Real cube root of a negative number

Question

I'm trying to see if there is a function to directly get the real cube root of a negative number. For example, in Java, there is the Math.cbrt() function. I'm looking for the equivalent in R.

Otherwise, my current hack is:

x <- -8
sign(x) * abs(x)^(1/3)

which is very inelegant and cumbersome to type every time. Thx!

Answer 1

Sounds like you just need to define your own Math.cbrt() function.

That will turn performing the operation from something inelegant and cumbersome to something clean, expressive, and easy to apply:

Math.cbrt <- function(x) {
    sign(x) * abs(x)^(1/3)
}

x <- c(-1, -8, -27, -64)

Math.cbrt(x)
# [1] -1 -2 -3 -4

Answer 2

In R you probably need to define a new function that limits the results to your goals:

> realpow <- function(x,rad) if(x < 0){ - (-x)^(rad)}else{x^rad}
> realpow(-8, 1/3)
[1] -2
> realpow(8, 1/3)
[1] 2

It's possible to make an infix operation if you quote the operator and use surrounding "%" signs in its name. Because its precedence is low, you will need to use parentheses, but you already appear to know that.

> `%r^%` <- function(x, rad) realpow(x,rad)
>  -8 %r^% 1/3
[1] -2.666667    # Wrong
> -8 %r^% (1/3)
[1] -2   #Correct

Agree with incorporating the questioner's version for its vectorized capacity:

`%r^%` <- function(x, rad) sign(x)*abs(x)^(rad)