[R] "[[" gotcha
Joerg van den Hoff
j.van_den_hoff at fzd.de
Tue Jan 16 12:02:24 CET 2007
Robin Hankin 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)?
>
>
not sure about "bulletproof", but:
you should tell R that B is really intended to be a list in the first place:
B <- list()
the rest then works as you intended. whether the 'simplification' of your 1x1 matrix to
a scalar in your example is canonical (and desirable) behaviour seems a question for some
of the experts (it's a bit reminiscent of the `drop = TRUE' vs. `drop = FALSE' problem)
joerg
>
>
>
>
>
>
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
> tel 023-8059-7743
>
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