Plotting multiple variable function in R

Question

I'm needing help with the following question:

Consider the following R function, named negloglike that has two input arguments: lam and x, in that order. Use this function to produce a plot of the log-likelihood function over a range of values λ ∈ (0, 2).

negloglike <- function(lam, x) {
  l = -sum(log(dexp(x, lam)))
  return(l)
}

Can anyone please help? Is it possible to do something like this with ggplot? I've been trying to do it with a set value of lam (like 0.2 here for example) using stat_function:

ggplot(data = data.frame(x = 0), mapping = aes(x = x)) +
  stat_function(fun = negloglike, args = list(lam = 0.2)) +
  xlim(0,10)

but the plot always returns a horizontal line at some y-value instead of returning a curve.

Should I be possibly using a different geom? Or even a different package altogether?

Much appreciated!

You should edit the question and post the code you have used. To know what you are doing is a first step to know what is wrong with it. Tip: I would start by swapping the order of the arguments, x first. — Rui Barradas
– Rui Barradas, Commented Aug 25, 2018 at 8:41
Thanks for the suggestion! I've added in my code to the question. The negloglike function works fine when I'm using it to calculate values but the graph that is produced by stat_function doesn't work the same (with the way that I've done it anyway). Changing it to -log(sum(.)) doesn't seem to make any difference, nor does swapping the order of x and lam — ethy_32
– ethy_32, Commented Aug 25, 2018 at 8:50
Note that the function sums over all values of dexp, so it will return one value only. — Rui Barradas
– Rui Barradas, Commented Aug 25, 2018 at 8:54
Yes, but isn't that usually the case when plotting a function? To plot one value for every x-value? Or am I misunderstanding the way the mechanics work? e.g. negloglike(x = 1, lam = 0.2) = 1.809438, negloglike(x = 2, lam = 0.2) = 2.009438 etc.? — ethy_32
– ethy_32, Commented Aug 25, 2018 at 8:57

Rui Barradas · Accepted Answer · 2018-08-25 13:41:50Z

2

The trick is to Vectorize the function over the argument of interest.
Thanks for the tip go to the most voted answer to this question. It uses base graphics only, so here is a ggplot2 equivalent.
First I will define the negative log-likelihood using function dexp

library(ggplot2)

negloglike <- function(lam, x) {negloglike <- function(lam, x) {
    l = -sum(dexp(x, lam, log = TRUE))
    return(l)
}
nllv <- Vectorize(negloglike, "lam")

But it's better to use the analytic form, which is easy to establish by hand.

negloglike2 <- function(lam, x) {
    l = lam*sum(x) - length(x)*log(lam)
    return(l)
}

nllv2 <- Vectorize(negloglike2, "lam")

ggplot(data = data.frame(lam = seq(0, 2, by = 0.2)), mapping = aes(x = lam)) +
  stat_function(fun = nllv2, args = list(x = 0:10))

Both nllv and nllv2 give the same graph.

edited Aug 25, 2018 at 13:41

answered Aug 25, 2018 at 9:08

Rui Barradas

78k8 gold badges41 silver badges75 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

ethy_32 Over a year ago

Awesome Rui, thanks so much! This helped heaps. I thought there had to be some vectorising in some shape or form after reading a couple of posts but I wasn't really sure about how to apply it here.

Rui Barradas Over a year ago

@ethy_32 Sorry but I made a mistake with the log-likelihood. See the new code and graph.

Collectives™ on Stack Overflow

Plotting multiple variable function in R

1 Answer 1

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Linked

Related