[R] How to extract from a matrix based on indices in a vector?

[David Winsemius]

On Jun 10, 2009, at 1:36 PM, Logickle wrote:

>
> Sorry, there may be some lingo for describing the extraction problem  
> I have,
> but I don't know it.
>
> I have a matrix of 2 rows and N columns, and a vector of N 1s and 2s.
>
> Matrix M:
>
>      1     2     3      4   ...  N
> 1    A     C     D      G  ...
> 2    B     D     F      H   ...
>
> Vector v:
>
> 1  2  2  1  2  1  1 ... N
>
> I'd like to apply v to M to get
>
> Vector w:
>
> A  D  F  G ...

Using length 10 M and v

 > w <- apply(cbind(v, 1:length(v) ), 1, function(x) M[x[1], x[2] ])
  [1] "A" "D" "F" "G" "I" "K" "N" "P" "R" "S"

 > M <- matrix(LETTERS[1:20], nrow=2)
 > M
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,] "A"  "C"  "E"  "G"  "I"  "K"  "M"  "O"  "Q"  "S"
[2,] "B"  "D"  "F"  "H"  "J"  "L"  "N"  "P"  "R"  "T"
 > v <- c(1,2,2,1, sample(1:2, 6, replace=TRUE) )
 > v
  [1] 1 2 2 1 1 1 2 2 2 1

The number of rows in M should not be a problem.

>
> I.e. each element of v is interpreted as a row-index used to extract  
> that
> row's value from the corresponding column in M into the  
> corresponding column
> of w.
>
> Also eventually nrow(M) > 2, in which case the value of the elements  
> of v
> would range over 1:nrow(M).
>
> Seems it should be simple, but maybe not?

Seems reasonably simple.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT