Merge csv-3.2.3

lib/csv/table.rb

@@ -3,31 +3,199 @@
 require "forwardable"
 
 class CSV
+  # = \CSV::Table
+  # A \CSV::Table instance represents \CSV data.
+  # (see {class CSV}[../CSV.html]).
   #
-  # A CSV::Table is a two-dimensional data structure for representing CSV
-  # documents. Tables allow you to work with the data by row or column,
-  # manipulate the data, and even convert the results back to CSV, if needed.
+  # The instance may have:
+  # - Rows: each is a Table::Row object.
+  # - Headers: names for the columns.
   #
-  # All tables returned by CSV will be constructed from this class, if header
-  # row processing is activated.
+  # === Instance Methods
   #
+  # \CSV::Table has three groups of instance methods:
+  # - Its own internally defined instance methods.
+  # - Methods included by module Enumerable.
+  # - Methods delegated to class Array.:
+  #   * Array#empty?
+  #   * Array#length
+  #   * Array#size
+  #
+  # == Creating a \CSV::Table Instance
+  #
+  # Commonly, a new \CSV::Table instance is created by parsing \CSV source
+  # using headers:
+  #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+  #   table = CSV.parse(source, headers: true)
+  #   table.class # => CSV::Table
+  #
+  # You can also create an instance directly. See ::new.
+  #
+  # == Headers
+  #
+  # If a table has headers, the headers serve as labels for the columns of data.
+  # Each header serves as the label for its column.
+  #
+  # The headers for a \CSV::Table object are stored as an \Array of Strings.
+  #
+  # Commonly, headers are defined in the first row of \CSV source:
+  #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+  #   table = CSV.parse(source, headers: true)
+  #   table.headers # => ["Name", "Value"]
+  #
+  # If no headers are defined, the \Array is empty:
+  #   table = CSV::Table.new([])
+  #   table.headers # => []
+  #
+  # == Access Modes
+  #
+  # \CSV::Table provides three modes for accessing table data:
+  # - \Row mode.
+  # - Column mode.
+  # - Mixed mode (the default for a new table).
+  #
+  # The access mode for a\CSV::Table instance affects the behavior
+  # of some of its instance methods:
+  # - #[]
+  # - #[]=
+  # - #delete
+  # - #delete_if
+  # - #each
+  # - #values_at
+  #
+  # === \Row Mode
+  #
+  # Set a table to row mode with method #by_row!:
+  #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+  #   table = CSV.parse(source, headers: true)
+  #   table.by_row! # => #<CSV::Table mode:row row_count:4>
+  #
+  # Specify a single row by an \Integer index:
+  #   # Get a row.
+  #   table[1] # => #<CSV::Row "Name":"bar" "Value":"1">
+  #   # Set a row, then get it.
+  #   table[1] = CSV::Row.new(['Name', 'Value'], ['bam', 3])
+  #   table[1] # => #<CSV::Row "Name":"bam" "Value":3>
+  #
+  # Specify a sequence of rows by a \Range:
+  #   # Get rows.
+  #   table[1..2] # => [#<CSV::Row "Name":"bam" "Value":3>, #<CSV::Row "Name":"baz" "Value":"2">]
+  #   # Set rows, then get them.
+  #   table[1..2] = [
+  #     CSV::Row.new(['Name', 'Value'], ['bat', 4]),
+  #     CSV::Row.new(['Name', 'Value'], ['bad', 5]),
+  #   ]
+  #   table[1..2] # => [["Name", #<CSV::Row "Name":"bat" "Value":4>], ["Value", #<CSV::Row "Name":"bad" "Value":5>]]
+  #
+  # === Column Mode
+  #
+  # Set a table to column mode with method #by_col!:
+  #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+  #   table = CSV.parse(source, headers: true)
+  #   table.by_col! # => #<CSV::Table mode:col row_count:4>
+  #
+  # Specify a column by an \Integer index:
+  #   # Get a column.
+  #   table[0]
+  #   # Set a column, then get it.
+  #   table[0] = ['FOO', 'BAR', 'BAZ']
+  #   table[0] # => ["FOO", "BAR", "BAZ"]
+  #
+  # Specify a column by its \String header:
+  #   # Get a column.
+  #   table['Name'] # => ["FOO", "BAR", "BAZ"]
+  #   # Set a column, then get it.
+  #   table['Name'] = ['Foo', 'Bar', 'Baz']
+  #   table['Name'] # => ["Foo", "Bar", "Baz"]
+  #
+  # === Mixed Mode
+  #
+  # In mixed mode, you can refer to either rows or columns:
