[R] Get a copy of a matrix only for TRUE entries of a matching size boolean matrix?

[Martin Maechler]

>>>>> DynV Montrealer 
>>>>>     on Fri, 3 May 2024 10:39:10 -0400 writes:

    > It's exactly what I was looking for, thanks.  I'm replying
    > to the whole list so others can skip this question, to not
    > waste time on it.

good.

    > Bonne fin de journée de Montréal (nous sommes le matin ici)

Note that the answer  Ben Bolker gave you (using "indexing")
is  much more efficient than the ifelse() answer below.

ifelse() may be convenient .. but is almost always suboptimal,
where as "indexing" is typically very often more efficient,
although often requiring a bit more thought than ifelse()
[so, if overall efficience is almost entirely your own human
 time, then using ifelse() may still be a good idea... ]

Martin

    > On Fri, May 3, 2024 at 10:30 AM Marc Girondot <marc_grt using yahoo.fr> wrote:

    >> Is it what you want ?
    >> mat_letters <- matrix(data=c('A', 'B', 'C', 'D'), ncol=2, byrow=TRUE)
    >> mat_bools <- matrix(data=c(FALSE, TRUE, TRUE, FALSE), ncol=2, byrow=TRUE)
    >> 
    >> ifelse(mat_bools, mat_letters, "")
    >> ifelse(mat_bools, mat_letters, NA)
    >> 
    >> > ifelse(mat_bools, mat_letters, "")
    >> [,1] [,2]
    >> [1,] ""   "B"
    >> [2,] "C"  ""
    >> > ifelse(mat_bools, mat_letters, NA)
    >> [,1] [,2]
    >> [1,] NA   "B"
    >> [2,] "C"  NA
    >> 
    >> Marc
    >> 
    >> 
    >> 
    >> Le 03/05/2024 à 14:47, DynV Montrealer a écrit :
    >> 
    >> Is there a way to get a copy of a matrix only for TRUE entries of a
    >> matching size boolean matrix? For **example**:
    >> 
    >> mat_letters <- matrix(data=c('A', 'B', 'C', 'D'), ncol=2, byrow=TRUE)
    >> mat_letters
    >> 
    >> [,1] [,2]
    >> [1,] "A"  "B"
    >> [2,] "C"  "D"
    >> 
    >> mat_bools <- matrix(data=c(FALSE, TRUE, TRUE, FALSE), ncol=2, byrow=TRUE)
    >> mat_bools
    >> 
    >> [,1]  [,2]
    >> [1,] FALSE  TRUE
    >> [2,]  TRUE FALSE
    >> *Reminder:* The following is only an example ; the solution might look very
    >> different.
    >> some_command(mat_letters, mat_bools, false=empty)
    >> [,1] [,2]
    >> [1,] ""  "B"
    >> [2,] "C"  ""
    >> some_command(mat_letters, mat_bools, false=na)
    >> [,1] [,2]
    >> [1,] NA  "B"
    >> [2,] "C"  NA
    >> 
    >> 
    >> 

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