Mutate by group with less boilerplate
    
    mutate_by(df, group_var, ...)
    
     Arguments
    
    
    
      | df | data frame | 
    
      | group_var | bare/unquoted column name to group by | 
    
      | ... | expressions as you would normally use in mutate | 
    
    
    Value
    ungrouped data frame containing results of grouped summary