table.rb in fat_table-0.4.2

- old
+ new

@@ -67,10 +67,27 @@
       @boundaries = []
     end
 
     # :category: Constructors
 
+    # Return an empty duplicate of self.  This allows the library to create an
+    # empty table that preserves all the instance variables from self.  Even
+    # though FatTable::Table objects have no instance variables, a class that
+    # inherits from it might.
+    def empty_dup
+      self.dup.__empty!
+    end
+
+    def __empty!
+      @columns = []
+      @boundaries = []
+      self
+    end
+
+
+    # :category: Constructors
+
     # Construct a Table from the contents of a CSV file named +fname+. Headers
     # will be taken from the first CSV row and converted to symbols.
     def self.from_csv_file(fname)
       File.open(fname, 'r') do |io|
         from_csv_io(io)
@@ -595,12 +612,16 @@
         key1 = sort_heads.map { |h| rev_heads.include?(h) ? r2[h] : r1[h] }
         key2 = sort_heads.map { |h| rev_heads.include?(h) ? r1[h] : r2[h] }
         key1 <=> key2
       end
       # Add the new rows to the table, but mark a group boundary at the points
-      # where the sort key changes value.
-      new_tab = Table.new
+      # where the sort key changes value.  NB: I use self.class.new here
+      # rather than Table.new because if this class is inherited, I want the
+      # new_tab to be an instance of the subclass.  With Table.new, this
+      # method's result will be an instance of FatTable::Table rather than of
+      # the subclass.
+      new_tab = empty_dup
       last_key = nil
       new_rows.each_with_index do |nrow, k|
         new_tab << nrow
         key = nrow.fetch_values(*sort_heads)
         new_tab.mark_boundary(k - 1) if last_key && key != last_key
@@ -711,11 +732,11 @@
       end
       ev = Evaluator.new(ivars: ivars,
                          before: before_hook,
                          after: after_hook)
       # Compute the new Table from this Table
-      result = Table.new
+      result = empty_dup
       normalize_boundaries
       rows.each_with_index do |old_row, old_k|
         # Set the group number in the before hook and run the hook with the
         # local variables set to the row before the new row is evaluated.
         grp = row_index_to_group_index(old_k)
@@ -768,11 +789,11 @@
     #
     #   tab.where('date > Date.today - 30') => rows with recent dates
     #   tab.where('@row.even? && shares > 500') => even rows with lots of shares
     def where(expr)
       expr = expr.to_s
-      result = Table.new
+      result = empty_dup
       headers.each do |h|
         col = Column.new(header: h)
         result.add_column(col)
       end
       ev = Evaluator.new(ivars: { row: 0, group: 0 })
@@ -790,11 +811,11 @@
     # :category: Operators
 
     # Return a new table with all duplicate rows eliminated. Resets groups. Same
     # as #uniq.
     def distinct
-      result = Table.new
+      result = empty_dup
       uniq_rows = rows.uniq
       uniq_rows.each do |row|
         result << row
       end
       result
@@ -902,11 +923,11 @@
       unless columns.map(&:type) == other.columns.map(&:type)
         msg = "can't apply a set ops to tables with different column types."
         raise UserError, msg
       end
       other_rows = other.rows.map { |r| r.replace_keys(headers) }
-      result = Table.new
+      result = empty_dup
       new_rows = rows.send(oper, other_rows)
       new_rows.each_with_index do |row, k|
         result << row
         result.mark_boundary if k == size - 1 && add_boundaries
       end
@@ -1009,11 +1030,11 @@
       self_row_nils = headers.map { |h| [h, nil] }.to_h
       other_row_nils = other.headers.map { |h| [h, nil] }.to_h
       join_exp, other_common_heads =
         build_join_expression(exps, other, join_type)
       ev = Evaluator.new
-      result = Table.new
+      result = empty_dup
       other_rows = other.rows
       other_row_matches = Array.new(other_rows.size, false)
       rows.each do |self_row|
         self_row_matched = false
         other_rows.each_with_index do |other_row, k|
@@ -1257,10 +1278,10 @@
     def group_by(*group_cols, **agg_cols)
       sorted_tab = order_by(group_cols)
       groups = sorted_tab.rows.group_by do |r|
         group_cols.map { |k| r[k] }
       end
-      result = Table.new
+      result = empty_dup
       groups.each_pair do |_vals, grp_rows|
         result << row_from_group(grp_rows, group_cols, agg_cols)
       end
       result.normalize_boundaries
       result