How to coerce a list object to type 'double'

Question

The code:

a <- structure(list(`X$Days` = c("10", "38", "66", "101", "129", "185", "283", 
                                 "374")), .Names = "X$Days")

Then a is like

$`X$Days`
[1] "10"  "38"  "66"  "101" "129" "185" "283" "374"

I would like to coerce a to an array of numeric values, but coercing functions return me

Error: (list) object cannot be coerced to type 'double'

Thanks,

Answer 1

If you want to convert all elements of a to a single numeric vector and length(a) is greater than 1 (OK, even if it is of length 1), you could unlist the object first and then convert.

as.numeric(unlist(a))
# [1]  10  38  66 101 129 185 283 374

Bear in mind that there aren't any quality controls here. Also, X$Days a mighty odd name.

Answer 2

If your list as multiple elements that need to be converted to numeric, you can achieve this with lapply(a, as.numeric).

Answer 3

You can also use list subsetting to select the element you want to convert. It would be useful if your list had more than 1 element.

as.numeric(a[[1]])

Answer 4

In this case a loop will also do the job (and is usually sufficiently fast).

a <- array(0, dim=dim(X))
for (i in 1:ncol(X)) {a[,i] <- X[,i]}