[R-pkg-devel] Inconsistent functionality of c++ code in MatchIt

[Bill Dunlap]

I see the problem when I compile the C++ code on Ubuntu 20.04 and the
latest R-devel with
   C++ compiler: ‘g++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
If I change all the unadorned 'abs' calls in src/nn_matchC_vec.cpp with the
prefix 'std::' the problem goes away.

-Bill


On Thu, May 11, 2023 at 11:12 AM Noah Greifer <noah.greifer using gmail.com>
wrote:

> Hello,
>
> I'm the mainter of the package *MatchIt*, which uses *Rcpp* to implement
> nearest neighbor matching. One way to customize nearest neighbor
> matching is to add a caliper, which is the largest distance two units can
> be from each other before they are not allowed to be matched. I've had some
> users complain recently that the caliper is not working for them, i.e.,
> even after specifying a caliper, units are being matched who are
> farther apart then the caliper width. I have been unable to replicate this
> problem on my Mac; the caliper always works as intended.
>
> One user noted that even when using the same package version, the
> performance varied across two machines: one obeyed the caliper and one
> didn't. I thought this might be related to the version of R installed, as
> for some users updating R fixed the issue, but for others it didn't. I'm
> kind of at a loss.
>
> I have a suspicion that the problem is related to the function abs() in the
> C++ functions find_right() and find_left() that I wrote to perform the
> matching, which are in nn_match_vec.cpp
> <https://github.com/kosukeimai/MatchIt/blob/master/src/nn_matchC_vec.cpp>.
> I have had problems with abs() before (seemingly related to a namespace
> conflict between std and Rcpp). In this case, abs() is used in the
> following way:
>
> if (abs(distance[ii] - distance[k]) > caliper_dist) {
>     //if closest is outside caliper, break; none can be found
>     break;
>  }
>
> Here, distance is a NumericVector, and ii and k are ints. My expectation is
> that this would dispatch to std::abs() with a double as its input. It's
> possible something is going wrong there. I'm wondering if this has to do
> with recent changes to R's C++ engine or compilers.
>
> If you want to run code to test whether the caliper is working correctly on
> your machine, you can run the following code:
>
> install.packages("MatchIt")
> data("lalonde", package = "MatchIt")
> m <- MatchIt::matchit(treat ~ age + educ + race + re74,
>                       data = lalonde, caliper = .01)
> summary(m)$nn
>
> If the caliper is working correctly, you should see a small matrix that has
> the row
>
> Matched            88      88
>
> If not, you would see the row
>
> Matched           185     185
>
> The GitHub issue of people complaining about this is here
> <https://github.com/kosukeimai/MatchIt/issues/163> along with their
> explanations about versions of *MatchIt* and R. The package code is also
> there.
>
> Any thoughts or insights about this would really help! Thank you so much!
>
> Noah
>
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