[R] "[[" gotcha
Robin Hankin
r.hankin at noc.soton.ac.uk
Tue Jan 16 11:10:42 CET 2007
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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