CODEWITHSUNDEEP
rmatrixsubsetindicespicking
Pau
Pau

Reputation: 25

R: Picking values from matrix by indice matrix

I have a datamatrix with n rows and m columns (in this case n=192, m=1142) and an indice matrix of nxp (192x114). Each row of the indice matrix shows the column numbers of the elements that I would like to pick from the matching row of the datamatrix. Thus I have a situation something like this (with example values):

data<-matrix(1:30, nrow=3)
data
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    1    4    7   10   13   16   19   22   25    28
[2,]    2    5    8   11   14   17   20   23   26    29
[3,]    3    6    9   12   15   18   21   24   27    30


columnindices<-matrix(sample(1:10,size=9, replace=TRUE),nrow=3)
columnindices
      [,1] [,2] [,3]
[1,]    8    7    4
[2,]   10    8   10
[3,]    8   10    2

I would like to pick values from the datamatrix rows using the in columnindices matrix, so that the resulting matrix would look like this

      [,1] [,2] [,3]
[1,]   22   19   10
[2,]   29   23   29
[3,]   24   30   6

I tried using a for loop:

result<-0
for(i in 1:3) {
 result[i]<-data[i,][columnindices[,i]]
 print[i]
}

but this doesn't show the wished result. I guess my problem should be rather simply solved, but unfortunately regardless many hours of work and multiple searches I still haven't been able to solve it (I am rookie). I would really appreciate some help!

Upvotes: 0

Views: 115

Answers (2)

Alexey
Alexey

Reputation: 190

The for-loop way described by @LAP is easier to understand and to implement.

If you would like to have something universal, i.e. you don't need to adjust row number every time, you may utilise the mapply function:

result <- mapply(
  FUN = function(i, j) data[i,j],
  row(columnindices),
  columnindices)
dim(result) <- dim(columnindices)

mapply loop through every element of two matrices,

  • row(columnindices) is for i row index
  • columnindices is for j column index.

It returns a vector, which you have to coerce to the initial columnindices dimension.

Upvotes: 0

LAP
LAP

Reputation: 6685

Your loop is just a little bit off:

result <- matrix(rep(NA, 9), nrow = 3)
for(i in 1:3){
  result[i,] <- data[i, columnindices[i,]]
}

> result
     [,1] [,2] [,3]
[1,]   25   13    7
[2,]   29   29   23
[3,]   15   15   18

Note that the matrix is not exactly the one you posted as expected result because the code for your example columnindices does not match the matrix you posted below. Code should work as you want it.

Upvotes: 0

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