[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)
   # Is the same as executing
   sin(1.5) - 0.08*1.5 + 0.2
   # Let's plot this

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 
talking about:

   # 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

   # 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'
  Además: Mensajes de aviso perdidos
  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?



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