[R] Confusing fori or ifelse result in matrix manipulation

[Eric Berger]

M[,x==1] is not the same as M[,x] :-)
However, M[,!!x] is the same as M[,x==1] and saves one character!
The point of this is "I can name that tune in ... " (as if that was not
obvious)



On Mon, Apr 25, 2022 at 5:30 PM Bert Gunter <bgunter.4567 using gmail.com> 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
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> > PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
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>
> ______________________________________________
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> https://stat.ethz.ch/mailman/listinfo/r-help
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> and provide commented, minimal, self-contained, reproducible code.
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