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<page id="basic-aggregate-examples"
type="topic"
xmlns="http://projectmallard.org/1.0/">
<title>Basic Aggregation Examples</title>
<p>This document provides some practical, simple, examples to demonstrate the <code>distinct</code> and <code>mapReduce</code> commands.</p>
<section id="setup">
<title>Setup</title>
<p>First we'll write some code to insert sample data:</p>
<code mime="text/x-csrc"><include parse="text" href="../examples/doc-common-insert.c" xmlns="http://www.w3.org/2001/XInclude" /></code>
</section>
<section id="distinct">
<title>"distinct" command</title>
<p>This is how to use the <code>distinct</code> command to get the distinct values of <code>x</code> which are greater than <code>1</code>:</p>
<code mime="text/x-csrc"><include parse="text" href="../examples/basic_aggregation/distinct.c" xmlns="http://www.w3.org/2001/XInclude" /></code>
</section>
<section id="map-reduce-basic">
<title>"mapReduce" - basic example</title>
<p>A simple example using the map reduce framework. It simply adds up the number of occurrences of each "tag".</p>
<p>First define the <code>map</code> and <code>reduce</code> functions:</p>
<code mime="text/x-csrc"><include parse="text" href="../examples/basic_aggregation/constants.c" xmlns="http://www.w3.org/2001/XInclude" /></code>
<p>Run the <code>mapReduce</code> command:</p>
<code mime="text/x-csrc"><include parse="text" href="../examples/basic_aggregation/map-reduce-basic.c" xmlns="http://www.w3.org/2001/XInclude" /></code>
</section>
<section id="map-reduce-advanced">
<title>"mapReduce" - more complicated example</title>
<p>You must have replica set running for this.</p>
<p>In this example we contact a secondary in the replica set and do an "inline" map reduce, so the results are returned immediately:</p>
<code mime="text/x-csrc"><include parse="text" href="../examples/basic_aggregation/map-reduce-advanced.c" xmlns="http://www.w3.org/2001/XInclude" /></code>
</section>
<section id="running">
<title>Running</title>
<p>Here's how to run the example code</p>
<listing>
<title>basic-aggregation.c</title>
<code mime="text/x-csrc"><include parse="text" href="../examples/basic_aggregation/basic-aggregation.c" xmlns="http://www.w3.org/2001/XInclude" /></code>
</listing>
<p>If you want to try the advanced map reduce example with a secondary, start a replica set (instructions for how to do this can be found <link href="https://docs.mongodb.com/manual/tutorial/deploy-replica-set-for-testing/">here</link>).</p>
<p>Otherwise, just start an instance of MongoDB:</p>
<screen><output style="prompt">$ </output><input>mongod</input></screen>
<p>Now compile and run the example program:</p>
<screen><output style="prompt">$ </output><input>cd examples/basic_aggregation/</input>
<output style="prompt">$ </output><input>gcc -Wall -o agg-example basic-aggregation.c $(pkg-config --cflags --libs libmongoc-1.0)</input>
<output style="prompt">$ </output><input>./agg-example localhost</input>
Inserting data
distinct
Next double: 2.000000
Next double: 3.000000
map reduce
{ "result" : "outCollection", "timeMillis" : 155, "counts" : { "input" : 84, "emit" : 126, "reduce" : 3, "output" : 3 }, "ok" : 1 }
{ "_id" : "cat", "value" : 63 }
{ "_id" : "dog", "value" : 42 }
{ "_id" : "mouse", "value" : 21 }
more complicated map reduce
{ "results" : [ { "_id" : "cat", "value" : 63 }, { "_id" : "dog", "value" : 42 }, { "_id" : "mouse", "value" : 21 } ], "timeMillis" : 14, "counts" : { "input" : 84, "emit" : 126, "reduce" : 3, "output" : 3 }, "ok" : 1 }
</screen>
</section>
</page>
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