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@maggiben
Forked from RedBeard0531/functions.js
Created April 4, 2014 08:40
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// derived from http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Parallel_algorithm
function map() {
emit(1, // Or put a GROUP BY key here
{sum: this.value, // the field you want stats for
min: this.value,
max: this.value,
count:1,
diff: 0, // M2,n: sum((val-mean)^2)
});
}
function reduce(key, values) {
var a = values[0]; // will reduce into here
for (var i=1/*!*/; i < values.length; i++){
var b = values[i]; // will merge 'b' into 'a'
// temp helpers
var delta = a.sum/a.count - b.sum/b.count; // a.mean - b.mean
var weight = (a.count * b.count)/(a.count + b.count);
// do the reducing
a.diff += b.diff + delta*delta*weight;
a.sum += b.sum;
a.count += b.count;
a.min = Math.min(a.min, b.min);
a.max = Math.max(a.max, b.max);
}
return a;
}
function finalize(key, value){
value.avg = value.sum / value.count;
value.variance = value.diff / value.count;
value.stddev = Math.sqrt(value.variance);
return value;
}
> load('functions.js')
> db.stuff.drop()
false
> db.stuff.insert({value:1})
> db.stuff.insert({value:2})
> db.stuff.insert({value:2})
> db.stuff.insert({value:2})
> db.stuff.insert({value:3})
> db.stuff.mapReduce(map, reduce, {finalize:finalize, out:{inline:1}}).results[0]
{
"_id" : 1,
"value" : {
"sum" : 10,
"min" : 1,
"max" : 3,
"count" : 5,
"diff" : 2,
"avg" : 2,
"variance" : 0.4,
"stddev" : 0.6324555320336759
}
}
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