+  # - An \Integer index refers to a row.
+  # - A \Range index refers to multiple rows.
+  # - A \String index refers to a column.
+  #
+  # Set a table to mixed mode with method #by_col_or_row!:
+  #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+  #   table = CSV.parse(source, headers: true)
+  #   table.by_col_or_row! # => #<CSV::Table mode:col_or_row row_count:4>
+  #
+  # Specify a single row by an \Integer index:
+  #   # Get a row.
+  #   table[1] # => #<CSV::Row "Name":"bar" "Value":"1">
+  #   # Set a row, then get it.
+  #   table[1] = CSV::Row.new(['Name', 'Value'], ['bam', 3])
+  #   table[1] # => #<CSV::Row "Name":"bam" "Value":3>
+  #
+  # Specify a sequence of rows by a \Range:
+  #   # Get rows.
+  #   table[1..2] # => [#<CSV::Row "Name":"bam" "Value":3>, #<CSV::Row "Name":"baz" "Value":"2">]
+  #   # Set rows, then get them.
+  #   table[1] = CSV::Row.new(['Name', 'Value'], ['bat', 4])
+  #   table[2] = CSV::Row.new(['Name', 'Value'], ['bad', 5])
+  #   table[1..2] # => [["Name", #<CSV::Row "Name":"bat" "Value":4>], ["Value", #<CSV::Row "Name":"bad" "Value":5>]]
+  #
+  # Specify a column by its \String header:
+  #   # Get a column.
+  #   table['Name'] # => ["foo", "bat", "bad"]
+  #   # Set a column, then get it.
+  #   table['Name'] = ['Foo', 'Bar', 'Baz']
+  #   table['Name'] # => ["Foo", "Bar", "Baz"]
   class Table
+    # :call-seq:
+    #   CSV::Table.new(array_of_rows, headers = nil) -> csv_table
     #
-    # Constructs a new CSV::Table from +array_of_rows+, which are expected
-    # to be CSV::Row objects. All rows are assumed to have the same headers.
+    # Returns a new \CSV::Table object.
     #
-    # The optional +headers+ parameter can be set to Array of headers.
-    # If headers aren't set, headers are fetched from CSV::Row objects.
-    # Otherwise, headers() method will return headers being set in
-    # headers argument.
+    # - Argument +array_of_rows+ must be an \Array of CSV::Row objects.
+    # - Argument +headers+, if given, may be an \Array of Strings.
     #
-    # A CSV::Table object supports the following Array methods through
-    # delegation:
+    # ---
     #
-    # * empty?()
-    # * length()
-    # * size()
+    # Create an empty \CSV::Table object:
+    #   table = CSV::Table.new([])
+    #   table # => #<CSV::Table mode:col_or_row row_count:1>
     #
+    # Create a non-empty \CSV::Table object:
+    #   rows = [
+    #     CSV::Row.new([], []),
+    #     CSV::Row.new([], []),
+    #     CSV::Row.new([], []),
+    #   ]
+    #   table  = CSV::Table.new(rows)
+    #   table # => #<CSV::Table mode:col_or_row row_count:4>
+    #
+    # ---
+    #
+    # If argument +headers+ is an \Array of Strings,
+    # those Strings become the table's headers:
+    #   table = CSV::Table.new([], headers: ['Name', 'Age'])
+    #   table.headers # => ["Name", "Age"]
+    #
+    # If argument +headers+ is not given and the table has rows,
+    # the headers are taken from the first row:
+    #   rows = [
+    #     CSV::Row.new(['Foo', 'Bar'], []),
+    #     CSV::Row.new(['foo', 'bar'], []),
+    #     CSV::Row.new(['FOO', 'BAR'], []),
+    #   ]
+    #   table  = CSV::Table.new(rows)
+    #   table.headers # => ["Foo", "Bar"]
+    #
+    # If argument +headers+ is not given and the table is empty (has no rows),
+    # the headers are also empty:
+    #   table  = CSV::Table.new([])
+    #   table.headers # => []
+    #
+    # ---
+    #
+    # Raises an exception if argument +array_of_rows+ is not an \Array object:
+    #   # Raises NoMethodError (undefined method `first' for :foo:Symbol):
+    #   CSV::Table.new(:foo)
+    #
+    # Raises an exception if an element of +array_of_rows+ is not a \CSV::Table object:
+    #   # Raises NoMethodError (undefined method `headers' for :foo:Symbol):
+    #   CSV::Table.new([:foo])
     def initialize(array_of_rows, headers: nil)
       @table = array_of_rows
       @headers = headers
@@ -54,88 +222,141 @@ class CSV
     extend Forwardable
     def_delegators :@table, :empty?, :length, :size
 
