I think changing nrow() to NROW() will get around that error. However, this,
as you can see, this goes a little astray at the end. Perhaps something like
this will work for you:

z.ext <- rbind(z,array(0,c(9,3)))
z.out <- array(0, c(3,3,9))
for(i in 1:9){
    z.out[,,i] <- cbind(z.ext[c(i,i+3,i+6),])
}
print(z.out)

Hope this helps,

Michael Weylandt


On Wed, Sep 14, 2011 at 3:42 PM, Kristian Lind <
kristian.langgaard.lind@gmail.com> wrote:

> Hi,
>
> I'm solve a problem where I want to select every 3rd row from a matrix. I
> do
> this for all rows in a loop. This gives me "i" matrices with different
> number of rows
>
> I want to stack these matrices in an array and fill the blanks with zero.
>
> Does anyone have any suggestions on how to go about this?
>
> So i want to go from
>
>       [,1] [,2] [,3]
>  [1,]    1    1    1
>  [2,]    2    2    2
>  [3,]    3    3    3
>  [4,]    4    4    4
>  [5,]    5    5    5
>  [6,]    6    6    6
>  [7,]    7    7    7
>  [8,]    8    8    8
>  [9,]    9    9    9
>
> to
>
> , , 1
>
>     [,1] [,2] [,3]
> [1,]    1    1    1
> [2,]    4    4    4
> [3,]    7    7    7
>
> , , 2
>
>     [,1] [,2] [,3]
> [1,]    2    2    2
> [2,]    5    5    5
> [3,]    8    8    8
>
> , , 3
>
>     [,1] [,2] [,3]
> [1,]    3    3    3
> [2,]    6    6    6
> [3,]    9    9    9
>
> , , 4
>
>     [,1] [,2] [,3]
> [1,]    4    4    4
> [2,]    7    7    7
> [3,]    0    0    0
>
> ...
>
> , , 9
>
>     [,1] [,2] [,3]
> [1,]    9    9    9
> [2,]    0    0    0
> [3,]    0    0    0
>
>
> Here's my go at it
>
> z <- array(c(1:9), c(9,3))
> z1 <- array(0, c(3,3,9))
>    for(i in 1:9){
>    if(nrow(z[seq(i,9,3),])  == 2)
>    z1[,,i] <- rbind(z[seq(i,9,3),],c(0,0,0))
>    else
>    z1[,,i] <- z[seq(i,9,3),]
>    }
> print(z1)
>
> I get the following error: Error in if (nrow(z[seq(i, 9, 3), ]) == 2) z1[,
> ,
> i] <- rbind(z[seq(i,  :
>  argument is of length zero
>
> This is because nrow(z[seq(i, 9, 3) becomes NULL.
> Furthermore, this doesn't take care of the case where nrow(z[seq(i,9,3),])
> ==1
>
> Sorry for the long email and thank you in advance for reading it ;)
>
> Kristian
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

	[[alternative HTML version deleted]]

