[R] Getting multiple matrix-values using a single command
David Winsemius
dwinsemius at comcast.net
Fri Mar 12 20:40:20 CET 2010
On Mar 12, 2010, at 2:31 PM, David Winsemius wrote:
>
> On Mar 12, 2010, at 9:26 AM, Nils Rüfenacht wrote:
>
>> Dear all!
>>
>> I'm trying to get multiple values from a matrix by using a single
>> command.
>>
>> Given a matrix A
>>
>> A <- matrix(seq(1,9),nrow=3,ncol=3)
>>
>> How can I get e.g. the values A[1,2] = 4 and A[3,3] = 9 with a
>> single command and without using any loop? My first idea was to
>> generate a row- and a column vector for the indices, i.e. c(1,3)
>> indicating row number 1 (for A[1,2]) and row number 3 (for A[3,3])
>> and similar for column-indices. Then I've tried to call
>>
>> A[c(1,3),c(2,3)]
>>
>> but instead of 4 , 9 the result is
>>
>> [,1] [,2]
>> [1,] 4 7
>> [2,] 6 9
>
> Pass the indices in a matrix:
>
> > A[matrix(c(c(1,3), c(2,3)), ncol=2)]
> [1] 4 9
Also works for higher dimensions:
> A <- array(seq(1,27),dim=c(3,3,3))
> A
, , 1
[,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9
, , 2
[,1] [,2] [,3]
[1,] 10 13 16
[2,] 11 14 17
[3,] 12 15 18
, , 3
[,1] [,2] [,3]
[1,] 19 22 25
[2,] 20 23 26
[3,] 21 24 27
> matrix(1:3, ncol=3, nrow=3)
[,1] [,2] [,3]
[1,] 1 1 1
[2,] 2 2 2
[3,] 3 3 3
> A[matrix(1:3, ncol=3, nrow=3)]
[1] 1 14 27
--
David.
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