How to count the number of times each value in the array appears in that array

How to count the number of times each value in the array appears in that array?

Source: StackOverflow.com

Question

I have an array of values. Essentially, I'd like to count the number of times each value in the array appears in that array. So for instance at a particular pixel of the world, the array might look like:

[1, 1, 0.7, 0.3, 1, 0.8, 1, 0.8, 0.7, 1]  

I need to count the number of times a particular value appears, then run the expression
Expression(value*(value-1)(value2+5));

So for that array, I want the distinct value counts...

    1: 5  
    0.7: 2  
    0.8: 2  
    0.3: 1  

And then compute the value of those "ties" as a part of the expression...

    5 * (5-1) * (5*2 -5) +   
    2 * (2-1) * (2*2 -5) +   
    2 * (2-1) * (2*2 -5) +   
    1 * (1-1) * (1*2 -5)  

As seen above, values with a count of 1 fall out, which is fine, however, I need to avoid double counting items so simply transforming the array into counts ([5, 5, 2, 1, 5, 2, 5, 2, 2, 5]) is incorrect.

Answer

AlaSQL library was originally designed to make calculations like in you question.

    var arr = [1, 1, 0.7, 0.3, 1, 0.8, 1, 0.8, 0.7, 1];

    var res = alasql('SELECT VALUE SUM(v*(v-1)*(v*2+5)) FROM 
                          (SELECT COUNT(*) AS v FROM ? GROUP BY _)',[arr]);

Try this example in jsFIddle