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
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