Summarize data.table by group

Question

I am working with a huge data table in R containing monthly measurements of temperature for multiple locations, taken by different sources.

The dataset looks like this:<

Answer 1

If we are using data.table, the CJ can be used

 CJ(Location=loc, date= dates,Model= mods)[, 
         Temperature:= temp][, .(Mean = mean(Temperature)), by = .(Location, date)]