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<CENTER><A HREF = "http://mapreduce.sandia.gov">MapReduce-MPI WWW Site</A> - <A HREF = "Manual.html">MapReduce-MPI Documentation</A>
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<H3>Writing a MapReduce program
</H3>
<P>The usual way to use the MR-MPI library is to write a small main
program that calls the library. In C++, your program includes two
library header files and uses the MapReduce namespace:
</P>
<PRE>#include "mapreduce.h"
#include "keyvalue.h"
using namespace MAPREDUCE_NS
</PRE>
<P>Follow these links for info on using the library from a <A HREF = "Interface_c.html">C
program</A> or from a <A HREF = "Interface_python.html">Python
program</A>.
</P>
<P>Arguments to the library's <A HREF = "map.html">map()</A> and <A HREF = "reduce.html">reduce()</A>
methods include function pointers to serial "mymap" and "myreduce"
functions in your code (named anything you wish), which will be
"called back to" from the library as it performs the parallel map and
reduce operations.
</P>
<P>A typical simple MapReduce program involves these steps:
</P>
<PRE>MapReduce *mr = new MapReduce(MPI_COMM_WORLD); // instantiate an MR object
mr->map(nfiles,&mymap); // parallel map
mr->collate() // collate keys
mr->reduce(&myreduce); // parallel reduce
delete mr; // delete the MR object
</PRE>
<P>The main program you write may be no more complicated than this. The
API for the MR-MPI library is a handful of methods which are
components of a MapReduce operation. They can be combined in more
complex sequences of calls than listed above. For example, one
<A HREF = "map.html">map()</A> may be followed by several <A HREF = "reduce.html">reduce()</A>
operations to massage your data in a desired way. Output of final
results is typically performed as part of a myreduce() function you
write which executes on one or more processors and writes to a file(s)
or the screen.
</P>
<P>The MR-MPI library operates on "keys" and "values" which are generated
and manipulated by your mymap() and myreduce() functions. A key and a
value are simply byte strings of arbitrary length which are logically
associated with each other, and can thus represent anything you wish.
For example, a key can be a text string or a particle or grid cell ID.
A value can be one or more numeric values or a text string or a
composite data structure that you create.
</P>
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