[R] avoiding loops

[Liaw, Andy]

I didn't read the question carefully.  You were asking all pairwise
distances between the two sets, right?

This isn't the most efficient way to do it, but it seems OK for problem of
the size you stated:

> pairdist <- function(m1, m2) {
+     idx1 <- rep(1:nrow(m1), each=nrow(m2))
+     idx2 <- rep(1:nrow(m2), nrow(m1))
+     d <- sqrt(rowSums((m1[idx1,] - m2[idx2,])^2))
+     dim(d) <- c(nrow(m1), nrow(m2))
+     d
+ }
> d1 <- matrix(rnorm(3e3), 1e3, 3)
> d2 <- matrix(rnorm(3e3), 1e3, 3)
> system.time(dm <- pairdist(d1, d2), gcFirst=TRUE)
[1] 0.73 0.13 0.92   NA   NA
> str(dm)
 num [1:1000, 1:1000] 2.44 2.21 2.45 1.58 3.58 ...

Andy


> From: Liaw, Andy
> 
> Doesn't this work?
> 
> > d1 <- data.frame(x=rnorm(100), y=rnorm(100), z=rnorm(100))
> > d2 <- data.frame(x=rnorm(100), y=rnorm(100), z=rnorm(100))
> > dist.12 <- sqrt(rowSums((d1 - d2)^2))
> 
> Andy
> 
> > From: dax42
> > 
> > Hi again,
> > 
> > thanks a lot for the quick answer. I just forgot the comma, always 
> > these stupid mistakes...
> > 
> > Anyways, as I said before, I have two data.frames containing 
> > about 1000 
> > rows and I would like to avoid looping through all of them...
> > 
> > In each data.frame are coordinates (x,y,z), so every row is 
> > giving the 
> > information on one single point.
> > I would like to calculate the distance from each point in the 
> > one frame 
> > to every point in the second...
> > But how to do this without loops, how to do it quickly? I would 
> > appreciate it very much, if someone would introduce me to the 
> > high art 
> > of R - avoiding loops :-).
> > 
> > Cheers, Dax.
> > 
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> >
> 
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