R help - Jun 2009 - How to extract from a matrix based on indices in a vector?

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

PS : I originally posted this on "R devel", then realized my mistake.
"Move
thread" didn't work when I tried it, though.
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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

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