How I can select rows from a dataframe that do not match?

Question

I'm trying to identify the values in a data frame that do not match, but can't figure out how to do this.

# make data frame 
a <- data.frame( x =  c(1,2,3,4)) 
b <- data.frame( y =  c(1,2,3,4,5,6))

# select only values from b that are not in 'a'
# attempt 1: 
results1 <- b$y[ !a$x ]

# attempt 2:  
results2 <- b[b$y != a$x,]

If a = c(1,2,3) this works, as a is a multiple of b. However, I'm trying to just select all the values from data frame y, that are not in x, and don't understand what function to use.

Answer 1

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

Answer 2

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

Answer 3

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