Find how many times duplicated rows repeat in R data frame [duplicate]

Question

I have a data frame like the following example

a = c(1, 1, 1, 2, 2, 3, 4, 4) b = c(3.5, 3.5, 2.5, 2, 2, 1, 2.2, 7) df <-data.frame(a,b) 

I can remove duplicated rows from R data frame by the following code, but how can I find how many times each duplicated rows repeated? I need the result as a vector.

unique(df) 

or

df[!duplicated(df), ] 

Answer 1

Here is solution using function ddply() from library plyr

library(plyr) ddply(df,.(a,b),nrow)    a   b V1 1 1 2.5  1 2 1 3.5  2 3 2 2.0  2 4 3 1.0  1 5 4 2.2  1 6 4 7.0  1 

Answer 2

You could always kill two birds with the one stone:

aggregate(list(numdup=rep(1,nrow(df))), df, length) # or even: aggregate(numdup ~., data=transform(df,numdup=1), length) # or even: aggregate(cbind(df[0],numdup=1), df, length)    a   b numdup 1 3 1.0      1 2 2 2.0      2 3 4 2.2      1 4 1 2.5      1 5 1 3.5      2 6 4 7.0      1