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The reduce function has two inputs, for every distinct key emitted by map the reduce function is called with a list of the corresponding values.
The reduce function has two inputs, for every distinct key emitted by map the reduce function is called with a list of the corresponding values.


==How many countries in each continent==
==emit all continents==
<div class=q data-lang="mongo">
<div class=q data-lang="mongo">
This example returns the number of countries in each continent.
This example returns the number of countries in each continent.
<pre class=def>
<pre class=def>
db.world.mapReduce(
db.world.mapReduce(
   function(){emit(this.continent, 1);},  
   function(){emit(this.continent, this.name);},
   function(k, v){ return v.length; }
   function(k, v){ return v.length; }
   out={"inline":1}
   {out:{"inline":1}}
)
)
</pre>
</pre>
</div>
</div>


<div class=q data-lang="py3"><code>query</code> can be used to filter the <b>input</b> documents to map.<br/>
==emit only some continents==
<p class="strong">Find the GDP for each continent, but only include data from countries that start with the letter A or B.</p>
<div class=q data-lang="mongo">
<pre class=def>
The map function may emit only sometimes.
from bson.code import Code
temp = db.world.map_reduce(
        query={"name": {"$regex":"^(A|B)"}},
        map=Code("function(){emit(this.continent, this.gdp)}"),
        reduce=Code("""function(key, values){
                          return Array.sum(values)
                      }
                  """),
        out={"inline":1},
)


pp.pprint(
In the example we are only counting the countries that have a large population
  temp["results"]
)
</pre>
<div class="ans">
from bson.code import Code;temp = db.world.map_reduce(query={"name": {"$regex":"^(A|B)"}},map=Code("function(){emit(this.continent, this.gdp)}"),reduce=Code("function(key, values){return Array.sum(values)}"),out={"inline":1});import operator;pp.pprint(temp["results"])
</div>
</div>
 
 
<div class=q data-lang="py3"><code>scope</code> takes in a <b>document</b>:<code>{}</code> and lets you create global variables.<br/>
It's syntax is: <code>scope={}</code>.<br/>
<p class="strong">Using <code>scope</code>, list all the countries with a higher population than Mexico.</p>
<pre class=def>
<pre class=def>
mexico_data = db.world.find_one({"name":"Mexico"})
db.world.mapReduce(
pp.pprint(mexico_data)
  function(){
 
    if (this.population>100000000)
from bson.code import Code
      emit(this.continent, this.name);},
temp = db.world.map_reduce(
  function(k, v){ return v.length; }
        scope = {"MEXICO":mexico_data},
  {out:{"inline":1}}
        map = Code("""function(){
                        if (this.population > MEXICO.population) emit(this.name, this.population)
                      }
                  """),  
        reduce=Code("function(key, values){return values}"),
        out={"inline":1},
)
pp.pprint(
  temp["results"]
)
</pre>
<div class="ans">
mexico_data = db.world.find_one({"name":"Mexico"}); pp.pprint(mexico_data);  from bson.code import Code; temp = db.world.map_reduce( scope={"MEXICO":mexico_data}, map=Code("function(){if (this.population > MEXICO.population) emit(this.name, this.population)}"), reduce=Code("function(key, values){return values}"), out={"inline":1});pp.pprint(temp['results'])
</div>
</div>
 
<div class=q data-lang="py3"><code>sort</code> and <code>limit</code><br/>
Sort allows us to sort the <b>input</b> documents that are passed to <b>map</b><br/>Limit is self explanatory and also applies to the <b>input</b> documents that are passed to <b>map</b>
<p class="strong">Get the five countries with the highest GDPs</p>
<pre class=def>
from bson.code import Code
temp = db.world.map_reduce(
        query={"gdp":{"$ne":None}},
        sort={"gdp":-1},
        limit=5,
        map=Code("function(){emit(this.name, this.gdp)}"),
        reduce=Code("function(key, values){return values}"),
        out={"inline":1},
)
 
