R: Transform dataframe column using dictionary/list?

Question

I have a dataframe column with numerous textual values (levels). I need to map those values to a predefined object-like structure in order to reduce the number of levels. I could easily achieve this in Python using a dictionary but could not do the same with a list in R.

For example, my dataframe column is something like:

df <- data.frame(weather = c('Clear','Snow','Clear','Rain','Rain','Other','Hail/sleet','Unknown'))

I need to map this to a list like

weather.levels <- list(
  dry = c('Clear', 'Cloudy'),
  wet = c('Snow', 'Rain', 'Hail/sleet'),
  other = c('Other','Unknown'))

so that my transformed dataframe looks like

    old.weather new.weather
1       Clear         dry
2        Snow         wet
3       Clear         dry
4        Rain         wet
5        Rain         wet
6      Other1       other
7  Hail/sleet         wet
8     Unknown       other

I have looked at solutions like this and this, but these do not answer my question. I cannot create a dataframe to use R's match function because the number of levels in each category of the preset dictionary weather.levels ('dry', 'wet', 'other') are different.

Answer 1

As there often is, there is a base R function designed to do exactly this. levels<- is what you want:

Note that the df$weather variable needs to be a factor variable for this to work appropriately (the below code without explicitly changing to a factor first worked pre R 4.0 because df$weather was a factor by default in the data.frame call).

df$new.weather <- `levels<-`(df$weather, weather.levels)
## if variable not already a factor, instead:
df$new.weather <- `levels<-`(factor(df$weather), weather.levels)
df
#     weather new.weather
#1      Clear         dry
#2       Snow         wet
#3      Clear         dry
#4       Rain         wet
#5       Rain         wet
#6      Other       other
#7 Hail/sleet         wet
#8    Unknown       other

In a slightly longer but simpler to read form this is equivalent to:

df$new.weather <- df$weather
levels(df$new.weather) <- weather.levels