+    # :call-seq:
+    #   table.by_col -> table_dup
     #
-    # Returns a duplicate table object, in column mode. This is handy for
-    # chaining in a single call without changing the table mode, but be aware
-    # that this method can consume a fair amount of memory for bigger data sets.
+    # Returns a duplicate of +self+, in column mode
+    # (see {Column Mode}[#class-CSV::Table-label-Column+Mode]):
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   table.mode # => :col_or_row
+    #   dup_table = table.by_col
+    #   dup_table.mode # => :col
+    #   dup_table.equal?(table) # => false # It's a dup
     #
-    # This method returns the duplicate table for chaining. Don't chain
-    # destructive methods (like []=()) this way though, since you are working
-    # with a duplicate.
+    # This may be used to chain method calls without changing the mode
+    # (but also will affect performance and memory usage):
+    #   dup_table.by_col['Name']
     #
+    # Also note that changes to the duplicate table will not affect the original.
     def by_col
       self.class.new(@table.dup).by_col!
     end
 
+    # :call-seq:
+    #   table.by_col! -> self
     #
-    # Switches the mode of this table to column mode. All calls to indexing and
-    # iteration methods will work with columns until the mode is changed again.
-    #
-    # This method returns the table and is safe to chain.
-    #
+    # Sets the mode for +self+ to column mode
+    # (see {Column Mode}[#class-CSV::Table-label-Column+Mode]); returns +self+:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   table.mode # => :col_or_row
+    #   table1 = table.by_col!
+    #   table.mode # => :col
+    #   table1.equal?(table) # => true # Returned self
     def by_col!
       @mode = :col
 
       self
     end
 
+    # :call-seq:
+    #   table.by_col_or_row -> table_dup
     #
-    # Returns a duplicate table object, in mixed mode. This is handy for
-    # chaining in a single call without changing the table mode, but be aware
-    # that this method can consume a fair amount of memory for bigger data sets.
+    # Returns a duplicate of +self+, in mixed mode
+    # (see {Mixed Mode}[#class-CSV::Table-label-Mixed+Mode]):
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true).by_col!
+    #   table.mode # => :col
+    #   dup_table = table.by_col_or_row
+    #   dup_table.mode # => :col_or_row
+    #   dup_table.equal?(table) # => false # It's a dup
     #
-    # This method returns the duplicate table for chaining.  Don't chain
-    # destructive methods (like []=()) this way though, since you are working
-    # with a duplicate.
+    # This may be used to chain method calls without changing the mode
+    # (but also will affect performance and memory usage):
+    #   dup_table.by_col_or_row['Name']
     #
+    # Also note that changes to the duplicate table will not affect the original.
     def by_col_or_row
       self.class.new(@table.dup).by_col_or_row!
     end
 
