有限制地删除r中重复的观测值

Question

I have a dataset with which contains duplicates of the ident variable. I need to select only 1 observation of each ident and it needs to be the newest value, ie the resulting data should contain the observation for the ident where the 'year' is the highest in the initial data set.

I believe a general case would look like this:

1. ident   value   year
 2. A       1       19X1
 3. A       2       19X2
 4. B       4       19X2
 5. B       2       19X1
 6. B       1       19X3
 7. C       1       19X4
 8. C       2       19X1

(I could not order it in a proper table here, so please disregard the numbered list on the left)

Only, I have several hundred thousands obs.

Order of the resulting data set is not important to me.

Answer 1

Using library dplyr you can do something like this:

library(dplyr)
df %>% group_by(ident) %>% arrange(desc(year)) %>% slice(1)

Output will be as follows:

Source: local data frame [3 x 4]
Groups: ident [3]

    X1. ident value  year
  (dbl) (chr) (int) (chr)
1     3     A     2  19X2
2     6     B     1  19X3
3     7     C     1  19X4

This assumes year is in a format where sorting in descending order makes it go from latest to oldest.

NOTE: that x1. column is a result of your input data above. I just read it as is.

Answer 2

Try

df <- do.call(rbind, lapply(split(df, df$ident), 
                            function(x) x[which.max(x$year), ]))