# [R] Confusing fori or ifelse result in matrix manipulation

Rui Barradas ru|pb@rr@d@@ @end|ng |rom @@po@pt
Mon Apr 25 17:25:13 CEST 2022

```Hello,

If !!x obfuscates what the is doing, there's

M[, as.logical(x)] <- 0

Hope this helps,

Às 15:52 de 25/04/2022, Ivan Calandra escreveu:
> Indeed, of course, my bad! I got confused by Bert's answer...
>
> So my first suggestion to do M[, x == 1] was correct :)
>
> --
> 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 16:44, Eric Berger wrote:
>> 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
>>     >> 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=
>>
>>
>> <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=>
>>
>>
>> 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=
>>
>>
>> <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
>>     http://www.R-project.org/posting-guide.html
>>     <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
>>     http://www.R-project.org/posting-guide.html
>>     <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
>>     http://www.R-project.org/posting-guide.html
>>     <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