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

[Zhenjiang Lan]

Thank you very much! that's exactly what I want.

On Tue, Jun 12, 2012 at 10:32 AM, R. Michael Weylandt
<michael.weylandt at gmail.com> wrote:
> 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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