+    # :call-seq:
+    #   table.by_col_or_row! -> self
     #
-    # Switches the mode of this table to mixed mode. All calls to indexing and
-    # iteration methods will use the default intelligent indexing system until
-    # the mode is changed again. In mixed mode an index is assumed to be a row
-    # reference while anything else is assumed to be column access by headers.
-    #
-    # This method returns the table and is safe to chain.
-    #
+    # Sets the mode for +self+ to mixed mode
+    # (see {Mixed Mode}[#class-CSV::Table-label-Mixed+Mode]); returns +self+:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true).by_col!
+    #   table.mode # => :col
+    #   table1 = table.by_col_or_row!
+    #   table.mode # => :col_or_row
+    #   table1.equal?(table) # => true # Returned self
     def by_col_or_row!
       @mode = :col_or_row
 
       self
     end
 
+    # :call-seq:
+    #   table.by_row -> table_dup
     #
-    # Returns a duplicate table object, in row mode.  This is handy for chaining
-    # in a single call without changing the table mode, but be aware that this
-    # method can consume a fair amount of memory for bigger data sets.
+    # Returns a duplicate of +self+, in row mode
+    # (see {Row Mode}[#class-CSV::Table-label-Row+Mode]):
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   table.mode # => :col_or_row
+    #   dup_table = table.by_row
+    #   dup_table.mode # => :row
+    #   dup_table.equal?(table) # => false # It's a dup
     #
-    # This method returns the duplicate table for chaining.  Don't chain
-    # destructive methods (like []=()) this way though, since you are working
-    # with a duplicate.
+    # This may be used to chain method calls without changing the mode
+    # (but also will affect performance and memory usage):
+    #   dup_table.by_row[1]
     #
+    # Also note that changes to the duplicate table will not affect the original.
     def by_row
       self.class.new(@table.dup).by_row!
     end
 
+    # :call-seq:
+    #   table.by_row! -> self
     #
-    # Switches the mode of this table to row mode. All calls to indexing and
-    # iteration methods will work with rows until the mode is changed again.
-    #
-    # This method returns the table and is safe to chain.
-    #
+    # Sets the mode for +self+ to row mode
+    # (see {Row Mode}[#class-CSV::Table-label-Row+Mode]); returns +self+:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   table.mode # => :col_or_row
+    #   table1 = table.by_row!
+    #   table.mode # => :row
+    #   table1.equal?(table) # => true # Returned self
     def by_row!
       @mode = :row
 
       self
     end
 
+    # :call-seq:
+    #   table.headers -> array_of_headers
     #
-    # Returns the headers for the first row of this table (assumed to match all
-    # other rows). The headers Array passed to CSV::Table.new is returned for
-    # empty tables.
+    # Returns a new \Array containing the \String headers for the table.
     #
+    # If the table is not empty, returns the headers from the first row:
+    #   rows = [
+    #     CSV::Row.new(['Foo', 'Bar'], []),
+    #     CSV::Row.new(['FOO', 'BAR'], []),
+    #     CSV::Row.new(['foo', 'bar'], []),
+    #   ]
+    #   table  = CSV::Table.new(rows)
+    #   table.headers # => ["Foo", "Bar"]
+    #   table.delete(0)
+    #   table.headers # => ["FOO", "BAR"]
+    #   table.delete(0)
+    #   table.headers # => ["foo", "bar"]
+    #
+    # If the table is empty, returns a copy of the headers in the table itself:
+    #   table.delete(0)
+    #   table.headers # => ["Foo", "Bar"]
     def headers
       if @table.empty?
         @headers.dup
@@ -145,17 +366,21 @@ class CSV
     end
 
