dplyr negative select if df not null-CodePudding

I run an api extract each morning. There is a variable bla that may or may not be null. If it's not null it is a dataframe that contains a field sessions, in which case I would like to deselect sessions.

Normally, bla is indeed a dataframe and not null and the following block runs fine:

bla |> ... dplyr chain here ... |> select(-sessions)

But in those cases where bla is just null, I need this code to still run and not buckle my workflow.

Tried:

bla |> ... dplyr chain here ... |> select(!any_of('sessions'))

But this errors with:

Error in UseMethod("select") : 
  no applicable method for 'select' applied to an object of class "NULL"

How can I tell dply to run the select command only if bla is not null?

CodePudding user response：

There are couple of ways to deal with this. Use an if condition on exists (if we have created an object earlier in the step)

library(dplyr)
if(exists('bla')) {
      bla |> 
       select(!any_of('sessions'))
  }

Or wrap with tryCatch

tryCatch(bla |> 
      select(!any_of('sessions')), error = function(e) NULL)

Based on the OP's update in comments, if this is part of a chain, we can check if the number of rows are greater than 0 before we select

ab_tests <- get_ga_df(viewId, c(start_date, end_date), ab_metrics, ab_dims) %>%  
     index_zen_name(dimension_lookup_marketing) %>%    
    dedup_key(c('session_id', 'date')) %>%  {  
      if(!is.null(.)) {          
        select(., !any_of('sessions'))
       } else .
   }