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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: function to get frequencies of a char array
Date: Fri, 23 Jan 2015 16:52:29 +0100
Organization: PointedEars Software (PES)
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Christoph M. Becker wrote:

> Evertjan. wrote:
>> Try this, using an object as target:
>> 
>> =============================
>> var arr = 'abjcdjjj'.split(''); // source array
>> var res = {};                   // result object
>> 
>> function frequencies() {
>>   for(i=0; i< arr.length; i++) {
>>     if (!res[arr[i]])
>>       res[arr[i]] = 0;
>>     res[arr[i]] +=1;
>>   };
>> };
>> frequencies();
>> 
>> console.log(res); // Object {a: 1, b: 1, j: 4, c: 1, d: 1}
>> =============================
> 
> Or maybe using Array.prototype.reduce:
> 
>     var a = ['a', 'b', 'j','c', 'd', 'j', 'j', 'j'];
>     a.reduce(function (prev, curr) {
>         if (prev.hasOwnProperty(curr)) {
>             prev[curr]++;
>         } else {
>             prev[curr] = 1;
>         }
>         return prev;
>     }, {});

While this works – but I would use “Object.create(null)” instead of “{}” to 
avoid name collisions –, I find a “for” or “for–in” statement more obvious 
and compatible:

  var counts = {};

  for (var i = 0, len = a.length; i < len; ++i)
  {
    /* NOTE: hasOwnProperty() may be required */
    var e = a[i];
    e ? ++counts[e] : (counts[e] = 1);
  }

  for (var i in a)
  {
    /* NOTE: hasOwnProperty() may be required */
    var e = a[i];
    e ? ++counts[e] : (counts[e] = 1);
  }

“for–in” is more efficient than “for“ and Array.prototype.forEach() with 
sparse arrays.

Array.prototype.forEach() can be used instead if it is supported and the 
array is insufficiently sparse:

  a.forEach(function (e, i, a) {
    var counts = a.counts_ || (a.counts_ = Object.create(null));
    (counts[e]) ? ++counts[e] : (counts[e] = 1);
  });
  
  a.counts_

The advantage of using the Array instance as container is that the scope 
chain is shorter, which is faster.  However, that method, too, can be passed 
a “this” value:

  var counts = Object.create(null);

  a.forEach(
    function (e) { (this[e]) ? ++this[e] : (this[e] = 1); },
    counts);
  
  counts

Array.prototype.reduce(), if I use it (it needs to be emulated in case it is 
not available), I reserve for getting a single *primitive* value from an 
Array instead.  For example:

  var sum = a.reduce(function (prev, curr) { return prev + (+curr); }, 0);

(Implicit type conversion is unnecessary if all elements are of type 
Number.)

-- 
PointedEars
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