[Rcpp-devel] Rcpp: Modification of input argument: Undefined behaviour

[William Dunlap]

> The `mode()` function was pretty useless for determining a matrix
object's type

Use the storage.mode() function.

Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Wed, Nov 26, 2014 at 9:06 AM, David Shih <david.shih at mail.utoronto.ca>
wrote:

>  Hi Yixuan,
>
>
>  You're right!
>
>
>  Whether the input matrix can be modified by a function that takes
> NumericMatrix is determined by whether the input matrix is a NumericMatrix
> or an IntegerMatrix (the latter needs to be copied and converted). The
> `mode()` function was pretty useless for determining a matrix
> object's type, but as at least `is.integer()` is predictive.
>
>
>  When I wrote another function that takes an IntegerMatrix, it was able
> to modify an input IntegerMatrix in-place.
>
>
>  Thank you,
>
> David
>
>
>  ------------------------------
> *From:* Yixuan Qiu <yixuan.qiu at cos.name>
> *Sent:* November 26, 2014 2:13 AM
> *To:* David Shih
> *Cc:* rcpp-devel at lists.r-forge.r-project.org
> *Subject:* Re: [Rcpp-devel] Rcpp: Modification of input argument:
> Undefined behaviour
>
>
> Hello David,
> The general answer to your question is, if the type of your matrix
> (integer or numeric) in R is different from the one you declare in Rcpp,
> Rcpp will make a copy and cast it to the appropriate type.
> For example, 1:12 is of type integer, and if you pass it as a
> NumericMatrix, Rcpp will implicitly copy the whole matrix, so no matter
> what modification you did, the original matrix will not change.
>
> Best,
> Yixuan
> On Nov 26, 2014 1:33 AM, "David Shih" <david.shih at mail.utoronto.ca> wrote:
>
>> Hello,
>>
>> There was an earlier post on this subject, but based on my
>> experimentation, the behaviour of modifying input argument is different
>> depending on how the matrix was initialization and other factors...
>>
>> I wrote a Rcpp function to modify an input matrix. After calling this
>> function, the input matrix is modified under some circumstances and not
>> modified under others. The behaviour is the same on repeat runs and on both
>> Linux (3.16.3-1-ARCH) and Mac (OSX 10.9).
>>
>> The R script, C++ code, and the results are available on Bitbucket:
>>
>> https://bitbucket.org/dshih/rcpp_inplace
>>
>> I don't quite understand when the input matrix is modified in place by
>> the Rcpp function and when the input matrix is be copied on write in the
>> Rcpp function.
>>
>> When I stay within Rcpp/C++, a input argument can be modified in-place by
>> a function. (This feature was critical to my optimization.)
>>
>> After the code returns to R, I would expect either copy-on-write or
>> in-place modification but not both.
>>
>> What is the correct behaviour?
>>
>>
>> Thank you,
>>
>> David J. H. Shih
>>
>> The Hospital for Sick Children
>> Peter Gilgan Centre for Research and Learning
>> 686 Bay St
>> 17th floor, Room 17.9707
>> Toronto, ON  M5G 0A4
>> Canada
>> Tel:  (416) 813-7654 x309157
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>>
>
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