[R] Confusing fori or ifelse result in matrix manipulation

[Bert Gunter]

Oh, I get it. The context was in  choosing columns to set to 0 via a
**predicate**.

Sorry for the noise.


Bert

On Mon, Apr 25, 2022 at 7:39 AM Bert Gunter <bgunter.4567 using gmail.com> wrote:
>
> Yes, sorry. But it's with the logical cast it's still M[, c(FALSE,
> TRUE, FALSE)] which is M[, 2], and so I still don't get the point.
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along
> and sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
>
> On Mon, Apr 25, 2022 at 7:33 AM Eric Berger <ericjberger using gmail.com> wrote:
> >
> > 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
> >> > >
> >> > > ______________________________________________
> >> > > R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see https://urldefense.proofpoint.com/v2/url?u=https-3A__stat.ethz.ch_mailman_listinfo_r-2Dhelp&d=DwICAg&c=sJ6xIWYx-zLMB3EPkvcnVg&r=9PEhQh2kVeAsRzsn7AkP-g&m=eyJm06tVDfKvtMDgz6oIWM-WVdoW3Szzb5G6rq0cCO_cB6ljj2x80E4oRkt3Vgba&s=K2RWPvtxaxwigGGH2oOrg8qiDWC5KTu60b8Wjybwsg4&e=
> >> > > PLEASE do read the posting guide https://urldefense.proofpoint.com/v2/url?u=http-3A__www.R-2Dproject.org_posting-2Dguide.html&d=DwICAg&c=sJ6xIWYx-zLMB3EPkvcnVg&r=9PEhQh2kVeAsRzsn7AkP-g&m=eyJm06tVDfKvtMDgz6oIWM-WVdoW3Szzb5G6rq0cCO_cB6ljj2x80E4oRkt3Vgba&s=L9VXAAYzIzrG2h17hBO-Qfg_EoS2mRQbjs3sRESp62Q&e=
> >> > > 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.
> >> >
> >> > ______________________________________________
> >> > 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.