Difference between revisions of "MAPREDUCE basics"
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<div class=q data-lang="py3"> | <div class=q data-lang="py3"> | ||
| − | <p class=strong> | + | <p class=strong>Return country name or capital city that starts with a letter 'M'</p> |
<pre class=def> | <pre class=def> | ||
</pre> | </pre> | ||
| Line 142: | Line 142: | ||
from bson.code import Code | from bson.code import Code | ||
temp = db.world.map_reduce( | temp = db.world.map_reduce( | ||
| − | map=Code("""function(){ if((this.name).startsWith(' | + | map=Code("""function(){ |
| + | if((this.name).startsWith('M')) | ||
| + | emit(this.name,null); | ||
| + | if((this.capital).startsWith('M')) | ||
| + | emit(this.capital,null); | ||
| + | }"""), | ||
reduce=Code("""function(key, values){ | reduce=Code("""function(key, values){ | ||
return values; | return values; | ||
}"""), | }"""), | ||
| + | |||
out={"inline":1}) | out={"inline":1}) | ||
pp.pprint(temp["results"]) | pp.pprint(temp["results"]) | ||
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</div> | </div> | ||
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<div class=q data-lang="py3"> | <div class=q data-lang="py3"> | ||
Revision as of 14:05, 30 July 2015
#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, width=160)
MapReduce the basics
This tutorial introduces the MapReduce command.
MapReduce examples are available.
Find the total population of the each continent
from bson.code import Code
temp = db.world.map_reduce(
map=Code("function(){emit(this.continent, this.population)}"),
reduce=Code("""function(key, values){
return Array.sum(values);
}"""),
out={"inline":1})
pp.pprint(temp["results"])
from bson.code import Code temp = db.world.map_reduce(
map=Code("function(){emit(this.continent, this.population)}"),
reduce=Code("""function(key, values){
return Array.sum(values);
}"""),
out={"inline":1})
pp.pprint(temp["results"])
Use the previous answer to find the population of the world to the nearest million
from bson.code import Code temp = db.world.map_reduce(
map=Code("function(){emit('World Population in Millions', this.population)}"),
reduce=Code("""function(key, values){
return Math.round(Array.sum(values)/1000000)*1000000;
}"""),
out={"inline":1})
pp.pprint(temp["results"])
Count number of countries by first letter
from bson.code import Code temp = db.world.map_reduce(
map=Code("""function(){ emit((this.name).substring(0,1), 1)}"""),
reduce=Code("""function(key, values){
return Array.sum(values);
}"""),
out={"inline":1})
pp.pprint(temp["results"])
Show the number of countries on each continent
from bson.code import Code temp = db.world.map_reduce(
map=Code("function(){emit(this.continent, 1)}"),
reduce=Code("""function(key, values){
return Array.sum(values);
}"""),
out={"inline":1})
pp.pprint(temp["results"])
Show the smallest 3 countries name and area (ignore areas of 0 or None)
from bson.code import Code temp = db.world.map_reduce(
query={"$and":[{"area":{"$ne":None}}, {"area":{"$ne":0}}]},
sort={"area":1},
limit=3,
map=Code("function(){emit(this.name, this.area)}"),
reduce=Code("function(key, values){}"),
out={"inline":1},
)
pp.pprint(
temp["results"]
)
Return the first and last country based on name order for each continent
from bson.code import Code temp = db.world.map_reduce(
map=Code("function(){emit(this.continent, {first:this.name,last:this.name})}"),
reduce=Code("""function(key, values){
var ret = {first:'ZZZ',last:'AAA'};
for(var i=0;i<values.length;i++){
if (ret.first>values[i].first) ret.first=values[i].first;
if (ret.last<values[i].last) ret.last=values[i].last;
}
return ret;
}"""),
out={"inline":1})
pp.pprint(temp["results"])
Return country name or capital city that starts with a letter 'M'
from bson.code import Code temp = db.world.map_reduce(
map=Code("""function(){
if((this.name).startsWith('M'))
emit(this.name,null);
if((this.capital).startsWith('M'))
emit(this.capital,null);
}"""),
reduce=Code("""function(key, values){
return values;
}"""),
out={"inline":1})
pp.pprint(temp["results"])
Show the first and last city for each letter and the count of cities
from bson.code import Code temp = db.world.map_reduce(
map=Code("""function(){ if(this.capital)emit((this.capital).substring(0,1), {first:this.capital, last:this.capital})}"""),
reduce=Code("""function(key, values){
var ret = {first:'ZZZ',last:'AAA', count:0};
for(var i=0;i<values.length;i++){
if (ret.first>values[i].first) ret.first=values[i].first;
if (ret.last<values[i].last) ret.last=values[i].last;
ret.count += 1;
}
return ret;
}"""),
finalize=Code("""function(key, val){
if(!val.count){
val.count = 1;
return val;
}else
return val;
}"""),
out={"inline":1})
pp.pprint(temp["results"])