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== Writing your own Pig functions ==

Pig has a number of built-in functions for loading, filtering, aggregating, 
etc. (A complete list is available at PigBuiltins.) However, if you want to do 
something specialized you may need to write your own function. This page will 
walk you through how to do this.

=== Types of functions ===
The most important type and commonly used type of functions are EvalFunction. 
Eval functions consume a tuple, do some computation, and produce some data 

Eval functions are very flexible, e.g. they can mimic "map" and "reduce" style 
      * ''"Map" behavior:'' The output type of an Eval Function is one of: a 
single value, a tuple, or a bag of tuples (a Map/Reduce "map" function produces 
a bag of tuples).
      * ''"Reduce" behavior:'' Recall that in the Pig data model, a tuple may 
contain fields of type ''bag''. Hence an Eval Function may perform aggregation 
or "reducing" by iterating over a bag of tuples nested within the input tuple. 
This is how the built-in aggregation function SUM(...) works, for example.   
The other types of functions are:
   * '''Filter Function:''' evalutes to True or False when given a tuple; used 
to eliminate unwanted tuples from a relation or bag
   * '''Group Function:''' assigns tuples to group(s) 
   * '''Load Function:''' controls reading of tuples from files
   * '''Store Function:''' controls storing of tuples to files

==== Example ====
The following example uses each of the five types of functions. It computes the 
set of unique IP addresses associated with "good" products drawn from a list of 
products found on the web.

register myFunctions.jar
products = LOAD '/productlist.txt' USING MyListStorage() AS (name, price, 
description, url);
goodProducts = FILTER products BY (price <= '19.99' AND MyFilter(description));
hostnames = FOREACH goodProducts GENERATE MyHostExtractor(url) AS hostname;
uniqueIPs = FOREACH (GROUP hostnames BY MyIPLookup(hostname)) GENERATE group AS 
STORE uniqueIPs INTO '/iplist.txt' USING MyListStorage();

In the above example, !MyListStorage() serves as a load function as well as a 
store function; !MyFilter() is a filter function; !MyHostExtractor() is an eval 
function; !MyIPLookup() is a group function. `myFunctions.jar` is a jar file 
that contains the classes for the user-defined functions.

=== How to write functions ===

Ready to write your own handy-dandy pig function? Before you start, you will 
need to know about the APIs for interacting with the data types (atom, tuple, 
bag). Click here: PigDataTypeAPIs.

Note: for Pig users with little or no experience in Java, here's a quick link 
to help you along: PigJavaForDummies.

Click below to learn how to build your own:
   * EvalFunction
   * FilterFunction
   * GroupFunction
   * StorageFunction (These are the most difficult to write, and usually, the 
inbuilt ones should be enough)

=== Ok, I have written my function, how to use it? ===

You can use your functions following the steps below:

   * Put all the compiled files used by your function together into a jar file
   * Tell Pig about that jar by the `register &lt;udfJar&gt;` command before 
using the function. (If you are using PigLatin in embedded mode, call 
   * Then use your function, as you would use a builtin! Its that simple.


1. Create your function `/src/myfunc/`

package myfunc;

import java.util.StringTokenizer;

public class MyEvalFunc extends EvalFunc<DataBag>
        public void exec(Tuple input, DataBag output) throws IOException
                String str = input.getAtomField(0).strval();
                StringTokenizer tok = new StringTokenizer(str, " \",()*", 
                while (tok.hasMoreTokens())
                        output.add(new Tuple(tok.nextToken()));

2. Compile your function. Make sure to point java compiler to pig jar file.

/src/myfunc $ javac -classpath /src/pig.jar

3. Create jar file 

/src/myfunc $ cd ..
/src $  jar cf myfunc.jar myfunc

4. Use the function through grunt (similar use from script). Note that there is 
no quotes around path in the `register` call.

/src $ java -jar pig.jar -
grunt> register /src/myfunc.jar
grunt> A = load 'students' using PigStorage('\t');
grunt> B = foreach A generate myfunc.MyEvalFunc($0);
grunt> dump B;
({(joe smith)})
({(john adams)})
({(anne white)})

See PigTutorial to see example of embeding Pig and your functions in Java. Use 
the same procedure outlined above to create your function jar file.

==== Advanced Features: ====
   * If you would like your function class to be instantiated with a 
non-default constructor, you can use the `define <alias> <funcSpec>` command. 
(If you are using PigLatin in embedded mode, call 
   * E.g., if I want my class `MyFunc` to be instantiated wih the string 
`'foo'`, I can write `define myFuncAlias myFunc('foo')`. I can then use 
`myFuncAlias` as a normal user-defined function.
   * Note that only string arguments to constructors are supported.

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