What's the difference between lapply and do.call?

Question

I\'m learning R recently and confused by two function: lapplyand do.call. It seems that they\'re just similar to map function in Lisp.

Answer 1

lapply applies a function over a list, do.call calls a function with a list of arguments. That looks like quite a difference to me...

To give an example with a list :

X <- list(1:3,4:6,7:9)

With lapply you get the mean of every element in the list like this :

> lapply(X,mean)
[[1]]
[1] 2

[[2]]
[1] 5

[[3]]
[1] 8

do.call gives an error, as mean expects the argument "trim" to be 1.

On the other hand, rbind binds all arguments rowwise. So to bind X rowwise, you do :

> do.call(rbind,X)
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
[3,]    7    8    9

If you would use lapply, R would apply rbind to every element of the list, giving you this nonsense :

> lapply(X,rbind)
[[1]]
     [,1] [,2] [,3]
[1,]    1    2    3

[[2]]
     [,1] [,2] [,3]
[1,]    4    5    6

[[3]]
     [,1] [,2] [,3]
[1,]    7    8    9

To have something like Map, you need ?mapply, which is something different alltogether. TO get eg the mean of every element in X, but with a different trimming, you could use :

> mapply(mean,X,trim=c(0,0.5,0.1))
[1] 2 5 8