Plotting CDF and PDF in R with custom function-CodePudding

I was wondering if there was any way to plot this PDF and CDF in R. I found these on a different question a user asked, and was curious.

pdfcdf

I know that I have to create a function and then plot this, but I'm struggling with the different parameters and am unsure how to translate this to R. I have only ever plotted PDF/CDF using a normal distribution, or from datasets.

CodePudding user response：

You can write the pdf and cdf as functions, using ifelse to specify the values within the ranges, then simply plot the functions over the desired range:

pdf <- function(x) {
  ifelse(x >= 0 & x < 1, 0.5, ifelse(x < 1.5 & x >= 1, 1, 0))
}

cdf <- function(x) {
  ifelse(x < 0, 0,
         ifelse(x >= 0 & x < 1, 0.5 * x,
                ifelse(x < 1.5 & x >= 1, x - 0.5, 1)))
}

plot(pdf, xlim = c(-1, 2), type = "s")


plot(cdf, xlim = c(-1, 2))

^{Created on 2022-10-27 with reprex v2.0.2}

CodePudding user response：

ifelse can be very slow, we can fill an output vector instead. numeric() creates a vector of zeroes of a specified length, we then simply change everything that should not yield zero.

pdf_vec <- function(x) {
  out <- numeric(length(x))
  out[x >= 0 & x < 1] <- .5
  out[x >= 1 & x < 1.5] <- 1
  out
}

cdf_vec <- function(x) {
  out <- numeric(length(x))
  tmp <- x >= 0 & x < 1
  out[tmp] <- .5*x[tmp]
  tmp <- x >= 1 & x < 1.5
  out[tmp] <- x[tmp] - .5
  tmp <- x >= 1.5
  out[tmp] <- 1
  out
}

set.seed(42)
x <- rnorm(1e6)


stopifnot(all.equal(cdf(x), cdf1(x)))
stopifnot(all.equal(pdf(x), pdf1(x)))

@Allan Camero already showed nicely how to plot it.

Microbenchmark

It's about three times faster than the ifelse solution.

microbenchmark::microbenchmark(
  cdf_ifelse=cdf(x), cdf_vec=cdf1(x), check='equal'
)
# Unit: milliseconds
#       expr       min        lq     mean    median        uq       max neval cld
# cdf_ifelse 110.66285 121.25428 133.0789 133.86041 142.53296 167.64401   100   b
#    cdf_vec  43.56277  45.08759  48.8924  46.83869  49.46047  74.95487   100  a 

microbenchmark::microbenchmark(
  pdf_ifelse=pdf(x), pdf_vec=pdf1(x), check='equal'
)
# Unit: milliseconds
#       expr      min       lq      mean   median       uq      max neval cld
# pdf_ifelse 71.39501 76.71747 101.17738 81.43037 87.82162 192.3919   100   b
#    pdf_vec 27.82642 30.02056  34.55301 30.38457 31.29751 133.2798   100  a