[Rd] combn implementation

[Giampiero Salvi]

Thank you!
It's clear now. I was not looking at the real source code, but at the
code you get from the print method (that is by simply typing combn
at the prompt), which has all the comments removed. Next time I'll
take my time looking for the real source code ;)

Thanks again,
Giampiero


On Fri, 19 Jan 2007, Martin Maechler wrote:

> >>>>> "Giampiero" == Giampiero Salvi <giampi at kth.se>
> >>>>>     on Fri, 19 Jan 2007 14:21:24 +0100 (CET) writes:
>
>     Giampiero> Hi,
>     Giampiero> I was checking the source code to the function combn that "generates
>     Giampiero> all combinations of the elements of 'x' taken 'm' at a time.",
>     Giampiero> because I wished to modify it. I have a doubt about a statement.
>     Giampiero> This is the main loop.
>
>   >     ._1 <- 1:1
>   >     nmmp1 <- n - m + ._1
>   >     while (a[1] != nmmp1) {
>   >         if (e < n - h) {
>   >             h <- ._1
>   >             e <- a[m]
>   >             j <- ._1
>   >         }
>   >         else {
>   >             e <- a[m - h]
>   >             h <- h + ._1
>   >             j <- 1:h
>   >         }
>   >         a[m - h + j] <- e + j
>   >         r <- if (nofun)
>   >             x[a]
>   >         else FUN(x[a], ...)
>   >         if (simplify)
>   >             out[, i] <- r
>   >         else out[[i]] <- r
>   >         i <- i + 1
>   >     }
>
> Not really!
> You are not showing all the comments that go with it!
>  --> https://svn.R-project.org/R/trunk/src/library/utils/R/combn.R
> is the current source file of the development version whereas
>      https://svn.r-project.org/R/tags/R-2-4-1/src/library/utils/R/combn.R
> is the R-2.4.1 version
>
>     Giampiero> I wonder what is the meaning of the statement "._1 <- 1:1".
>
> The "real" source in 2.4.1 says
>    ._1 <- 1:1 # to keep integer arithmetic
> 	      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> So: It assigns  "1:1" {the infamous short form for as.integer(1)}
>     to the variable '._1'
>
> The variable name is a bit unusual and I had a little chuckle when I
> invented it to put there.
>
> However with '1L' in R-devel, i.e. from R-2.5.0 on,
> code like the above will be definitely be simplified to use '1L'
> in such places, and then assigning it does not make sense
> anymore (of course it's questionable if it ever did make sense,
> but then, as I said, a had a chuckle and I think I found back
> then that it *did* increase speed slightly) ....
>
>
>     Giampiero> First question: what is the difference with "._1 <- 1"?
> explained in the comment
>
>     Giampiero> Second question (perhaps answerd by the answer to the first):
>
> indeed it is.
>
>     Giampiero> why not using "1" directly? E.g.:
>
>     Giampiero> [...]
>     Giampiero> h <- 1
>     Giampiero> e <- a[m]
>     Giampiero> e <- 1
>     Giampiero> [...]
>
>     Giampiero> Sorry if this is covered by the manuals, but it's not easy to search
>     Giampiero> for the string "._1" with the usual search engines.
>     Giampiero> Thank you!
>     Giampiero> Giampiero
>
> You're welcome.
>
> Martin Maechler, ETH Zurich
>