pp.pprint(
  temp["results"]
)
)
</pre>
</pre>
<div class="ans">
from bson.code import Code; temp = db.world.map_reduce( query={"gdp":{"$ne":None}}, sort={"gdp":-1}, limit=5, map=Code("function(){emit(this.name, this.gdp)}"), reduce=Code("function(key, values){return values}"), out={"inline":1}, );pp.pprint(temp["results"])
</div>
</div>
<div class=q data-lang="py3"><code>finalize</code> is an optional additional step that allows you to modify the data produce by <code>reduce</code><br/>
<p class="strong">Show the top 15 countries by population, then show their population as a percentage of Mexico's population.</p>
<pre class=def>
mexico_data = db.world.find_one({"name":"Mexico"})
from bson.code import Code
temp = db.world.map_reduce(
        scope = {"MEXICO":mexico_data},
        query={"population":{"$ne":None}},
        sort={"population":-1},
        limit=15,
        map=Code("function(){emit(this.name, this.population)}"),
        reduce=Code("function(key, values){return values}"),
        out={"inline":1},
        finalize=Code("""function(key, values){
                            return 100*(values/MEXICO.population)+"%"
                        }
                      """)
)
pp.pprint(
  temp["results"]
)
</pre>
<div class="ans">
mexico_data = db.world.find_one({"name":"Mexico"});from bson.code import Code; temp = db.world.map_reduce( scope = {"MEXICO":mexico_data}, query={"population":{"$ne":None}}, sort={"population":-1}, limit=15, map=Code("function(){emit(this.name, this.population)}"), reduce=Code("function(key, values){return values}"), out={"inline":1}, finalize=Code("""function(key, values){return 100*(values/MEXICO.population)+"%"} """) );pp.pprint(temp["results"] );
</div>
</div>
<div class=q data-lang="py3">Rounding can also be performed by using JavaScript.<br/>
<p class="strong">Show the top 15 countries by population, then show their population as a whole number percentage of Mexico's population.</p>
<pre class=def>
mexico_data = db.world.find_one({"name":"Mexico"})
from bson.code import Code
temp = db.world.map_reduce(
        scope = {"MEXICO":mexico_data},
        query={"population":{"$ne":None}},
        sort={"population":-1},
        limit=15,
        map=Code("function(){emit(this.name, this.population)}"),
        reduce=Code("function(key, values){return values}"),
        out={"inline":1},
        finalize=Code("""function(key, values){
                            return Math.round(100*(values/MEXICO.population))+"%"
                        }
                      """)
)
pp.pprint(
  temp["results"]
)
</pre>
<div class="ans">
mexico_data = db.world.find_one({"name":"Mexico"});from bson.code import Code;temp=db.world.map_reduce(scope ={"MEXICO":mexico_data},query={"population":{"$ne":None}},sort={"population":-1},limit=15,map=Code("function(){emit(this.name,this.population)}"),reduce=Code("function(key, values){return values}"), out={"inline":1},finalize=Code("function(key,values){return Math.round(100*(values/MEXICO.population))+'%'}"));pp.pprint(temp["results"])
</div>
</div>

Revision as of 19:49, 2 August 2016

#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)
Top Tips

Introducing the MapReduce function

The MapReduce function is an aggregate function that consists of two functions: Map and Reduce.

The map is always performed before the reduce.

The map function examines every document in the collection and emits (key,value) pairs.

The map function takes no input however the current document can be accessed as this

The reduce function has two inputs, for every distinct key emitted by map the reduce function is called with a list of the corresponding values.

emit all continents

This example returns the number of countries in each continent. 

db.world.mapReduce(
  function(){emit(this.continent, this.name);},
  function(k, v){ return v.length; }
  {out:{"inline":1}}
)

emit only some continents

The map function may emit only sometimes.

In the example we are only counting the countries that have a large population 

db.world.mapReduce(
  function(){
    if (this.population>100000000)
      emit(this.continent, this.name);},
  function(k, v){ return v.length; }
  {out:{"inline":1}}
)
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