`x`

.
If `na.rm`

is `TRUE`

then missing values are removed before
computation proceeds.
`sd(x, na.rm = FALSE)`

x

a numeric vector or an R object which is coercible to one
by

`as.double(x)`

.na.rm

logical. Should missing values be removed?

`var`

this uses denominator $n - 1$. The standard deviation of a zero-length vector (after removal of
`NA`

s if `na.rm = TRUE`

) is not defined and gives an error.
The standard deviation of a length-one vector is `NA`

.

`var`

for its square, and `mad`

, the most
robust alternative.
sd(1:2) ^ 2