sapply() in R: why returns input?

Question

Why does this code:

sapply(c(1, 3, 4, 0), print)

Returns:

[1] 1
[1] 3
[1] 4
[1] 0
[1] 1 3 4 0

Why it returns the input too?

Answer 1

sapply() is returning a vector of your printed values but first printing each one as they're called. This may be more clear if you look at this example:

> x <- sapply(1:4,print)
[1] 1
[1] 2
[1] 3
[1] 4
> x
[1] 1 2 3 4
> y <- sapply(1:4,function(x) x)
> y
[1] 1 2 3 4
> identical(x,y)
[1] TRUE