sapply problem

c <- apply(b, c(1,2), unlist)
c

class(c[1,1])

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)?

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

regards

joerg

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)?

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

regards

joerg

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