Replace a subset of a data frame with dplyr join operations

Question

Suppose that I gave a treatment to some column values of a data frame like this:

  id animal weight   height ...
  1    dog     23.0
  2    cat     NA
  3           


        

Answer 1

What you describe is a join operation in which you update some values in the original dataset. This is very easy to do with great performance using data.table because of its fast joins and update-by-reference concept (:=).

Here's an example for your toy data:

library(data.table)
setDT(df)             # convert to data.table without copy
setDT(sub_df)         # convert to data.table without copy

# join and update "df" by reference, i.e. without copy 
df[sub_df, on = c("id", "animal"), weight := i.weight]

The data is now updated:

#   id animal weight
#1:  1    dog   23.0
#2:  2    cat    2.2
#3:  3   duck    1.2
#4:  4  fairy    0.2
#5:  5  snake    1.3

You can use setDF to switch back to ordinary data.frame.