I have the following data frame:
dat <- data.frame(toys = c("yoyo", "doll", "duckie", "tractor", "airplaine", "ball", "racecar", "dog", "jumprope", "car", "elephant", "bear", "xylophone", "tank", "checkers", "boat", "train", "jacks", "truck", "whistle", "pinwheel"),
price = c(1.22, 2.75, 1.85, 5.97, 6.47, 2.16, 7.13, 4.57, 1.46, 5.18, 3.16, 4.89, 7.11, 6.45, 4.77, 8.04, 6.71, 2.31, 6.21, 0.98, 0.87))
I now want to get all combination of toys for 7 to 14 selected toys. Following this thread (Unordered combinations in R), I'm using the combinations function in the arrangements package:
library(arrangements)
combs <- lapply(7:14, combinations, x = dat$toys)
Looking at the results with str(combs) it gives a list of length 8, where each list element is a two-dimensional factor, e.g.
test <- combs[[1]]
dim(test)
However, if I want to convert the list elements to a data frame now it just gives me a data frame with one column, whereas I would expect 7 columns for as.data.frame(combs[[1]]).
If I use an integer or character vector in the combinations function above, all works as expected, e.g. with:
combs2 <- lapply(7:14, combinations, x = as.character(dat$toys)) # or
combs3 <- lapply(7:14, combinations, x = 1:21)
test2 <- as.data.frame(combs2[[1]])
test3 <- as.data.frame(combs3[[1]])
I get a proper data frame with several columns.
Why is my code working with integers and characters, but not with factors?
When you call combinations, the underlying c code set the dim attributes on the output. When it is a character, numeric or integer, it is converted into a matrix and then you can get a data.frame from it:
We can try this in R for characters and integers (like you have shown):
x = 1:4
attr(x,"dim") <- c(2,2)
class(x)
[1] "matrix"
dim(data.frame(x))
1] 2 2
x = as.character(1:4)
attr(x,"dim") <- c(2,2)
class(x)
[1] "matrix"
dim(data.frame(x))
[1] 2 2
Note for the above, you get the correct dimensions and class (matrix) back. For factors, it doesn't complain, you get a two dimensional factor:
x = factor(1:4)
attr(x,"dim") <- c(2,2)
class(x)
[1] "factor"
str(x)
Factor[1:2, 1:2] w/ 4 levels "1","2","3","4": 1 2 3 4
However, it is not a matrix although it looks like one:
x
[,1] [,2]
[1,] 1 3
[2,] 2 4
Levels: 1 2 3 4
However, converting it into a data.frame fails:
as.data.frame(x)
x.1 x.2
1 1 3
2 2 4
3 <NA> <NA>
4 <NA> <NA>
Warning message:
In format.data.frame(if (omit) x[seq_len(n0), , drop = FALSE] else x, :
corrupt data frame: columns will be truncated or padded with NAs
My guess is that you were very lucky with the combinations of 7 to 14. If you try lower numbers, it fails:
data.frame(combinations(dat$toys,5))
Error in `[.default`(xj, i, , drop = FALSE) : subscript out of bounds
data.frame(combinations(dat$toys,2))
#throws same erros as above