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

[Marc Schwartz]

On Jun 10, 2009, at 12:36 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 ...
>
> 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?
>
> Thanks!
>
> Doug

Doug,

Try this:

# M is a character matrix
 > M
   1   2   3   4
1 "A" "C" "D" "G"
2 "B" "D" "F" "H"

# v is a vector, equal in length to the number of columns in M
 > v
[1] 1 2 2 1

# Get the diagonal of the matrix that results from the combinations of  
indices
 > diag(M[v, 1:ncol(M)])
[1] "A" "D" "F" "G"


I created a larger 2 row matrix to test further:

set.seed(1)
M <- matrix(sample(LETTERS, 40, replace = TRUE), 2, 20)

 > M
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12]
[1,] "G"  "O"  "F"  "Y"  "Q"  "F"  "R"  "U"  "S"  "J"   "Y"   "Q"
[2,] "J"  "X"  "X"  "R"  "B"  "E"  "J"  "M"  "Z"  "U"   "F"   "D"
      [,13] [,14] [,15] [,16] [,17] [,18] [,19] [,20]
[1,] "G"   "A"   "W"   "M"   "M"   "V"   "U"   "S"
[2,] "K"   "J"   "I"   "P"   "E"   "R"   "C"   "K"


and 'v':

set.seed(2)
v <- sample(2, 20, replace = TRUE)

 > v
  [1] 1 2 2 1 2 2 1 2 1 2 2 1 2 1 1 2 2 1 1 1


Then:

 > diag(M[v, 1:ncol(M)])
  [1] "G" "X" "X" "Y" "B" "E" "R" "M" "S" "U" "F" "Q" "K" "A" "W" "P"  
"E"
[18] "V" "U" "S"


Looks like it might work.

HTH,

Marc Schwartz