[R] How to index a matrix with different row-number for each column?

[R. Michael Weylandt]

I think you are looking for

mt[cbind(vt, seq_along(vt))]

which uses some trickines known as matrix indexing (basically, the two
columns created by cbind() are considered row & col indices -- then
seq_along() gives the col1,col2, bit as needed.

HTH,

Michael

On Tue, Jun 12, 2012 at 10:28 AM, Zhenjiang Lan <lan.zhenjiang at gmail.com> wrote:
> here's my question: suppose I have a matrix:
>
>    mt<-matrix(1:12,ncol=6)
>
> now I have a vector
>
>    vt<-c(1,2,2,2,1,2)
>
> which means I want to get:
> the 1st row for column1;
> the 2nd row for column2;
> the 2nd row for column3;
> the 2nd row for column4;
> ...
>
> that what I want is this vector:
> 1,4,6,8,9,12
>
> Does anyone know how to do this fast?
> I know I can use for-loop to travel all columns,but that's not what I want.
>
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