Tests for empty data frames.

[liori]

> inner_join(data.frame(x=c()), data.frame(x=c(1)), by="x")
Error: index out of bounds

I would expect the result to be equivalent to data.frame(x=c()).

[hs3180]

and summarise doesn't works too
summarise(data.frame(a = numeric(0)), sum(a))

[romainfrancois]

The problem for the original issue is that c() gives NULL and not a vector, e.g this works:

inner_join(data.frame(x=numeric(0)), data.frame(x=c(1)), by="x")

The original data not only has no rows but also no columns:

> data <- data.frame(x = c())
> data
tableau de données (data frame) avec 0 colonnes et 0 lignes

[romainfrancois]

And I just fixed the second problem, i.e. the one reported by @hs3180 which was hidden deep inside.

> summarise(data.frame(a = numeric(0)), sum(a) )
  sum(a)
1      0