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<pre class=setup>
#ENCODING
import io
import sys
sys.stdout = io.TextIOWrapper(sys.stdout.buffer, encoding='utf-16')
#MONGO
from pymongo import MongoClient
client = MongoClient()
client.progzoo.authenticate('scott','tiger')
db = client['progzoo']
#PRETTY
import pprint
pp = pprint.PrettyPrinter(indent=4)
#CODE
from bson.code import Code
</pre>
{{TopTenTips}}
{{TopTenTips}}
Rounding is easy inside a <code>MapReduce</code> as it is possible to use the <code>Math</code> object provided by JavaScript.
<div style="min-height:25em">
At the time of writing the <code>aggregation()</code> method has no rounding functions, though it is still doable.
There are various ways of rounding in MongoDB.<br/>
<div class=q data-lang="py3">MapReduce
Rounding is easy inside of a <code>MapReduce</code> as it is possible to use the <code>Math</code> object provided by JavaScript.<br/>
<pre class=def>
More information on <code>Math</code> can be found [https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Math here].<br/><br/>
from bson.code import Code
In MongoDB versions prior to 3.2 the <code>aggregation()</code> method has no rounding functions, though it is still doable with <code>$mod</code> and <code>$multiply</code>, as shown [http://www.kamsky.org/stupid-tricks-with-mongodb/rounding-numbers-in-aggregation-framework here] by the lead project manager at MongoDB.<br/>
temp = db.world.map_reduce(
As of MongoDB 3.2, it is possible to round up, down, and truncate via <code>$ceil</code>, <code>$floor,</code> and <code>$trunc</code> respectively.
        query={"population":{"$ne":None}},
</div>
        limit=1,
==MapReduce==
        map=Code("function(){emit(this.name, this.population)}"),  
<div class="q nonum" data-lang="mongo">
        reduce=Code("function(key, values){return Math.round(values/1000000)}"),
<pre class="def"><nowiki>
        out={"inline":1},
db.world.mapReduce(
)
  function () {
    emit(this.name, this.population);
  },
  function (key, values) {
    return values;
  },
  {
    finalize: function (key,values) {
      return {
          "population": values,
          "population in millions": Math.round(values / 1000000)
      };
    },
    out: {inline: 1},
    query: {"name": "United Kingdom"}
  }
);</nowiki></pre>
</div>


pp.pprint(
==Aggregate==
  temp["results"]
<div class="q nonum" data-lang="mongo">
)
<pre class="def"><nowiki>
</pre>
db.world.aggregate([
  {"$match": {"name": "United Kingdom"}},
  {"$project": {
    "_id": 1,
    "name": 1,
    "population": {"$divide": ["$population", 1000000]}
  }},
  {"$project": {
    "_id": 0,
    "name": 1,
    "population in millions": '$population',
    "population in millions (floored)": {"$floor": "$population"},
    "population in millions (ceiling)": {"$ceil": "$population"},
    "population in millions (truncated)": {"$trunc": "$population"},
    "population in millions (1dp with multiply / mod)":{
      "$divide":[
        {"$subtract":[
          {"$multiply": ['$population', 100]},
          {"$mod": [{"$multiply": ['$population', 100]}, 1]}
        ]},
        100
      ]
    }
  }}
]).pretty();</nowiki></pre>
</div>
</div>

Latest revision as of 16:45, 18 July 2018

Top Tips

There are various ways of rounding in MongoDB.
Rounding is easy inside of a MapReduce as it is possible to use the Math object provided by JavaScript.
More information on Math can be found here.

In MongoDB versions prior to 3.2 the aggregation() method has no rounding functions, though it is still doable with $mod and $multiply, as shown here by the lead project manager at MongoDB.
As of MongoDB 3.2, it is possible to round up, down, and truncate via $ceil, $floor, and $trunc respectively.

MapReduce

db.world.mapReduce(
  function () {
    emit(this.name, this.population);
  },
  function (key, values) {
    return values;
  },
  {
    finalize: function (key,values) {
      return {
          "population": values,
          "population in millions": Math.round(values / 1000000)
      };
    },
    out: {inline: 1},
    query: {"name": "United Kingdom"}
  }
);

Aggregate

db.world.aggregate([
  {"$match": {"name": "United Kingdom"}},
  {"$project": {
    "_id": 1,
    "name": 1,
    "population": {"$divide": ["$population", 1000000]}
  }},
  {"$project": {
    "_id": 0,
    "name": 1,
    "population in millions": '$population',
    "population in millions (floored)": {"$floor": "$population"},
    "population in millions (ceiling)": {"$ceil": "$population"},
    "population in millions (truncated)": {"$trunc": "$population"},
    "population in millions (1dp with multiply / mod)":{
      "$divide":[
        {"$subtract":[
          {"$multiply": ['$population', 100]},
          {"$mod": [{"$multiply": ['$population', 100]}, 1]}
        ]},
        100
      ]
    }
  }}
]).pretty();
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