如何根据R中的索引更改data.frame中的列？

Question

I have a data.frame with two columns: Name and Index and 2 million rows.

I am sure that all index were written correctly, but I need to verify the 'Name' column.

How do I do to change all Name values based on Index values.

Let me give an example. Suppose we have the following data.frame 'db':

db
Index Name
1      Carlos
2      John
3      Bill
4      Mary
1      Cerlas

As it is shown, 'Name' should be equal for every Index value, but someone write it incorrectly.

How would I correct it? Is there a solution employing 'dplyr' or 'tidyr'?

I tried the following code, but it has not worked.

for (i in unique(db$Index)) {
    db$Nome[db$Index==i] <- db$Nome[db$Index==i][1]
}

Thanks

Answer 1

If somebody named it correctly for the first element in 'Name' for each 'Index', we can use data.table to assign the 'Name' as the first element of 'Name' grouped by 'Index'.

library(data.table)
setDT(db)[, Name:= Name[1L], by = Index]

Answer 2

You could do it in dplyr. Here we are making the first name given for each Index the name for the entire index:

library(dplyr)
dat %>% group_by(Index) %>%
        mutate(Name = Name[1])