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From Gary Gregory <garydgreg...@gmail.com>
Subject Re: [CSV] Headers and the first record
Date Wed, 31 Jul 2013 12:58:57 GMT
On Jul 31, 2013, at 3:38, Benedikt Ritter <britter@apache.org> wrote:

> 2013/7/31 Gary Gregory <garydgregory@gmail.com>
>
>> On Tue, Jul 30, 2013 at 5:29 PM, Emmanuel Bourg <ebourg@apache.org> wrote:
>>
>>> Le 30/07/2013 23:26, Gary Gregory a écrit :
>>>> And another thing: internally, the header should be a Set<String>,
not
>> a
>>>> String[]. I plan on fixing that later too.
>>>
>>> Why should it be a set? Is there an impact on the performance?
>>
>> Well, I did not finish my though on that one, sorry about that, please
>> allow me to walk through my use cases. The issue is about the feature, not
>> performance.
>>
>> At first glance, using a set avoids an inherent problem with any non-set
>> data structure: defining duplicates. What does the following mean?
>>
>> withHeader("A", "B", "C", "A");
>>
>> It's is a recipe for garbage results: record.get("A") returns what?
>>
>> Today, I added some CSVFormat validation code that checks for duplicate
>> column names. If you build a format with withHeader("A", "B", "C", "A");
>> you will get an ISE when validate() is called.
>>
>> If we had withHeader(Set) and document it as the 'main' way to specify
>> column names, then we can say that withHeader(String...) is just a
>> syntactical convenience and turn the String[] into a Set. But that will not
>> work.
>>
>> The problem with a Java Set is that it is not ordered and the current
>> implementation relies on order of the String[]. But why? What the current
>> implementation says is: ignore what the header line of the file is and use
>> the given column names at the given positions. A perfectly good user story.
>> So for withHeader("A", "B", "C"), "A" is column 0, "B" is column 1, and so
>> on. Ok, that's one usage.
>>
>> Taking a step back, I want to talk about why should the column name order
>> matter when you are calling withHeader(). I would like to be able to tell
>> the parser that I want to use a Set of column names and have it figure out,
>> based on the header line, the columns indices. This is quite different than
>> what we have now.
>>
>> A use case I have now is a CSV file with a lot of columns (~90) but I only
>> care about a small subset of the columns (~10). I'd like to be able to say
>> withHeader(Set) where the Set may be a subset of the actual column names in
>> the header line. This is different from withHeader(String[]) because the
>> names in the Set must match the names in the header record.
>
> I'm not sure if we should try to build in all this different cases
> (guessing headers, using the first record as headers, only use a subset of
> the available headers) into one implementation.
>
> What you are talking about sounds more like a view or a projection of the
> actual content being parsed.
> Do we really need this for 1.0 or can it be postponed?

This is a real scenario and a real need, not some imaginary complication ;)

Even if it is not implemented for 1.0, we should talk about how it
should be done such that it fits in and does not cause API problems
later. And if I can get it done by then, then that much the better.

Gary

>
>
>>
>> So I think it boils down to ignoring my comment about using a Set
>> internally and adding a feature where I can tell the parser that I want to
>> use a set of column names and not worry about the order, because the parser
>> will match up the column names when it reads the header line.
>>
>> Gary
>>
>>
>>>
>>>
>>> Emmanuel Bourg
>>>
>>>
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>>
>>
>> --
>> E-Mail: garydgregory@gmail.com | ggregory@apache.org
>> Java Persistence with Hibernate, Second Edition<
>> http://www.manning.com/bauer3/>
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>
>
>
> --
> http://people.apache.org/~britter/
> http://www.systemoutprintln.de/
> http://twitter.com/BenediktRitter
> http://github.com/britter

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