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Ebert,Timothy Aaron

2022-Apr-25 14:01 UTC

[R] Confusing fori or ifelse result in matrix manipulation

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 at r-project.org> On Behalf Of Uwe Freier
Sent: Sunday, April 24, 2022 11:06 AM
To: r-help at 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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provide commented, minimal, self-contained, reproducible code.

Ivan Calandra

2022-Apr-25 14:18 UTC

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[R] Confusing fori or ifelse result in matrix manipulation

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 at r-project.org> On Behalf Of Uwe
Freier
> Sent: Sunday, April 24, 2022 11:06 AM
> To: r-help at 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 at 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 at 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 - Apr 2022 - Confusing fori or ifelse result in matrix manipulation

[R] Confusing fori or ifelse result in matrix manipulation

[R] Confusing fori or ifelse result in matrix manipulation