How to remove rows that have only 1 combination for a given ID

Question

I have a dataframe like this

ID <- c(\"A\",\"A\",\"A\",\"B\",\"B\",\"C\",\"C\")
Measurement <- c (\"Length\",\"Breadth\",\"Breadth\",\"Breadth\",\"Leng         


        

Answer 1

Base R: Give a logical value created by ave to the i parameter of "[":

 df[ ave( as.character(df$Measurement), df$ID, FUN=function(x) length(unique(x)) ) > 1, ]
  ID Measurement Value
1  A      Length   4.5
2  A     Breadth   6.6
3  A     Breadth   7.5
4  B     Breadth   3.3
5  B      Length   5.6

This is a case where I think the base solution is "dominated" in terms of easy of programming and intelligibility by both the data.table and the dplyr solutions offered. That is not often the case in my experience.

Answer 2

In dplyr, it would be

library(dplyr)

df %>% group_by(ID) %>% filter(n_distinct(Measurement) > 1)
##       ID Measurement Value
##   <fctr>      <fctr> <dbl>
## 1      A      Length   4.5
## 2      A     Breadth   6.6
## 3      A     Breadth   7.5
## 4      B     Breadth   3.3
## 5      B      Length   5.6

Answer 3

Using data.table:

library(data.table)
DT <- data.table(df)
DT[, Count := length(unique(Measurement)), ID][Count > 1]

Edit

Alternatively, a much better one-liner suggested by @DavidArenburg:

setDT(df)[, if(uniqueN(Measurement) > 1) .SD, by = ID]

Answer 4

Another option is using anyDuplicated

df[with(df, ave(as.character(Measurement), ID, FUN = anyDuplicated)!=0),]