     # :call-seq:
-    #   table[n] -> row
-    #   table[range] -> array_of_rows
-    #   table[header] -> array_of_fields
+    #   table[n] -> row or column_data
+    #   table[range] -> array_of_rows or array_of_column_data
+    #   table[header] -> array_of_column_data
     #
     # Returns data from the table;  does not modify the table.
     #
     # ---
     #
-    # The expression <tt>table[n]</tt>, where +n+ is a non-negative \Integer,
-    # returns the +n+th row of the table, if that row exists,
-    # and if the access mode is <tt>:row</tt> or <tt>:col_or_row</tt>:
+    # Fetch a \Row by Its \Integer Index::
+    # - Form: <tt>table[n]</tt>, +n+ an integer.
+    # - Access mode: <tt>:row</tt> or <tt>:col_or_row</tt>.
+    # - Return value: _nth_ row of the table, if that row exists;
+    #   otherwise +nil+.
+    #
+    # Returns the _nth_ row of the table if that row exists:
     #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
     #   table = CSV.parse(source, headers: true)
     #   table.by_row! # => #<CSV::Table mode:row row_count:4>
@@ -168,20 +393,45 @@ class CSV
     #
     # Returns +nil+ if +n+ is too large or too small:
     #   table[4] # => nil
-    #   table[-4] => nil
+    #   table[-4] # => nil
     #
     # Raises an exception if the access mode is <tt>:row</tt>
-    # and +n+ is not an
-    # {Integer-convertible object}[https://docs.ruby-lang.org/en/master/implicit_conversion_rdoc.html#label-Integer-Convertible+Objects].
+    # and +n+ is not an \Integer:
     #   table.by_row! # => #<CSV::Table mode:row row_count:4>
     #   # Raises TypeError (no implicit conversion of String into Integer):
     #   table['Name']
     #
     # ---
     #
-    # The expression <tt>table[range]</tt>, where +range+ is a Range object,
-    # returns rows from the table, beginning at row <tt>range.first</tt>,
-    # if those rows exist, and if the access mode is <tt>:row</tt> or <tt>:col_or_row</tt>:
+    # Fetch a Column by Its \Integer Index::
+    # - Form: <tt>table[n]</tt>, +n+ an \Integer.
+    # - Access mode: <tt>:col</tt>.
+    # - Return value: _nth_ column of the table, if that column exists;
+    #   otherwise an \Array of +nil+ fields of length <tt>self.size</tt>.
+    #
+    # Returns the _nth_ column of the table if that column exists:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   table.by_col! # => #<CSV::Table mode:col row_count:4>
+    #   table[1] # => ["0", "1", "2"]
+    #
+    # Counts backward from the last column if +n+ is negative:
+    #   table[-2] # => ["foo", "bar", "baz"]
+    #
+    # Returns an \Array of +nil+ fields if +n+ is too large or too small:
+    #   table[4] # => [nil, nil, nil]
+    #   table[-4] # => [nil, nil, nil]
+    #
+    # ---
+    #
+    # Fetch Rows by \Range::
+    # - Form: <tt>table[range]</tt>, +range+ a \Range object.
+    # - Access mode: <tt>:row</tt> or <tt>:col_or_row</tt>.
+    # - Return value: rows from the table, beginning at row <tt>range.start</tt>,
+    #   if those rows exists.
+    #
+    # Returns rows from the table, beginning at row <tt>range.first</tt>,
+    # if those rows exist:
     #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
     #   table = CSV.parse(source, headers: true)
     #   table.by_row! # => #<CSV::Table mode:row row_count:4>
@@ -191,11 +441,11 @@ class CSV
     #   rows = table[1..2] # => #<CSV::Row "Name":"bar" "Value":"1">
     #   rows # => [#<CSV::Row "Name":"bar" "Value":"1">, #<CSV::Row "Name":"baz" "Value":"2">]
     #
-    # If there are too few rows, returns all from <tt>range.first</tt> to the end:
+    # If there are too few rows, returns all from <tt>range.start</tt> to the end:
     #   rows = table[1..50] # => #<CSV::Row "Name":"bar" "Value":"1">
     #   rows # => [#<CSV::Row "Name":"bar" "Value":"1">, #<CSV::Row "Name":"baz" "Value":"2">]
     #
-    # Special case:  if <tt>range.start == table.size</tt>, returns an empty \Array:
+    # Special case: if <tt>range.start == table.size</tt>, returns an empty \Array:
     #   table[table.size..50] # => []
     #
     # If <tt>range.end</tt> is negative, calculates the ending index from the end:
@@ -211,9 +461,41 @@ class CSV
     #
     # ---
     #
-    # The expression <tt>table[header]</tt>, where +header+ is a \String,
-    # returns column values (\Array of \Strings) if the column exists
-    # and if the access mode is <tt>:col</tt> or <tt>:col_or_row</tt>:
+    # Fetch Columns by \Range::
+    # - Form: <tt>table[range]</tt>, +range+ a \Range object.
+    # - Access mode: <tt>:col</tt>.
+    # - Return value: column data from the table, beginning at column <tt>range.start</tt>,
+    #   if those columns exist.
+    #
+    # Returns column values from the table, if the column exists;
+    # the values are arranged by row:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   table.by_col!
+    #   table[0..1] # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]
+    #
+    # Special case: if <tt>range.start == headers.size</tt>,
+    # returns an \Array (size: <tt>table.size</tt>) of empty \Arrays:
+    #   table[table.headers.size..50] # => [[], [], []]
+    #
+    # If <tt>range.end</tt> is negative, calculates the ending index from the end:
+    #   table[0..-1] # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]
+    #
+    # If <tt>range.start</tt> is negative, calculates the starting index from the end:
+    #   table[-2..2] # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]
+    #
+    # If <tt>range.start</tt> is larger than <tt>table.size</tt>,
+    # returns an \Array of +nil+ values:
+    #   table[4..4] # => [nil, nil, nil]
+    #
+    # ---
+    #
+    # Fetch a Column by Its \String Header::
+    # - Form: <tt>table[header]</tt>, +header+ a \String header.
+    # - Access mode: <tt>:col</tt> or <tt>:col_or_row</tt>
+    # - Return value: column data from the table, if that +header+ exists.
+    #
+    # Returns column values from the table, if the column exists:
     #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
     #   table = CSV.parse(source, headers: true)
     #   table.by_col! # => #<CSV::Table mode:col row_count:4>
@@ -238,22 +520,132 @@ class CSV
       end
     end
 
