[R] Confusing fori or ifelse result in matrix manipulation

[Eric Berger]

M[,x] <- 0 *does not do the same* !! even if x only contains 0's and 1's

To understand this, note that
M[,c(2,3)]
gives columns 2 and 3
M[,c(0,3)]
gives the 3rd column and
M[,c(0,1,0)]
gives the *first* column!!



On Mon, Apr 25, 2022 at 5:41 PM Ivan Calandra <ivan.calandra using rgzm.de> wrote:

> Indeed, M[, x] <- 0 does the same, but only if x is 0's and 1's only,
> right? I thought that it might not always be the case so I choose this
> maybe superflous approach.
>
> M[, 2] does the same of course in the example, but I was assuming that
> the columns to change to zero are not known in advance and are based on
> data contained in another vector. Where that vector comes from is
> important too because the whole thing might be unnecessary. Is that
> maybe what you were hinting at, Bert?
>
> Maybe Uwe can tell us more about what/why he wants to do!
>
> Ivan
>
> --
> Dr. Ivan Calandra
> Imaging lab
> RGZM - MONREPOS Archaeological Research Centre
> Schloss Monrepos
> 56567 Neuwied, Germany
> +49 (0) 2631 9772-243
> https://www.researchgate.net/profile/Ivan_Calandra
>
> On 25/04/2022 16:30, Bert Gunter wrote:
> > x == 1 is the same as M[, x] so your expression is the same as
> > M[, c(FALSE, TRUE, FALSE)] <- 0
> > which is the same as M[, 2]  <- 0
> >
> > So what is the point of all this, exactly?
> >
> > Bert
> >
> > On Mon, Apr 25, 2022 at 7:18 AM Ivan Calandra <ivan.calandra using rgzm.de>
> wrote:
> >> Hi Uwe,
> >>
> >> If I understood the problem completely and building up on Tim's answer,
> >> this is even easier:
> >> M <- A <- matrix(1:9, ncol = 3)
> >> x <- c(0, 1, 0)
> >> M[, x == 1] <- 0
> >> M
> >>
> >> The original issue was with the way ifelse works. The explanation is in
> >> the help page: "ifelse returns a value with the same shape as test||".
> >> So, because x[i] == 0 returns a single value (TRUE or FALSE), ifelse
> >> will also return a single value (either A[, i][1] or 0) and not a vector
> >> of length 3 as you wanted. This single value is recycled to fill M[, i],
> >> hence the result.
> >>
> >> HTH,
> >> Ivan
> >>
> >> --
> >> Dr. Ivan Calandra
> >> Imaging lab
> >> RGZM - MONREPOS Archaeological Research Centre
> >> Schloss Monrepos
> >> 56567 Neuwied, Germany
> >> +49 (0) 2631 9772-243
> >> https://www.researchgate.net/profile/Ivan_Calandra
> >>
> >> On 25/04/2022 16:01, Ebert,Timothy Aaron wrote:
> >>> A <- matrix(1:9,ncol=3)
> >>> x <- c(0,1,0)
> >>> M <- matrix(ncol=3,nrow=3)
> >>> M<-A
> >>> for(i in 1:3) {
> >>>     if(x[i]){
> >>>       M[,i] <-0
> >>>       }
> >>>     }
> >>> }
> >>> M
> >>>
> >>> The outcome you want is to set all of the middle column values to
> zero. So I used x as a logical in an if test and when true everything in
> that column is set to zero.
> >>>
> >>> Your approach also works but you must go through each element
> explicitly.
> >>> A <- matrix(1:9,ncol=3)
> >>> x <- c(0,1,0)
> >>> M <- matrix(ncol=3,nrow=3)
> >>> for(j in 1:3){
> >>>     for(i in 1:3){
> >>>       ifelse(x[i]==1, M[j,i]<-0, M[j,i]<-A[j,i])
> >>>     }
> >>> }
> >>> M
> >>>
> >>>
> >>>
> >>> Tim
> >>>
> >>> -----Original Message-----
> >>> From: R-help <r-help-bounces using r-project.org> On Behalf Of Uwe Freier
> >>> Sent: Sunday, April 24, 2022 11:06 AM
> >>> To: r-help using r-project.org
> >>> Subject: [R] Confusing fori or ifelse result in matrix manipulation
> >>>
> >>> [External Email]
> >>>
> >>> Hello,
> >>>
> >>> sorry for the newbie question but I can't find out where I'm wrong.
> >>>
> >>> A <- matrix(1:9,ncol=3)
> >>> x <- c(0,1,0)
> >>> M <- matrix(ncol=3,nrow=3)
> >>> for(i in 1:3) {
> >>>     M[,i] <- ifelse(x[i] == 0, A[,i], 0)
> >>> }
> >>>
> >>> expected:
> >>>
> >>>> M
> >>>         [,1] [,2] [,3]
> >>> [1,]    1    0    7
> >>> [2,]    2    0    8
> >>> [3,]    3    0    9
> >>>
> >>>
> >>> but the result is:
> >>>
> >>>> M
> >>>         [,1] [,2] [,3]
> >>> [1,]    1    0    7
> >>> [2,]    1    0    7
> >>> [3,]    1    0    7
> >>>
> >>>
> >>> If I do it "manually":
> >>>
> >>>> M[,1] <- A[,1]
> >>>> M[,2] <- 0
> >>>> M[,3] <- A[,3]
> >>> M is as expected, where is my misconception?
> >>>
> >>> Thanks for any hint and best regards,
> >>>
> >>> Uwe
> >>>
> >>> ______________________________________________
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> >> ______________________________________________
> >> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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