How can I sample from a two-dimensional grid of points
you can still use sample(), e.g., n.grid <- 500 muA.grid <- seq(-4, 4, length = n.grid) muB.grid <- seq(-4, 4, length = n.grid) vals <- data.matrix(expand.grid(muA.grid, muB.grid)) mu.p <- dnorm(vals[, 1]) * dnorm(vals[, 2], sd = 0.5) mu.p <- mu.p / sum(mu.p) ind <- 1:nrow(vals) samp.ind <- sample(ind, 1000, TRUE, mu.p) mu <- vals[samp.ind, ] head(mu, 10) I hope it helps. Best, Dimitris ---- Dimitris Rizopoulos Biostatistical Centre School of Public Health Catholic University of Leuven Address: Kapucijnenvoer 35, Leuven, Belgium Tel: +32/(0)16/336899 Fax: +32/(0)16/337015 Web: http://med.kuleuven.be/biostat/ http://www.student.kuleuven.be/~m0390867/dimitris.htm ----- Original Message ----- From: "Gregory Gentlemen" <gregory_gentlemen at yahoo.ca> To: <r-help at stat.math.ethz.ch> Sent: Monday, March 10, 2008 3:32 PM Subject: [R] How can I sample from a two-dimensional grid of points
Hi everyone,
My goal is to sample from a two-dimensional grid. Consider the
following example of code:
n.grid <- 500
muA.grid <- seq(-4,4, length=n.grid)
muB.grid <- seq(-4,4, length=n.grid)
mu.p <- matrix(NA, nrow=n.grid, ncol=n.grid)
for(i in 1:n.grid){
for(j in 1:n.grid){
mu.p[i,j] <- dnorm(muA.grid[i], 0, 1)*dnorm(muB.grid[j], 0, 0.5)
}
}
mu.p <- mu.p/sum(mu.p)
I would now like to sample the grid of points from the
probabilities in mu.p. Im using the multivariate normal here for
illustration as my real problem is a more complicated probability
density. If this problem were only one-dimensional, this is easy:
n.samples <- 1000
# assuming mu.p and muA.grid are now the appropriate vectors
mu <- sample(muA.grid, n.samples, replace=T, prob=mu.p)
However, im not sure how to do this in two-dimensions in R.
Thanks in advance for any help.
All the best,
Gregory Gentlemen
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