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Date: Wed, 21 Nov 2012 14:50:51 -0500
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Subject: fundamental doubt
From: jamal sasha <jamalshasha@gmail.com>
To: "user@hadoop.apache.org" <user@hadoop.apache.org>
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Hi..
I guess i am asking alot of fundamental questions but i thank you guys for
taking out time to explain my doubts.
So i am able to write map reduce jobs but here is my mydoubt
As of now i am writing mappers which emit key and a value
This key value is then captured at reducer end and then i process the key
and value there.
Let's say i want to calculate the average...
Key1 value1
Key2 value 2
Key 1 value 3

So the output is something like
Key1 average of value  1 and value 3
Key2 average 2 = value 2

Right now in reducer i have to create a dictionary with key as original
keys and value is a list.
Data = defaultdict(list) == // python usrr
But i thought that
Mapper takes in the key value pairs and outputs key: ( v1,v2....)and
Reducer takes in this key and list of values and returns
Key , new value..

So why is the input of reducer the simple output of mapper and not the list
of all the values to a particular key or did i  understood something.
Am i making any sense ??

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Hi..<br> I guess i am asking alot of fundamental questions but i thank you =
guys for taking out time to explain my doubts.<br>So i am able to write map=
 reduce jobs but here is my mydoubt<br>As of now i am writing mappers which=
 emit key and a value<br>
This key value is then captured at reducer end and then i process the key a=
nd value there.<br>Let&#39;s say i want to calculate the average...<br>Key1=
 value1<br>Key2 value 2<br>Key 1 value 3<br><br>So the output is something =
like<br>
Key1 average of value =A01 and value 3<br>Key2 average 2 =3D value 2<br><br=
>Right now in reducer i have to create a dictionary with key as original ke=
ys and value is a list.<br>Data =3D defaultdict(list) =3D=3D // python usrr=
<br>But i thought that <br>
Mapper takes in the key value pairs and outputs key: ( v1,v2....)and <br>Re=
ducer takes in this key and list of values and returns <br>Key , new value.=
.<br><br>So why is the input of reducer the simple output of mapper and not=
 the list of all the values to a particular key or did i =A0understood some=
thing.<br>
Am i making any sense ??

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