Difference between revisions of "Round"

Latest revision as of 16:45, 18 July 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]}
  }},
  {"$project": {
    "_id": 0,
    "name": 1,
    "population in millions": '$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();

@@ Line 1: / Line 1: @@
-<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}}
-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='extra_space' style='width:1em; height:20em;'></div>
+Rounding is easy inside of a <code>MapReduce</code> as it is possible to use the <code>Math</code> object provided by JavaScript.<br/>
-<div class=q data-lang="py3">MapReduce
+More information on <code>Math</code> can be found [https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Math here].<br/><br/>
-<pre class=def>
+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/>
-from bson.code import Code
+As of MongoDB 3.2, it is possible to round up, down, and truncate via <code>$ceil</code>, <code>$floor,</code> and <code>$trunc</code> respectively.
-temp = db.world.map_reduce(
+</div>
-        query={"population":{"$ne":None}},
+==MapReduce==
-        limit=1,
+<div class="q nonum" data-lang="mongo">
-        map=Code("function(){emit(this.name, this.population)}"),
+<pre class="def"><nowiki>
-        reduce=Code("function(key, values){return Math.round(values/1000000)}"),
+db.world.mapReduce(
-        out={"inline":1},
+  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]}
+        ]},
+      ]
+    }
+  }}
+]).pretty();</nowiki></pre>
 </div>