Le 17/11/12 14:42, Hadley Wickham a écrit :
Hi all,
I've included what seems to be a simple application of Rcpp sugar
below, but I'm getting some very strange results. Any help would be
much appreciate!
Thanks,
Hadley
library(Rcpp)
library(microbenchmark)
# Compute distance between single point and vector of points
pdist1 <- function(x, ys) {
(x - ys) ^ 2
}
cppFunction('
NumericVector pdist2(double x, NumericVector ys) {
int n = ys.size();
NumericVector out(n);
for(int i = 0; i < n; ++i) {
out[i] = pow(ys[i] - x, 2);
}
return out;
}
')
ys <- runif(1e4)
all.equal(pdist1(0.5, ys), pdist2(0.5, ys))
library(microbenchmark)
microbenchmark(
pdist1(0.5, ys),
pdist2(0.5, ys)
)
# C++ version about twice as fast, presumably because it avoids a
# complete vector allocation.
# Sugar version:
cppFunction('
NumericVector pdist3(double x, NumericVector ys) {
return pow((x - ys), 2);
}
')
all.equal(pdist1(0.5, ys), pdist3(0.5, ys))
microbenchmark(
pdist1(0.5, ys),
pdist2(0.5, ys),
pdist3(0.5, ys)
)
# 10-fold slower?? Maybe it's because I'm using a double instead of
# a numeric vector?
That's weird. Not sure where the ineffiency is. Some hunting.
My guess is that sugar pow needs a refresh.
cppFunction('
NumericVector pdist4(NumericVector x, NumericVector ys) {
return pow((x - ys), 2);
}
')
all.equal(pdist1(0.5, ys), pdist4(0.5, ys))
# Is this a bug in sugar? Should recycle to length of longest vector.
# Let's try flipping the order of operations:
There's no recycling. We pick the size of "x" and consider that y is of
the same length. Something worth mentionning in documentation. Recycling
would add some expense.
cppFunction('
NumericVector pdist5(NumericVector x, NumericVector ys) {
return pow((ys - x), 2);
}
')
all.equal(pdist1(0.5, ys), pdist5(0.5, ys))
# Where are the missing values coming from??
Same here. We pick the length of ys (the lhs of the binary operation).
Both pdist4 and pdist5 are undeterminate behaviour.
I would prefer to keep it that way and document it that vector should
have the same size, ... rather than introduce the extra cost of recyling.
--
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