumalator, value) {
    accumalator[value] = true;
    return accumalator;
}, {});

answered Feb 17, 2013 at 5:03

alex

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lmortenson Over a year ago

IE 8 doesn't support reduce, and it's still fairly prevalent

kufudo · Accepted Answer · 2013-02-17 05:03:50Z

1

Why not: var arr = {"a":1, "b":1, "c":1};

answered Feb 17, 2013 at 5:03

kufudo

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jsmdnq Over a year ago

because I'd have to add :1 to about 1000 + elements and to every element I manually added to the array.

jsmdnq · Accepted Answer · 2013-02-17 05:11:53Z

1

Converting arrays to associative arrays seems to be the easiest and very fast:

var arr = ["a", "b", "c"];
var arrA = {}; for(var i = 0; i < arr.length; i++) arrA[arr[i]] = 0;

then just use key in arrA for O(1) lookup. (it would be easier to have the ability not have to explicitly supply a value to a key but....)

Essentially

if (key in arrA) ...

replaces

for(var i = 0; i < arr.length; i++) if (key == arr[i]) ...

which essentially is O(n) vs O(n^2) when uses inside a loop.

answered Feb 17, 2013 at 5:11

jsmdnq

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the system Over a year ago

If you really want to shorten it, you could do eval("({" + arr.join(":1,") + ":1})")

Aesthete · Accepted Answer · 2013-02-17 05:13:48Z

1

Most modern browsers support Array.indexOf(). This will return -1 if your search bears no results.

Are you actually having speed issues, or are you just pre-optimizing? The container you should be using is an array. You have an array of elements - they don't associate with any other values, so why put them in a map container?

Also, it sounds like you want a set, where you have a unique set of elements.

edited Feb 17, 2013 at 5:13

answered Feb 17, 2013 at 5:05

Aesthete

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2 Comments

ATOzTOA Over a year ago

OP wants to compare against the value, not the a,b,c keys.

jsmdnq Over a year ago

Yes, I have speed issues, I've fixed them by converting my arrays to associative arrays and it's about 1000 times faster now. (instant)

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