[R] sapply problem

[Sundar Dorai-Raj]

Joerg van den Hoff said the following on 12/14/2006 7:30 AM:
> I have encountered the following problem: I need to extract from
> a list of lists equally named compenents who happen to be 'one row'
> data frames. a trivial example would be:
> 
> a <- list(list(
> df = data.frame(A = 1, B = 2, C = 3)), list(df = data.frame(A = 4,B = 5,C = 6)))
> 
> I want the extracted compenents to fill up a matrix or data frame row by row.
> the obvious thing to do seems:
> 
> b <- sapply(a, "[[", "df")
> b <- t(b)
> 
> now `b' looks all right:
> 
> b
> class(b)
> 
> but it turns out that all elements in this matrix are one element lists:
> 
> class(b[1,1])
> 
> which prevents any further standard processing of `b' (like `colMeans', e.g.)
> 
> question 1: is their a straightforward way to enforce that `b' contains
> simple numbers as elements right from the start (instead of something like
> apply(b, 1:2, "class<-", "numeric") afterwards)?
> 

Try this:

a <- list(list(df = data.frame(A = 1, B = 2, C = 3)),
           list(df = data.frame(A = 4, B = 5, C = 6)))
b <- do.call("rbind", sapply(a, "[", "df"))
b


> question 2: should not sapply do this further 'simplification' anyway in a situation
> like this (matrix elements turn out to be one-element lists)?
> 

I think it does as it much as it knows how. I think you might believe 
that matrix elements can only contain numeric values. This is not a 
valid assumption. Take this example:

 > a <- list(1)
 > b <- list(2)
 > (m <- matrix(c(a, b), 2, 1))
      [,1]
[1,] 1
[2,] 2
 > class(m[1, 1])
[1] "list"
 > class(m[2, 1])
[1] "list"

HTH,

--sundar

> regards
> 
> joerg
> 
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