atan() function returns the radian arctangent of number data.

atan(x)

x: Numeric value, array or vector

> atan(1)

[1] 0.7853982

> atan(0)

[1] 0

> atan(0.5)

[1] 0.4636476

> x <- c(1, 0, 0.5)
> atan(x)

[1] 0.7853982 0.0000000 0.4636476

Let's plot the function:

> plot(x, atan(x), typ="l", col="blue")
> abline(h=0,col="green",lty=6)