在R中满足条件时向数据框添加值

Question

I have a dataframe like this:

ID1   ID2   Position  Grade   Day
234   756      2        87     27
245   486      4        66     26
321   275      1        54     20
768   656      6        51     7
421   181      1        90     14
237   952      8        68     23
237   553      4        32     30

And I have another dataframe like this:

ID1   ID2   Day  Count
234   756    2     3 
245   486    2     1
209   706    2     1
124   554    2     2
237   553    2     4

I need to add the Counts to the first dataframe where the ID1, ID2 and Day are matched. However, I also need to have it so that if there is no match (no Counts in the second dataframe for the set of ID1, ID2 and Day in the first dataframe) then a zero is put in that place. So the final dataframe would be something like:

ID1   ID2   Position  Grade   Day   Count
234   756      2        87     27     3
245   486      4        66     26     1
321   275      1        54     20     0
768   656      6        51     7      0
421   181      1        90     14     0
237   952      8        68     23     0
237   553      4        32     30     4

Answer 1

This can be useful

> # First, merge df1 and df2
> df3 <- merge(df1, df2, by=c("ID1", "ID2"), all.x=TRUE)
> # Replace NA with 0's
> transform(df3[, -6], Count=ifelse(is.na(Count), 0, Count))
  ID1 ID2 Position Grade Day.x Count
1 234 756        2    87    27     3
2 237 553        4    32    30     4
3 237 952        8    68    23     0
4 245 486        4    66    26     1
5 321 275        1    54    20     0
6 421 181        1    90    14     0
7 768 656        6    51     7     0