Question

I have a set of objects in Mongodb that each have a set of values embedded in them, e.g.:

[1.22, 12.87, 1.24, 1.24, 9.87, 1.24, 87.65] // ... up to about 150 values

Is a map/reduce the best solution for finding the median (average) and mode (most common value) in the embedded arrays? The reason that I ask is that the map and the reduce both have to return the same (structurally) set of values. It looks like in my case I want to take in a set of values (the array) and return a set of two values (median, mode).

If not, what’s the best way to approach this? I want it to run in a rake task, if that’s relevant. It’d be an overnight data crunching kind of thing.

Answer 1

I assume you want to find the mode & median of each document, you can do this with map reduce. In this case you calculate median & mode in the map function and reduce will return the map result untouched

map = function() {
   var res = 0;
   for (i = 0; i < this.marks.length; i++) {
      res = res + this.marks[i];
   }
   var median = res/this.marks.length;
   emit(this._id,{marks:this.marks,median:median});
}


reduce = function (k, values) {
    values.forEach(function(value) {
      result = value;
    });
    return result;
}

and for this collection

{ "_id" : ObjectId("4f02be1f1ae045175f0eb9f1"), "name" : "ram", "marks" : [ 1.22, 12.87, 1.24, 1.24, 9.87, 1.24, 87.65 ] }
{ "_id" : ObjectId("4f02be371ae045175f0eb9f2"), "name" : "sam", "marks" : [ 1.32, 11.87, 12.4, 4.24, 9.37, 3.24, 7.65 ] }
{ "_id" : ObjectId("4f02be4c1ae045175f0eb9f3"), "name" : "pam", "marks" : [ 3.32, 10.17, 11.4, 2.24, 2.37, 3.24, 30.65 ] }

you can get the median by

  db.test.mapReduce(map,reduce,{out: { inline : 1}})

{
    "results" : [
        {
            "_id" : ObjectId("4f02be1f1ae045175f0eb9f1"),
            "value" : {
                "marks" : [
                    1.22,
                    12.87,
                    1.24,
                    1.24,
                    9.87,
                    1.24,
                    87.65
                ],
                "median" : 16.475714285714286
            }
        },
        {
            "_id" : ObjectId("4f02be371ae045175f0eb9f2"),
            "value" : {
                "marks" : [
                    1.32,
                    11.87,
                    12.4,
                    4.24,
                    9.37,
                    3.24,
                    7.65
                ],
                "median" : 7.155714285714285
            }
        },
        {
            "_id" : ObjectId("4f02be4c1ae045175f0eb9f3"),
            "value" : {
                "marks" : [
                    3.32,
                    10.17,
                    11.4,
                    2.24,
                    2.37,
                    3.24,
                    30.65
                ],
                "median" : 9.055714285714286
            }
        }
    ],
    "timeMillis" : 1,
    "counts" : {
        "input" : 3,
        "emit" : 3,
        "reduce" : 0,
        "output" : 3
    },
    "ok" : 1,
}