How to find number of unique ids corresponding to each date in a data drame

Question

I have a data frame that looks like this:

      date         time              id            datetime    
1 2015-01-02 14:27:22.130 999000000007628 2015-01-02 14         


        

Answer 1

You can use the uniqueN function from data.table:

library(data.table)
setDT(df)[, uniqueN(id), by = date]

or (as per the comment of @Richard Scriven):

aggregate(id ~ date, df, function(x) length(unique(x)))