[R] Ask for help: find corresponding elements between matrix

Thu Feb 21 16:21:46 CET 2013

Hi Zhengyu

Surely not the most elegant piece of coding, but this should do the trick:

> m=rep(NA,nrow(seq)*ncol(seq))

> mat1=matrix(m,3)

> for(i in 1:nrow(seq)){
+ for(j in 1:ncol(seq)){
+ ifelse((seq[i,j]==1 | seq[i,j]==-1),
+         mat1[i,j]<-mat[i,j],mat1[i,j]<-NA)
+ }
+ }

> mat1

e.g. With
> mat
          [,1]      [,2]       [,3]       [,4]
[1,] 1.8937156 0.9461121  0.9446139  0.1662869
[2,] 1.0417071 1.5253203 -0.1476443  0.2010656
[3,] 0.8777728 2.2808083 -0.6486065 -0.6618795

and 

>seq
     [,1] [,2] [,3] [,4]
[1,]    1    1    0    1
[2,]   -1    1    0    1
[3,]    0   -1   -1   NA

You get

         [,1]      [,2]       [,3]      [,4]
[1,] 1.893716 0.9461121         NA 0.1662869
[2,] 1.041707 1.5253203         NA 0.2010656
[3,]       NA 2.2808083 -0.6486065        NA

Regards,

José

José Iparraguirre
Chief Economist
Age UK

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-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of JiangZhengyu
Sent: 21 February 2013 14:39
To: r-help at r-project.org
Subject: [R] Ask for help: find corresponding elements between matrix

Dear R experts,

I have two matrix (seq & mat) & I want to retrieve in a new matrix all the numbers from mat that =1 (corresponding to the same row/ column position) in seq, or all the numbers in mat that =-1 in seq. - Replace all the numbers with NA if it's not 1/-1 in seq.  There are some "NA"s in seq. 

seq=matrix(c(1,-1,0,1,1,-1,0,0,-1,1,1,NA),3,4)
mat=matrix(rnorm(12),3)
 I made this code but I don't know where's the problem..

seq=matrix(c(1,-1,0,1,1,-1,0,0,-1,1,1,NA),3,4)
mat=matrix(rnorm(12),3)

m=rep(NA,nrow(seq))
for(i in 1:nrow(seq))
 {m[i]=mat[seq[i]==1]
}

Error in m[i] = mat[seq[i] == 1] : replacement has length zero
In addition: Warning message:
In m[i] = mat[seq[i] == 1] :
  number of items to replace is not a multiple of replacement length
> 

I was wondering if anyone can help me with this.
Thanks in advance, 
Zhengyu
eg. here is what I want

 > head(seq)
     [,1] [,2] [,3] [,4]
[1,]    1    1    0    1
[2,]   -1    1    0    1
[3,]    0   -1   -1   NA

> head(mat)
           [,1]       [,2]       [,3]      [,4]
[1,]  0.1006950  0.3354360  0.1796888 1.3198761
[2,]  1.0948252  0.5350797 -0.5739454 0.2744478
[3,] -0.5409936 -0.3820084 -0.5601096 0.9239680

This is a new new matrix.
> head(mat1)
           [,1]       [,2]       [,3]      [,4]
[1,]  0.1006950  0.3354360  NA 1.3198761
[2,]  NA  0.5350797 NA 0.2744478
[3,]  NA NA NA NA

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