How to replace values with table map in R

Question

I have two dataframes, one with 'encoded' values and another one that acts as a dictionary:

> head( encoded_values )
value
1
2
1
3

> head( dict )
id    name
1     foo
2     bar
3     baz

I want to replace the values in the first data frame with the 'decoded' values by looking up the second data frame. This should be the result:

> head( encoded_values )
foo
bar
foo
baz

I've found many similar posts but nothing specific to my case. Maybe it's quite a common operation but I'm very new to R and quite lost with all the many possibilities I've tried so far (none of which worked).

Many thanks.

Answer 1

This is what match (note, much faster than merge) is made for:

dict[match(encoded_values$value, dict$id), 2, drop=F]

produces (we need drop=F so that a data.frame is returned instead of a vector since we're selecting only one column):

    name
1    foo
2    bar
1.1  foo
3    baz

match returns the location of the values in it's first argument, in the second argument. You can then use this to index the second argument.

To actually replace:

encoded_values$value <- with(dict, name[match(encoded_values$value, id)])

Note, in this simple case, because your ids match with row numbers in dict, you can also do:

dict[encoded_values$value, 2, drop=F]

but this only works because of the special nature of the id variable in dict (starts at 1, increments by 1 each).