[R] matrix problem

[Sarah Goslee]

How about:

y <- c(1,1,1,3,2)
m <- matrix(0, nrow=length(y), ncol=4)
m[y==1, ] <- matrix(1:4, nrow=sum(y == 1), ncol=4, byrow=TRUE)

or, depending on your actual problem
y <- c(1,1,1,3,2)
m <- matrix(0, nrow=length(y), ncol=4)
m[y == 1,] <- col(m[y == 1,])

Sarah


On Mon, Jun 20, 2011 at 3:54 PM, Costis Ghionnis <conighion at gmail.com> wrote:
> Hallo everyone! I have a problem about creating a matrix...
>
> Suppose we have a vector y<-c(1,1,1,3,2)
>
> and a zero matrix, m ,with nrows=length(y) and ncol=4.
>
> The matrix would look like this:
> 0       0       0       0
> 0       0       0       0
> 0       0       0       0
> 0       0       0       0
> 0       0       0       0
>
> I want to change the first three rows with the vector c(1,2,3,4).
> I thought that with the command m[y==1,1:4]<-c(1,2,3,4) i would get
>
> 1       2       3       4
> 1       2       3       4
> 1       2       3       4
> 0       0       0       0
> 0       0       0       0
>
> but instead i am getting
>
> 1       4       3       2
> 2       1       4       3
> 3       2       1       4
> 0       0       0       0
> 0       0       0       0
>
> It seems it is filling the data by col instead by row. I want to use this technique in more complicated problems.
> So i do not want to have to work with the transpose matrix. Do you know another way to make this work. Thank you...
>


-- 
Sarah Goslee
http://www.functionaldiversity.org