+    # :call-seq:
+    #   table[n] = row -> row
+    #   table[n] = field_or_array_of_fields -> field_or_array_of_fields
+    #   table[header] = field_or_array_of_fields -> field_or_array_of_fields
     #
-    # In the default mixed mode, this method assigns rows for index access and
-    # columns for header access. You can force the index association by first
-    # calling by_col!() or by_row!().
+    # Puts data onto the table.
     #
-    # Rows may be set to an Array of values (which will inherit the table's
-    # headers()) or a CSV::Row.
+    # ---
     #
-    # Columns may be set to a single value, which is copied to each row of the
-    # column, or an Array of values. Arrays of values are assigned to rows top
-    # to bottom in row major order. Excess values are ignored and if the Array
-    # does not have a value for each row the extra rows will receive a +nil+.
+    # Set a \Row by Its \Integer Index::
+    # - Form: <tt>table[n] = row</tt>, +n+ an \Integer,
+    #   +row+ a \CSV::Row instance or an \Array of fields.
+    # - Access mode: <tt>:row</tt> or <tt>:col_or_row</tt>.
+    # - Return value: +row+.
     #
-    # Assigning to an existing column or row clobbers the data. Assigning to
-    # new columns creates them at the right end of the table.
+    # If the row exists, it is replaced:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   new_row = CSV::Row.new(['Name', 'Value'], ['bat', 3])
+    #   table.by_row! # => #<CSV::Table mode:row row_count:4>
+    #   return_value = table[0] = new_row
+    #   return_value.equal?(new_row) # => true # Returned the row
+    #   table[0].to_h # => {"Name"=>"bat", "Value"=>3}
     #
+    # With access mode <tt>:col_or_row</tt>:
+    #   table.by_col_or_row! # => #<CSV::Table mode:col_or_row row_count:4>
+    #   table[0] = CSV::Row.new(['Name', 'Value'], ['bam', 4])
+    #   table[0].to_h # => {"Name"=>"bam", "Value"=>4}
+    #
+    # With an \Array instead of a \CSV::Row, inherits headers from the table:
+    #   array = ['bad', 5]
+    #   return_value = table[0] = array
+    #   return_value.equal?(array) # => true # Returned the array
+    #   table[0].to_h # => {"Name"=>"bad", "Value"=>5}
+    #
+    # If the row does not exist, extends the table by adding rows:
+    # assigns rows with +nil+ as needed:
+    #   table.size # => 3
+    #   table[5] = ['bag', 6]
+    #   table.size # => 6
+    #   table[3] # => nil
+    #   table[4]# => nil
+    #   table[5].to_h # => {"Name"=>"bag", "Value"=>6}
+    #
+    # Note that the +nil+ rows are actually +nil+, not a row of +nil+ fields.
+    #
+    # ---
+    #
+    # Set a Column by Its \Integer Index::
+    # - Form: <tt>table[n] = array_of_fields</tt>, +n+ an \Integer,
+    #   +array_of_fields+ an \Array of \String fields.
+    # - Access mode: <tt>:col</tt>.
+    # - Return value: +array_of_fields+.
+    #
+    # If the column exists, it is replaced:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   new_col = [3, 4, 5]
+    #   table.by_col! # => #<CSV::Table mode:col row_count:4>
