data.table replace data using values from another data.table, conditionally

Question

This is similar to Update values in data.table with values from another data.table and R data.table replacing an index of values from another data.table, except in my situation

Answer 1

This code should work in the current format given your criteria.

dt_original[dt_newdata, names(dt_newdata) := Map(pmax, mget(names(dt_newdata)), dt_newdata)]

It joins to the IDs that match between the data.tables and then performs an assignment using := Because we want to return a list, I use Map to run pmax through the columns of data.tables matching by the name of dt_newdata. Note that it is necessary that all names of dt_newdata are in dt_original data.

Following Frank's comment, you can remove the first column of the Map list items and the column names using [-1] because they are IDs, which don't need to be computed. Removing the first column from Map avoids one pass of pmax and also preserves the key on id. Thanks to @brian-stamper for pointing out the key preservation in the comments.

dt_original[dt_newdata,
            names(dt_newdata)[-1] := Map(pmax,
                                         mget(names(dt_newdata)[-1]),
                                         dt_newdata[, .SD, .SDcols=-1])]

Note that the use of [-1] assumes that the ID variable is located in the first position of new_data. If it is elsewhere, you could change the index manually or use grep.