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From Apache Wiki <wikidi...@apache.org>
Subject [Lucene-hadoop Wiki] Update of "Grep" by HairongKuang
Date Wed, 28 Jun 2006 01:23:05 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Lucene-hadoop Wiki" for change notification.

The following page has been changed by HairongKuang:

New page:
= Grep Example =
'''Grep''' example extracts matching strings from text files and count how many time they

The program runs two map/reduce jobs in sequence. The first job counts how many times a matching
string occured and the second job sorts matching strings by their frequency and stores the
output in a single output file.
Each mapper of the first job takes a line as input and matches the user-provided regular expression
against the line. It extracts all matching strings and emits (matching string, 1) pairs. Each
reducer sums the frequencies of each matching string. The output is sequence files contaning
the matching string and count. The reduce phase is optimized by running a combiner that sums
the frequency of strings from local map output. As a result it reduces the amount of data
that needs to be shipped to a reduce task.

The second job takes the output of the first job as input. The mapper is an inverse map, while
the reducer is an indentity reducer. The number of reducers is one, so the output is stored
in one file, and it is sorted by the count in a descending order. The output file is text,
each line of which contains count and a matching string.

The example also demonstrates how to pass a command-line parameter to a mapper or a reducer.
This is done by adding (key, value) pairs to the job's configuration before the job is submitted.
Map or reduce tasks are able to access the value by getting it from the job's configuration
in the method ''configure''.

To run the example, type the following command:[[BR]]
bin/hadoop org.apache.hadoop.examples.Grep <indir> <outdir> <regex> [<group>]


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