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From jaylac <Jayalakshmi.Munias...@cognizant.com>
Subject MapReduce
Date Fri, 02 Mar 2007 04:24:44 GMT


I was just going thro abt MapReduce for my final year project work.....

I got confused in the middle.... What i thought is "MapReduce deals greatly
with key/value pairs only... For fitting a problem into mapreduce we should
find the key/value pairs"

I want to know whether im right or wrong....

I got confused after looking at the explanation in wikipedia... The
following is the content in wikipedia abt mapreduce...

"A map function iterates over a list of independent elements and performs a
specified operation on each element. The list of answers is stored
independently from the original list. Because each element is operated on
independently and the original list is not being modified, it is very easy
to perform a map operation in parallel. On appropriate hardware this allows
extremely large data sets to be processed in short amounts of elapsed time.

For example consider a list of test scores where each score has been found
to be 1 too high. A map function of s − 1 could be applied to correct every
score s.

A reduce operation takes a list and combines elements according to some
algorithm. Since a reduce always ends up with a single answer, it is not as
parallelizable as a map function, but the large number of relatively
independent calculations means that reduce functions are still useful in
highly parallel environments.

Continuing the previous example, what if one wanted to know the average of
the test scores? One could define a reduce function which halved the size of
the list by adding an entry in the list to its neighbor, recursively
continuing until there is only one (large) entry, and dividing the total sum
by the original number of elements to get the average."


Here in map function we are simply adding up the test scores.... we are not
using any key/value pair..... Im totally confused....

I might be wrong at any point... please someone help me out..... Am i wrong
in the basic understanding of MapReduce itself..... Ill be thankful if
anyone explains me clearly...

please help me out to successfully complete my final year project....


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