subsetting and which

[jeroen]

When first learning subsetting, a common mistake is to use x[which(y)] instead of x[y]. Here the which() achieves nothing: it switches from logical to integer subsetting, but the result will be exactly the same.

I'm not sure I agree; if length(which(y)) << length(y) there can be obvious performance benefits:

x <- runif(1e8)
x[1] <- NA;

system.time(x[is.na(x)])
system.time(x[which(is.na(x))])

[hadley]

I agree, but I think this is a subtlety beyond the scope of the text at this point.