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Sum and Count: Difference between revisions

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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}}
<div style="min-height:25em">
<div style="min-height:25em">
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mapReduce can produce a sum or a count during the results stage by using JavaScript.
mapReduce can produce a sum or a count during the results stage by using JavaScript.
</div>
</div>
<div class="q nonum" data-lang="py3">.count()
<div class="q nonum" data-lang="mongo">.count()
<div class="def">print(db.world.count({"continent":"Africa"}))
<div class="def">db.world.count({"continent":"Africa"})
</div>
</div>
</div>
</div>


<div class="q nonum" data-lang="py3">$sum
<div class="q nonum" data-lang="mongo">$sum
<div class="def">pp.pprint(list(
<div class="def">
    db.world.aggregate([
db.world.aggregate([
        {"$group":{
    {"$group":{
            "_id":"$continent",
        "_id":"$continent",
            "sum of populations":{"$sum":"$population"},
        "sum of populations":{$sum:"$population"},
            "count of countries":{"$sum":1}
        "count of countries":{$sum:1}
        }}
    }}
    ])
])
))
</div>
</div>
</div>
</div>


<div class="q nonum" data-lang="py3">Array.sum()
<div class="q nonum" data-lang="mongo">Array.sum()
<div class="def">temp = db.world.map_reduce(
<div class="def">
     map=Code("function(){emit(this.continent, this.population)}"),
db.world.mapReduce(
     reduce=Code("""function(k,v){
     function(){emit(this.continent, this.population)},
                      return Array.sum(v)
     function(k,v){ return Array.sum(v) },
                  }
     {out:{inline:1}}
                """),
     out={"inline":1}
)
)


Revision as of 15:33, 3 December 2015

Top Tips

There are many ways to do this in MongoDB.

count() is a cursor method that takes a query and returns a number equal to the amount of documents that matched the query.
$sum is an aggregation operator availible in the $group stage, that can be used to both sum values and count the number of documents.
mapReduce can produce a sum or a count during the results stage by using JavaScript.

.count()
db.world.count({"continent":"Africa"})
$sum

db.world.aggregate([ 

   {"$group":{
       "_id":"$continent",
       "sum of populations":{$sum:"$population"},
       "count of countries":{$sum:1}
   }}

])

Array.sum()

db.world.mapReduce( 

   function(){emit(this.continent, this.population)},
   function(k,v){ return Array.sum(v) },
   {out:{inline:1}}

)

pp.pprint(temp['results'])

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