How to plot a function that calculates the PDF?

问题:

So this is the PDF that my function calculates:

fx = 0.3 if (0<=x<1) 0.1 if (1<=x<2) 0.25 if (2<=x<3) 0.15 if (3<=x<4) 0.2 if (4<=x<5) 0 otherwise

And this is my coding for it:

    fx = function(x)
    { 
    if ((0<=x) & (x<1)) 0.3
    else if ((1<=x) & (x<2)) 0.1
    else if ((2<=x) & (x<3)) 0.25
    else if ((3<=x) & (x<4)) 0.15
    else if ((4<=x) & (x<5)) 0.2
    else 0
    }

Now how would I go about plotting y=fx?
I've tried:

    x <- runif(n,0,5)
    y <- fx(x)
    plot(x, y, type='1', xlim=c(0,5), ylim=c(0,5))

But I get an error that 'x' and 'y' have differing lengths?

回答1:

Your problems comes down to the fact your function isn't vectorized properly (it doesn't deal with a vector well).

If you use the accepted solution from your previous question about exactly the same problem then you won't have any issues

eg

# a solution that will work and be properly vectorized
fx <- function(x) c(0, 0.3,0.1,0.25,0.15,0.20, 0)[findInterval(x, c(-Inf, 0:5, Inf))]


 x <- runif(n,0,5)

plot(x, fx(x))

If you want to plot a step function (which is what this pdf is), you can use stepfun

eg

fx <- stepfun(x = 0:5, y = c(0,0.3,0.1,0.25,0.15,0.20,0))
plot(fx, ylim = c(0,0.4),xlim = c(0,5), main = 'f(x)')

If you don't want the points added then

plot(fx, ylim = c(0,0.4),xlim = c(0,5), main = 'f(x)', do.points=FALSE)

If you want to vectorize a step function, then use Vectorize

 vfx <- Vectorize(fx)

回答2:

Your PDF is not vectorized. Try this:

fx <- function(x) {
  ifelse((0<=x) & (x<1), 0.3,
  ifelse((1<=x) & (x<2), 0.1,
  ifelse((2<=x) & (x<3), 0.25,
  ifelse((3<=x) & (x<4), 0.15,
  ifelse((4<=x) & (x<5), 0.2,
  0)))))
}

x <- seq(0, 6, length.out=n)
plot(x, fx(x))