r - Difference between mean(c(1,2,21)) and mean(1,2,21)

Question

What's the difference between these two?

mean(c(1,2,21))

and

mean(1,2,21)

The answers are different, but what's the meaning of each one?

Answer 1

mean(c(1,2,21))
#[1] 8

This passes a vector of three elements to the mean function and the mean value of these three elements is calculated.

mean(1,2,21)
#[1] 1

This passes 1 as the first argument, 2 as the second argument and 21 as the third argument to the mean function. mean passes these arguments to mean.default. In help("mean.default") you can find the arguments of this function:

1. The object you want the mean for.
2. the fraction (0 to 0.5) of observations to be trimmed from each end of x before the mean is computed. Values of trim outside that range are taken as the nearest endpoint.
3. a logical value indicating whether NA values should be stripped before the computation proceeds. (Since you pass a numeric value, it is coerced to logical automatically).

So you calculate this:

mean.default(1, 0.5, TRUE)
[1] 1