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From "Kiyan Ahmadizadeh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-57) Add a length function to PCollection
Date Thu, 06 Sep 2012 20:10:07 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-57?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13450003#comment-13450003
] 

Kiyan Ahmadizadeh commented on CRUNCH-57:
-----------------------------------------

On a similar note, is it expected that for a PCollection<T>, T is comparable?  Aggregate.max
and Aggregate.min contain implementations that check that the type class for the PCollection
is Comparable.  Given the MapReduce framework's reliance on sorting, however, it seems like
T should always be Comparable.  

If it's not expected that T be Comparable, I would argue that max, min, and sort functionality
should be in a PCollection decorator that contains operations for PCollections that contain
comparable elements.  
                
> Add a length function to PCollection
> ------------------------------------
>
>                 Key: CRUNCH-57
>                 URL: https://issues.apache.org/jira/browse/CRUNCH-57
>             Project: Crunch
>          Issue Type: New Feature
>          Components: Core
>    Affects Versions: 0.3.0
>            Reporter: Kiyan Ahmadizadeh
>            Assignee: Josh Wills
>         Attachments: CRUNCH-57.patch
>
>
> Sometimes it's useful and interesting to compute the number of elements in a PCollection.
>  
> For example, suppose there was an initial PCollection that was then filtered into another.
 If I'm interested in how many elements of the original PCollection matched the filter, I'll
have to write extra code to compute this.
> PCollections should have a length method that, when called, computes the number of elements
in the PCollection and returns the result. 

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