Question

I have the following matrix

mat<-read.csv("mat.csv")
sel<-c(135, 211)

I would like to select the rows in ‘mat’ that correspond to ‘sel’

I do it in the following way:

subset(mat, mat$V2==c(sel))

and I get the following error:

Warning message:
In l[, 2] == c(135, 211) :
  longer object length is not a multiple of shorter object length

And also it only selects one of the two.

Answer 1

Try this (credits go to Roland)

mat[mat$V2 %in% sel,]
    X V1  V2 V3 V4  V5 V6 V7 V8 V9 V10
11 11  1 135  2  7 100  2  0  0  0   0
15 15  1 211  5  7 100  2  0  0  0   0

from ?'%in% you can read:

    %in% is a more intuitive interface as a binary operator, which returns
a logical vector indicating if there is a match or not for its left operand.

If you have a logical vector indicating the matching, then you can use it for indexing and selecting the elements you want. In this case mat$V2 %in% sel matches all elements of mat$V2 that are in sel it will give you a logical vector, then using it in mat[row, col] you’ll get ontly those desired elements as in mat[mat$V2 %in% sel,] this means: Give all the columns for those rows which elements meeting the condition mat$V2 %in% sel.