R how to retrieve data from data.frame with multiple conditions

Question

I am wondering how to perform some basic data manipulation in R. What i want to do is the following.

I have a data table with the following pattern :

  V1     V2     V3    
  ABC     X     24
  ABC     Y     30
  EFG     X     4
  EFG     Y     28
  HIJ     P     40
  HIJ     Y     41
  PKL     X     32

1. Now i want to retrieve all the values/pairs of V1 where it doesn't have a corresponding value which is not X on V2. In the above dataset this subset would be

   HIJ     P    40
   HIJ     Y    41
   

Since neither of the pair of HIJ have a V2 value of X.

2. I would also like to retrieve all values of V1 that don't repeat twice. In the above example it would be

   PKL  X 32
   

Answer 1

You mentioned data.table, so here's two possible approaches for both requests

library(data.table)

For 1.

setDT(df)[, .SD[all(V2 != "X")], by = V1]
#     V1 V2 V3
# 1: HIJ  P 40
# 2: HIJ  Y 41

For 2.

df[, .SD[.N == 1L], by = V1]
#     V1 V2 V3
# 1: PKL  X 32

Or (a bit more optimized version)

indx <- df[, .(indx = .I[.N == 1L]), by = V1]$indx
df[indx]
#     V1 V2 V3
# 1: PKL  X 32