[R] not understanding a do.call

[Tony Plate]

Suppose you have a list of equal-length numeric vectors and you want to 
bind them together in a matrix.  You want to a piece of code that will 
work no matter how many vectors are in the list.  That's what this 
construct with do.call() is useful for,

e.g.:

 > a <- 1:3
 > b <- 4:6
 > c <- 7:9
 > x1 <- list(a=a,b=b)
 > x2 <- list(a=a,b=b,c=c)
 > do.call("cbind", x1)
      a b
[1,] 1 4
[2,] 2 5
[3,] 3 6
 > do.call("cbind", x2)
      a b c
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9
 >

-- Tony Plate

Leeds, Mark (IED) wrote:
> I did a ?do.call but i don't think i understand it.
>  
> if a, b,c,d are numeric vectors then could someone explain the
> difference between
>  
> do.call("cbind",list(a,b,c,d))
>  
> and cbind(a,b,c,d).
>  
> or point to an archive on it.
>  
> the return value of cbind is a matrix or dataframe depending on what is
> sent in but i don't 
> understand wheen it would be useful to use do.call. i realize it takes a
> list but that's
> all i know about why one use it ? thanks.
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