How I can select rows from a dataframe that do not match?
If I understand correctly, you need the negation of the %in% operator. Something like this should work:
subset(b, !(y %in% a$x))
> subset(b, !(y %in% a$x))
y
5 5
6 6
Try the set difference function setdiff. So you would have
results1 = setdiff(a$x, b$y) # elements in a$x NOT in b$y
results2 = setdiff(b$y, a$x) # elements in b$y NOT in a$x
You could also use dplyr for this task. To find what is in b but not a:
library(dplyr)
anti_join(b, a, by = c("y" = "x"))
# y
# 1 5
# 2 6