+    #   return_value = table[1] = new_col
+    #   return_value.equal?(new_col) # => true # Returned the column
+    #   table[1] # => [3, 4, 5]
+    #   # The rows, as revised:
+    #   table.by_row! # => #<CSV::Table mode:row row_count:4>
+    #   table[0].to_h # => {"Name"=>"foo", "Value"=>3}
+    #   table[1].to_h # => {"Name"=>"bar", "Value"=>4}
+    #   table[2].to_h # => {"Name"=>"baz", "Value"=>5}
+    #   table.by_col! # => #<CSV::Table mode:col row_count:4>
+    #
+    # If there are too few values, fills with +nil+ values:
+    #   table[1] = [0]
+    #   table[1] # => [0, nil, nil]
+    #
+    # If there are too many values, ignores the extra values:
+    #   table[1] = [0, 1, 2, 3, 4]
+    #   table[1] # => [0, 1, 2]
+    #
+    # If a single value is given, replaces all fields in the column with that value:
+    #   table[1] = 'bat'
+    #   table[1] # => ["bat", "bat", "bat"]
+    #
+    # ---
+    #
+    # Set a Column by Its \String Header::
+    # - Form: <tt>table[header] = field_or_array_of_fields</tt>,
+    #   +header+ a \String header, +field_or_array_of_fields+ a field value
+    #   or an \Array of \String fields.
+    # - Access mode: <tt>:col</tt> or <tt>:col_or_row</tt>.
+    # - Return value: +field_or_array_of_fields+.
+    #
+    # If the column exists, it is replaced:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   new_col = [3, 4, 5]
+    #   table.by_col! # => #<CSV::Table mode:col row_count:4>
+    #   return_value = table['Value'] = new_col
+    #   return_value.equal?(new_col) # => true # Returned the column
+    #   table['Value'] # => [3, 4, 5]
+    #   # The rows, as revised:
+    #   table.by_row! # => #<CSV::Table mode:row row_count:4>
+    #   table[0].to_h # => {"Name"=>"foo", "Value"=>3}
+    #   table[1].to_h # => {"Name"=>"bar", "Value"=>4}
+    #   table[2].to_h # => {"Name"=>"baz", "Value"=>5}
+    #   table.by_col! # => #<CSV::Table mode:col row_count:4>
+    #
+    # If there are too few values, fills with +nil+ values:
+    #   table['Value'] = [0]
+    #   table['Value'] # => [0, nil, nil]
+    #
+    # If there are too many values, ignores the extra values:
+    #   table['Value'] = [0, 1, 2, 3, 4]
+    #   table['Value'] # => [0, 1, 2]
+    #
+    # If the column does not exist, extends the table by adding columns:
+    #   table['Note'] = ['x', 'y', 'z']
+    #   table['Note'] # => ["x", "y", "z"]
+    #   # The rows, as revised:
+    #   table.by_row!
+    #   table[0].to_h # => {"Name"=>"foo", "Value"=>0, "Note"=>"x"}
+    #   table[1].to_h # => {"Name"=>"bar", "Value"=>1, "Note"=>"y"}
+    #   table[2].to_h # => {"Name"=>"baz", "Value"=>2, "Note"=>"z"}
+    #   table.by_col!
+    #
+    # If a single value is given, replaces all fields in the column with that value:
+    #   table['Value'] = 'bat'
+    #   table['Value'] # => ["bat", "bat", "bat"]
     def []=(index_or_header, value)
       if @mode == :row or  # by index
          (@mode == :col_or_row and index_or_header.is_a? Integer)
@@ -463,6 +855,9 @@ class CSV
       end
     end
 
