Difference between revisions of "Round"

Revision as of 03:00, 6 April 2018

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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]}
  }},
  {"$project":{
    "_id": 0,
    "name": 1,
    "population": '$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()

@@ Line 1: / Line 1: @@
 {{TopTenTips}}
 <div style="min-height:25em">
-Rounding is easy inside a <code>MapReduce</code> as it is possible to use the <code>Math</code> object provided by JavaScript.<br/>
+There are various ways of rounding in MongoDB.<br/>
+Rounding is easy inside of a <code>MapReduce</code> as it is possible to use the <code>Math</code> object provided by JavaScript.<br/>
 More information on <code>Math</code> can be found [https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Math here].<br/><br/>
-At the time of writing 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/>
+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/>
+As of MongoDB 3.2, it is possible to round up, down, and truncate via <code>$ceil</code>, <code>$floor,</code> and <code>$trunc</code> respectively.
 </div>
 ==MapReduce==
@@ Line 43: / Line 45: @@
      "name": 1,
      "population": '$population',
-     "population in millions":{
+     "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":[