R devel - May 2018 - Understanding the sequence of events when calling the R dpois function

Hello all,

I am trying to get a better understanding of the underlying code for the
stats::dpois function in R and, specifically, what happens under the hood when
it is called. I finally managed to track down the C course at:
https://github.com/wch/r-source/blob/trunk/src/nmath/dpois.c. It would seem that
the dpois C function is taking a double for each of the x and lambda arguments
so I am a bit confused why I can provide a vector and matrix to dpois in R, e.g.

dpois(1:5, matrix(runif(2*5, 1, 10), nrow = 5)))

and get a matrix back with the results. Due to this, I was expecting to see a
loop, or similar, in the underlying C source but? to no avail. So I have to
conclude that either a) there is a step between when I call dpois in R and the C
code getting executed that loops over the inputs or b) that there is a construct
in the C code (my proficiency here is limited) that is called per input. Any
help in enlightening me on what code is responsible for iterating over the
multiple inputs (or if someone is feeling energetic, the exact stepwise code
that is executed when calling dpois) would be greatly appreciated!!

Kind Regards,
Jason Serviss


	[[alternative HTML version deleted]]

> On May 31, 2018, at 8:25 AM, Jason Serviss <jason.serviss at ki.se>
wrote:
> 
> Hello all,
> 
> I am trying to get a better understanding of the underlying code for the
stats::dpois function in R and, specifically, what happens under the hood when
it is called. I finally managed to track down the C course at:
https://github.com/wch/r-source/blob/trunk/src/nmath/dpois.c. It would seem that
the dpois C function is taking a double for each of the x and lambda arguments
so I am a bit confused why I can provide a vector and matrix to dpois in R, e.g.
> 
> dpois(1:5, matrix(runif(2*5, 1, 10), nrow = 5)))
> 
> and get a matrix back with the results. Due to this, I was expecting to see
a loop, or similar, in the underlying C source but? to no avail. So I have to
conclude that either a) there is a step between when I call dpois in R and the C
code getting executed that loops over the inputs or b) that there is a construct
in the C code (my proficiency here is limited) that is called per input. Any
help in enlightening me on what code is responsible for iterating over the
multiple inputs (or if someone is feeling energetic, the exact stepwise code
that is executed when calling dpois) would be greatly appreciated!!
Have a look at arithmetic.c. The math2 function handles calling dpois in a
vectorized fashion.

You need to trace how it is that the compiler knows to use dpois.c in
constructing `C_dpois' which is what the R function dpois is calling.

If you know how to grep the sources for dpois, that will get you started. You
will need to look over a bunch of #define's and a few C utilities to see how
it all fits together.

If you dig in and find the `do { ... } while(0)' construct confusing you
might look at

https://stackoverflow.com/questions/257418/do-while-0-what-is-it-good-for

HTH,

Chuck

Jason,

as Chuck Berry (to whom, *thanks* for 'do {...} while(0)'!) suggested,
using grep, or even grep executed from find, such as
----
find . -type f -exec grep -H "dpois" \{\} \; | less
----
(executed from the root of an R source tree), is your friend.

cheers, Greg

Chuck and Greg,

Thanks a lot for your help! I have a much better understanding now of what is
happening ?under the hood?.

Kind Regards,
Jason

> On 31 May 2018, at 20:08, Greg Minshall <minshall at acm.org> wrote:
> 
> Jason,
> 
> as Chuck Berry (to whom, *thanks* for 'do {...} while(0)'!)
suggested,
> using grep, or even grep executed from find, such as
> ----
> find . -type f -exec grep -H "dpois" \{\} \; | less
> ----
> (executed from the root of an R source tree), is your friend.
> 
> cheers, Greg