Question

I would like to add a column to a data frame with the count of matches in another data frame, this seems pretty trivial but I can’t seem to get it to work. Example:

smaller_df$CountOfMatches <- nrow(subset(larger_df, Date == smaller_df$Date))

This gives me the error:

In `==.default`(Date, smaller_df$Date) :
  longer object length is not a multiple of shorter object length

I know the data frames are different lengths, I’m not asking for a merge, I simply need for that for every row/date (valid date object) in the smaller_df; a count how many matches in the larger_df.

I’m very new to R so there must be something fundamental and very trivial I’m missing here.

Thanks in advance

Answer 1

Here’s what seems fairly straightforward:

smaller_df$bigDfCount <-sapply( smaller_df$Date,  
                        FUN=function(x) length(larger_df[larger_df$Date==x, "Date"] ) )
smaller_df

         Date bigDfCount
1  2000-01-01          4
2  2000-01-02          2
3  2000-01-03          5
4  2000-01-04          4
5  2000-01-05          5
6  2000-01-06          6
7  2000-01-07          2
8  2000-01-08          5
9  2000-01-09          3
10 2000-01-10          3