The other day I got stuck working with a huge data set using data.table in R. It took me a little while to realise that I had to produce a minimal reproducible example to actually understand why I got stuck in the first place. I know, this is the mantra I should follow before I reach out to R-help, Stack Overflow or indeed the package authors. Of course, more often than not, by following this advise, the problem becomes clear and with that the solution obvious.
The example I present here is a little silly, yet it illustrates how to join tables with data.table in R. Mapping old data to new dataCategories in general are never fixed, they always change at some point. And then the trouble starts with the data. For example not that long ago we didn’t distinguish between smartphones and dumbphones, or video on demand and video rental shops. I would like to back track price change data for smartphones and online movie rental shops, assuming that their earlier development can be set to the categories they were formerly part of, namely mobile and video rental shops to create indices.