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What is Foreach Operator in Apache Pig ?

  • FOREACH gives us a simple way to apply transformations which is done based on columns.
  • The FOREACH operator of Apache pig is used to create unique function as per the column data which is available.
  • FOREACH operator evaluates an expression for each possible combination of values of some iterator variables, and returns all the results
  • FOREACH operator generates data transformations which is done based on columns of data.
  • The FOREACH operator is used to generate specified data transformations which is done based on the column data.

Pig Operations - Projection

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  • FOREACH instruction:
    • Generate new relations by projecting data of a relation
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  • FOREACH instruction:
    • Let us execute the instruction and… it seems that nothing happens!
    • We had some tracing output with LOAD, DUMP, and ILLUSTRATE…
  • Nested FOREACH:
    • Operate on data in bags inside a relation and then project
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    Syntax:

    grunt> Relation_name2 = FOREACH Relatin_name1 GENERATE (required data);
    

    Example:

    wikitechy_student_details.txt

    001,Suresh,Reddy,21,9848022337,Hyderabad
    002,Arvin,Battacharya,22,9848022338,Kolkata
    003,AanchalKhanna,22,9848022339,Delhi 
    004,Preethi,Agarwal,21,9848022330,Pune 
    005,Vanitha,Mohanthy,23,9848022336,Bhuwaneshwar 
    006,Sruti,Mishra,23,9848022335,Chennai 
    007,Kamal,Nayak,24,9848022334,trivendram 
    008,Barath,Nambiayar,24,9848022333,Chennai
    • And we have loaded this file into Pig with the relation name wikitechy_student_details which is given below:
    grunt> student_details = LOAD 'hdfs://localhost:9000/pig_data/wikitechy_student_details.txt' USING PigStorage(',') as shown below.
       as (id:int, firstname:chararray, lastname:chararray,age:int, phone:chararray, city:chararray);
    
    • We need to get the id, age, and city values of each student from the relation wikitechy_student_details and store it into another relation named foreach_data which is given below:
    grunt> foreach_data = FOREACH wikitechy_student_details GENERATE id,age,city;
    

    Verification:

    grunt> Dump foreach_data;
    

    Output:

    (1,21,Hyderabad)
    (2,22,Kolkata)
    (3,22,Delhi)
    (4,21,Pune) 
    (5,23,Bhuwaneshwar)
    (6,23,Chennai) 
    (7,24,trivendram)
    (8,24,Chennai)
    

    FOREACH ... GENERATE

  • field referenced by name or by position (starting from 0)
  • all fields (*) or a range of field can be referenced (..)
  • BinCond (the ternary operator) is supported
  • a custom UDF can be invoked
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