Question

While writing convenience functions for subset(), I ran into a strange situation where using equivalent logical statements returns different subsets. So, for example:

dat = data.frame(ttl.stims = c(4,4,8,8), change = c('big', 'small'))
dat
ttl.stims = 4

#logical statements are equivalent
dat$ttl.stims == 4
dat$ttl.stims == ttl.stims

#subset evaluates differently
subset(dat, dat$ttl.stims == 4)
subset(dat, dat$ttl.stims == ttl.stims)

I’ve been working around this by doing:

index = dat$ttl.stims == ttl.stims
subset(dat, index)

But I’m so curious about why the first two subsets don’t produce identical results! Ideas? Thoughts? Pontifications?

Answer 1

Because inside the call to subset the symbol ttl.stims gets interpreted in the environment of dat, so it becomes (after interpretation) dat$ttl.stims. I predict that the second call to subset returns the entire dataframe.