as.data.table behavior inconsistent with as.data.frame for matrix list members

[ghost]

See http://stackoverflow.com/questions/26070187/why-does-data-table-recycle-matrices-into-a-single-vector-when-data-frame-does-n for background.

Reproduce:

a.matrix <- matrix(seq_len(25),ncol = 5, nrow = 5)

a.list <- list(seq_len(5),a.matrix)

a.dt <- as.data.table(a.list)
a.df <- as.data.frame(a.list)
a.dt.df <- as.data.table(a.df)

str(a.dt)
str(a.df) # different from a.dt
str(a.dt.df) # same as a.df

[arunsrinivasan]

This SO post from @rsaporta has the answer, I think.

[rsaporta]

The issue arrises when calling as.data.table on any list(matrix)
eg:

Mat43 <- matrix(1:12, ncol=3)

list_of_matrix <- list(Mat43)

identical(as.data.frame(as.data.table(list_of_matrix)), as.data.frame(list_of_matrix))
# [1] FALSE

as.data.table(list_of_matrix)
#     V1
#  1:  1
#  2:  2
#  3:  3
#  4:  4
#  5:  5
#  6:  6
#  7:  7
#  8:  8
#  9:  9
# 10: 10
# 11: 11
# 12: 12

as.data.frame(list_of_matrix)
#   X1 X2 X3
# 1  1  5  9
# 2  2  6 10
# 3  3  7 11
# 4  4  8 12