msh855 msh855 - 2 months ago 8
R Question

Finding factors that correspond to more than one values

Suppose, that one has the following dataframe:

x=data.frame(c(1,1,2,2,2,3),c("A","A","B","B","B","B"))
names(x)=c("v1","v2")

x
v1 v2
1 1 A
2 1 A
3 2 B
4 2 B
5 2 B
6 3 B


In this dataframe a value in
v1
I want to correspond into a label in
v2
. However, as one can see in this example
B
has more than one corresponding values.

Is there any elegant and fast way to find which labels in
v2
correspond to more than one values in
v1
?

The result I want ideally to show, the values - which in our example should be
c(2,3)
- as well as the row number - which in our example should be
r=c(5,6)
.

Answer

Assuming that we want the index of the unique elements in 'v1' grouped by 'v2' and that should have more than one unique elements, we create a logical index with ave and use that to subset the rows of 'x'.

i1 <- with(x, ave(v1, v2, FUN = function(x) 
    length(unique(x))>1 & !duplicated(x, fromLast=TRUE)))!=0
x[i1,]
#   v1 v2
#5  2  B
#6  3  B

Or a faster option is data.table

library(data.table)
i1 <- setDT(x)[, .I[uniqueN(v1)>1 & !duplicated(v1, fromLast=TRUE)], v2]$V1
x[i1, 'v1', with = FALSE][, rn := i1][]
#   v1 rn
#1:  2  5
#2:  3  6
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