Using cbind on an arbitrarily long list of objects

Question

I would like to find a way to create a data.frame by using cbind() to join together many separate objects. For example, if A, B, C & D are all vectors of eq

Answer 1

First you need to get the objects you want and store them together as a list; if you can construct their names as strings, you use the get function. Here I create two variables, A and B:

> A <- 1:4
> B <- rep(LETTERS[1:2],2)

I then construct a character vector containing their names (stored as ns) and get these variables using lapply. I then set the names of the list to be the same as their original names.

> (ns <- LETTERS[1:2])
[1] "A" "B"
> obj.list <- lapply(ns, get)
> names(obj.list) <- ns
> obj.list
$A
[1] 1 2 3 4

$B
[1] "A" "B" "A" "B"

Then you can use do.call; the first argument is the function you want and the second is a list with the arguments you want to pass to it.

> do.call(cbind, obj.list)
     A   B  
[1,] "1" "A"
[2,] "2" "B"
[3,] "3" "A"
[4,] "4" "B"

However, as aL3xa correctly notes, this makes a matrix, not a data frame, which may not be what you want if the variables are different classes; here my A has been coerced to a character vector instead of a numeric vector. To make a data frame from a list, you just call data.frame on it; then the classes of the variables are retained.

> (AB <- data.frame(obj.list))
  A B
1 1 A
2 2 B
3 3 A
4 4 B
> sapply(AB, class)
        A         B 
"integer"  "factor" 
> str(AB)
'data.frame':   4 obs. of  2 variables:
 $ A: int  1 2 3 4
 $ B: Factor w/ 2 levels "A","B": 1 2 1 2