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

[Zhenjiang Lan]

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.