[R] Confusing fori or ifelse result in matrix manipulation

Bert Gunter bgunter@4567 @end|ng |rom gm@||@com
Mon Apr 25 16:39:33 CEST 2022

```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
>> > >
>> > > ______________________________________________
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