[R] "[[" gotcha

[Joerg van den Hoff]

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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