Magritttr + lapply where first argument isn't to LHS

Question

I'd like to pass a data frame into lapply via %>%, but I need to be able to access the names of the columns, so my lapply arguments are something like this:

mydf %>%
   lapply( 1:length(.), function(x) {
        manipulate_df( mydf[x], using_column_names(names(mydf)[x] )
   })

However, when I try that, I get the following error:

Error in match.fun(FUN) :
   '1:length(.)' is not a function, character or symbol

As far as I can tell R and lapply don't like 1:length(.). I suppose a valid option is breaking the chain, but I'd like to learn how to do it properly.

Answer 1

Your issue here is that %>% is inserting mydf as the first argument (so that three arguments are getting passed to lapply. Try wrapping the entire lapply expression in brackets. This prevents the insertion behavior:

mydf %>%
   { lapply( 1:length(.), function(x) {
        manipulate_df( mydf[x], using_column_names(names(mydf)[x] )
   }) }

I think the prettiest fix would be to make a new function:

manipulate_whole_df = function(mydf)
  lapply( 1:length(mydf), function(x)
            manipulate_df( mydf[x], using_column_names(names(mydf)[x] ) ) )

mydf %>%
  manipulate_whole_df

Or even

library(tidyr)

mydf %>%
  gather(variable, value) %>%
  group_by(variable) %>%
  do(manipulate_df(.$value, 
                   .$variable %>% first %>% using_column_name ) )