Simple lookup to insert values in an R data frame

Question

This is a seemingly simple R question, but I don't see an exact answer here. I have a data frame (alldata) that looks like this:

Case     zip     market
1        44485   NA
2        44488   NA
3        43210   NA

There are over 3.5 million records.

Then, I have a second data frame, 'zipcodes'.

market    zip
1         44485
1         44486
1         44488
...       ... (100 zips in market 1)
2         43210
2         43211
...       ... (100 zips in market 2, etc.)

I want to find the correct value for alldata$market for each case based on alldata$zip matching the appropriate value in the zipcode data frame. I'm just looking for the right syntax, and assistance is much appreciated, as usual.

Answer 1

Since you don't care about the market column in alldata, you can first strip it off using and merge the columns in alldata and zipcodes based on the zip column using merge:

merge(alldata[, c("Case", "zip")], zipcodes, by="zip")

The by parameter specifies the key criteria, so if you have a compound key, you could do something like by=c("zip", "otherfield").