# [R] A strange behaviour in the graphical function "curve"

Julio Sergio juliosergio at gmail.com
Fri Apr 12 05:15:41 CEST 2013

```I thought the curve function was a very flexible way to draw functions. So I
could plot funtions like the following:

# I created a function to produce functions, for instance:
fp <- function(m,b) function(x) sin(x) + m*x + b
# So I can produce a function like this
ff <- fp(-0.08, 0.2)
ff(1.5)
# Is the same as executing
sin(1.5) - 0.08*1.5 + 0.2
# Let's plot this
plot(fp(0.1,0.1),xlim=c(-2*pi,2*pi),col="red")

When I get to plot some more complex functions, "curve", instead of taking
the argument function as a black-box, i.e., something that takes an argument
(the x) and returns a value (the y), seems to inspect the inner code of the
argument function in a way that even R itself doesn't do. See what I'm

# A function that returns a 2-element vector, given a
# single argument
zetas <- function(alpha) {z <- qnorm(alpha/2); c(z,-z)}

# A transformation function - it can take a vector as
# its z argument
Tzx <- function(z, sigma_p, mu_p) sigma_p*z + mu_p

# Another transformation function similar to the
# previous one - it can take a vector as its x argument
Txz <- function(x, sigma_p, mu_p) (x - mu_p)/sigma_p

# The general function with several arguments
BetaG <- function(mu, alpha, n, sigma, mu_0) {
lasZ <- zetas(alpha) # It is a vector
sigma_M <- sigma/sqrt(n)
lasX <- Tzx(lasZ, sigma_M, mu_0) # Another vector(transf. from lasZ)
NewZ <- Txz(lasX, sigma_M, mu) # A new vector:transf. from lasX
# And the result is a single value:
pnorm(NewZ[2]) - pnorm(NewZ[1])
}

# Now, let's have a function of a single argument, giving
# particular values to all other arguments; so miBeta depends
# only on the value of the argument 'mu'
miBeta <- function(mu) BetaG(mu, 0.05, 36, 48, 400)

# I can call this function with 420 and it works
miBeta(420)

# But when the time comes to plot the function, it doesn't work
curve(miBeta,xlim=c(370,430), xlab="mu", ylab="L(mu)")

When I called miBeta with any value the R interpreter didn't complain.
However, "curve" seems to go deeper than the R interpreter and issues
several error messages:

Error en curve(miBeta, xlim = c(370, 430), xlab = "mu", ylab = "L(mu)") :
'expr' did not evaluate to an object of length 'n'
In x - mu_p :
longitud de objeto mayor no es múltiplo de la longitud de uno menor

Do you have any idea on why "curve" behaves this way?

Thanks,

-Sergio.

```