aggregation framework - mongodb aggregate average of array elements -

i have collection of documents in mongodb 2.6.11 'cpu' array showing [cpu0,cpu1], see example below

{ cpu: [ '100','20' ], hostname: 'host1',_id: 1 }, { cpu: [ '40','30' ], hostname: 'host1',_id: 2 }, etc 

i'm looking average cpu on 'host1' ['70','25'] ( because '100'+'40'/2='70' , '20'+'30'='25' ). trying aggregate cpu1 not giving me right result

db.collection.aggregate(     [         {             $group:             {                 _id:"hostname",                 avgcpu: {$avg: "$cpu.1"}             }         }     ] ); 

in mongodb 2.6 positional notation in aggregation unsupported, need $unwind document, , on next stage perform $avg.

in mondodb 3.2 however, have $arrayelemat operator return element @ specified index { $arrayelemat: [ <array>, <idx> ] }

based on document structure posted, query work:

db.cpu.aggregate([ {$unwind: "$cpu"}, {$group: {    _id: "$_id",    first: {$first: "$cpu"},    last: {$last: "$cpu"} }}, {$group: {    _id: 0,    firstavg: {$avg: "$first"},    lastavg: {$avg: "$last"} }} ]);