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What is Eval Functions in Apache Pig ?
- Eval functions extend the Java class ‘org.apache.pig.EvalFunc and it is parameterized with the return type of the UDF which is a Java String which is a part of java class
- Eval Functions will take one record and returns one result, which will be invoked for every record that will pass through the execution pipeline.
- Eval Function takes the tuple which contains the fields for the script which is passed to the UDF (User Defined Function) as an input.
- Eval function is invoked on every input tuple and the input is done into the function which is a tuple with input parameters where they are passed to the function which is done in the Pig script.
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An example which is given below done to explain the Eval Functions
Example:
package myudfs;
import java.io.IOException;
import org.apache.pig.EvalFunc;
import org.apache.pig.data.Tuple;
public class UPPER extends EvalFunc<String>
{
public String exec(Tuple input) throws IOException {
if (input == null || input.size() == 0)
return null;
try{
String str = (String)input.get(0);
return str.toUpperCase();
}catch(Exception e){
throw new IOException("Caught exception processing input row ", e);
}
}
}
The table which is given below give us the eval functions and their description
| Function | Syntax | Description |
|---|---|---|
| AVG | AVG(expression | Computes the average of the numeric values in a single-column bag. |
| CONCAT | CONCAT (expression, expression) | Concatenates two expressions of identical type. |
| COUNT | COUNT(expression) | Computes the number of elements in a bag, it ignores null. |
| COUNT_STAR | COUNT_STAR(expression) | Computes the number of elements in a bag, it includes null. |
| DIFF | DIFF (expression, expression) | Compares two fields in a tuple, any tuples that are in one bag but not the other are returned in a bag. |
| IsEmpty | IsEmpty(expression) | Checks if a bag or map is empty |
| MAX | MAX(expression) | Computes the maximum of the numeric values or chararrays in a single-column bag |
| MIN | MIN(expression) | Computes the minimum of the numeric values or chararrays in a single-column bag. |
| SIZE | SIZE(expression) | Computes the number of elements based on any Pig data type. SIZE includes NULL values in the size computation |
| SUM | SUM(expression) | Computes the sum of the numeric values in a single-column bag. |
| TOKENIZE | TOKENIZE(expression [, ‘field_delimiter’]) | Splits a string and outputs a bag of words. |