generating a vector of difference between two vectors

Question

I have two csv files, and each of which consists of one column of data

For instance, vecA.csv is like

id 1 2 

vecB.csv is like

id 3 2 

I read the data set as follows:

vectorA<-read.table("vecA.csv",sep=",",header=T) vectorB<-read.table("vecB.csv",sep=",",header=T) 

I want to generate a vector consisting of elements belonging to B only.

Answer 1

You are looking for the function setdiff

setdiff(vectorB$id, vectorA$id) 

If you did not want this reduced to unique values, you could create a not in function

(kudos to @joran here Match with negation)

'%nin%' <- Negate('%in%')  vectorB$id[vectorB$id %nin% vectorA$id] 

Answer 2

If your vector's are instead data.tables, then all you need are five characters:

B[!A] 

---

library(data.table)  # read in your data, wrap in data.table(..., key="id")  A <- data.table(read.table("vecA.csv",sep=",",header=T), key="id") B <- data.table(read.table("vecB.csv",sep=",",header=T), key="id")  # Then this is all you need B[!A] 

---

[Matthew] And in v1.8.7 it's simpler and faster to read the file as well :

A <- setkey(fread("vecA.csv"), id) B <- setkey(fread("vecB.csv"), id) B[!A]