[R] "[[" gotcha

[Henrik Bengtsson]

To create a empty list do:

   B <- list()

/H


On 1/16/07, Robin Hankin <r.hankin at noc.soton.ac.uk> wrote:
> The following gotcha caught me off-guard just now.
>
> I have two matrices, a and b:
>
>
> a <- matrix(1,3,3)
> b <- matrix(1,1,1)
>
> (note that both "a" and "b" are matrices).
>
> I want them in a list:
>
>  > B <- NULL
>  > B[[1]] <- a
>  > B[[2]] <- b
>  > B
> [[1]]
>       [,1] [,2] [,3]
> [1,]    1    1    1
> [2,]    1    1    1
> [3,]    1    1    1
>
> [[2]]
>       [,1]
> [1,]    1
>
>  >
>
> This is fine.
>
> But swapping "a" and "b" over does not behave as desired:
>
>
>  > B <- NULL
>  > B[[1]] <- b
>  > B[[2]] <- a
> Error in B[[2]] <- a : more elements supplied than there are to replace
>  >
>
>
>
> The error is given because after B[[1]] <- a,   the variable B is
> just a scalar and
> not a matrix (why is this?)
>
> What's the bulletproof method for assigning matrices to a list (whose
> length is
> not known at runtime)?
>
>
>
>
>
>
>
>
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
>   tel  023-8059-7743
>
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