[Rcpp-devel] Efficient DataFrame access by row & column

[Romain Francois]

Le 20/02/13 00:09, Yan Zhou a écrit :
> The most inefficient part I see is the creation of a new NumericVector inside the inner most loop. You copied each column n times, of which n-1 times are unnecessary.

No. This does not copy data. This uses time to protect it, etc ... but 
the data in the vector is not copied.

> Yan Zhou
>
> On Feb 19, 2013, at 11:02 PM, Dirk Eddelbuettel <edd at debian.org> wrote:
>
>>
>> Ken,
>>
>> On 19 February 2013 at 22:35, Ken Williams wrote:
>> | I have a need to loop through all the entries of a DataFrame by row, then
>> | column.  I know two different ways:
>>
>> There have been prior discussions of this topic, as well as example posts --
>> even leading to a Rcpp Gallery article. Did you read any of these?  It wasn't
>> clear from your post.
>>
>> | I?m also curious why it?s a syntax error in Case A to just write `df[j][i]` or
>>
>> Eeeek.  I prefer the more C++-y way of writing df(j,i).  Square brackets only
>> work for vectors, and even then you may be better off with x(i) for
>> consistency.
>>
>> Overall, your premise may be wrong too.  "We all know" that a data.frame is
>> not the fastest data structure in R, so by forcing ourselves to the same
>> access are we not handycapping ourselves.
>>
>> Once you are in C++, you can use whatever C++ datatype you like.  A
>> data.frame really is just a list of vectors, each of the vectors has eg a
>> begin(0 iterator which you can (fairly costlessly) instantiate STL types.
>>
>> And those give you performance guarantees.
>>
>> Hope this helps,  Dirk
>>
>> --
>> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com
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