Two random variables $X$ and $Y$ are distributed according to \begin{align} f_{XY}(x,y)= \begin{cases} x+y & 0\leq x \leq 1, 0\leq y \leq 1 \\ 0 & otherwise \end{cases} \end{align}

I want to generate rvs of the given function using python. Is there any way to do it ?

  • $\begingroup$ What does "rvs" stand for? $\endgroup$ – Wolfgang Bangerth Apr 4 at 1:59
  • $\begingroup$ "Random values" $\endgroup$ – Robert Kern Apr 5 at 3:52

Rejection sampling is straightforward to implement for this case.

import numpy as np

def rng_xpy(n, rng=None, chunk_size=1024):
    rng = np.random.default_rng(rng)
    rvs = []
    n_drawn = 0
    while n_drawn < n:
        # Draw numbers from U(0, 1) for x, y, and z
        # We draw numbers in chunks for efficiency
        x, y, z = rng.random((3, chunk_size))
        # Scale z to the maximum value of x+y
        z *= 2
        # We only want to use (x, y) values when z is less than x+y
        good_mask = z <= (x + y)
        xy = np.column_stack([x, y])[good_mask]
        n_drawn += len(xy)
    # Because we draw in chunks, we probably drew too many.
    # Assemble the final array and trim it to the required number.
    xy = np.concatenate(rvs, axis=0)[:n]
    return xy
  • $\begingroup$ Thank your for your answer but I dont really understand this part:# Scale z to the maximum value of x+y. can you please elaborate $\endgroup$ – leo Apr 4 at 16:21
  • $\begingroup$ As described in the article, the proposal distribution that we sample (x,y,z) from needs to always be above the target distribution function. We are using a very simple proposal distribution that is just uniformly sampling from the bounding box defined by f(x, y). The maximum value of f(x,y) is at f(1, 1)=1+1=2, so we need to sample z from the U(0,2). Sampling from U(0,1) using the random() method then multiplying by 2 is just a convenience. There are lots of other ways to write that code. $\endgroup$ – Robert Kern Apr 5 at 3:52

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