+    # :call-seq:
+    #   table.delete_if {|row_or_column| ... } -> self
+    #
     # Removes rows or columns for which the block returns a truthy value;
     # returns +self+.
     #
@@ -506,6 +901,9 @@ class CSV
 
     include Enumerable
 
+    # :call-seq:
+    #  table.each {|row_or_column| ... ) -> self
+    #
     # Calls the block with each row or column; returns +self+.
     #
     # When the access mode is <tt>:row</tt> or <tt>:col_or_row</tt>,
@@ -534,7 +932,9 @@ class CSV
       return enum_for(__method__) { @mode == :col ? headers.size : size } unless block_given?
 
       if @mode == :col
-        headers.each { |header| yield([header, self[header]]) }
+        headers.each.with_index do |header, i|
+          yield([header, @table.map {|row| row[header, i]}])
+        end
       else
         @table.each(&block)
       end
@@ -542,6 +942,9 @@ class CSV
       self # for chaining
     end
 
+    # :call-seq:
+    #   table == other_table -> true or false
+    #
     # Returns +true+ if all each row of +self+ <tt>==</tt>
     # the corresponding row of +other_table+, otherwise, +false+.
     #
@@ -565,10 +968,14 @@ class CSV
       @table == other
     end
 
+    # :call-seq:
+    #   table.to_a -> array_of_arrays
     #
-    # Returns the table as an Array of Arrays. Headers will be the first row,
-    # then all of the field rows will follow.
-    #
+    # Returns the table as an \Array of \Arrays;
+    # the headers are in the first row:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   table.to_a # => [["Name", "Value"], ["foo", "0"], ["bar", "1"], ["baz", "2"]]
     def to_a
       array = [headers]
       @table.each do |row|
@@ -578,16 +985,29 @@ class CSV
       array
     end
 
+    # :call-seq:
+    #   table.to_csv(**options) -> csv_string
     #
-    # Returns the table as a complete CSV String. Headers will be listed first,
-    # then all of the field rows.
+    # Returns the table as \CSV string.
+    # See {Options for Generating}[../CSV.html#class-CSV-label-Options+for+Generating].
     #
-    # This method assumes you want the Table.headers(), unless you explicitly
-    # pass <tt>:write_headers => false</tt>.
+    # Defaults option +write_headers+ to +true+:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   table.to_csv # => "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
     #
-    def to_csv(write_headers: true, **options)
+    # Omits the headers if option +write_headers+ is given as +false+
+    # (see {Option +write_headers+}[../CSV.html#class-CSV-label-Option+write_headers]):
+    #   table.to_csv(write_headers: false) # => "foo,0\nbar,1\nbaz,2\n"
+    #
+    # Limit rows if option +limit+ is given like +2+:
+    #   table.to_csv(limit: 2) # => "Name,Value\nfoo,0\nbar,1\n"
+    def to_csv(write_headers: true, limit: nil, **options)
       array = write_headers ? [headers.to_csv(**options)] : []
-      @table.each do |row|
+      limit ||= @table.size
+      limit = @table.size + 1 + limit if limit < 0
+      limit = 0 if limit < 0
+      @table.first(limit).each do |row|
         array.push(row.fields.to_csv(**options)) unless row.header_row?
       end
 
@@ -613,9 +1033,24 @@ class CSV
       end
     end
 
-    # Shows the mode and size of this table in a US-ASCII String.
+    # :call-seq:
+    #   table.inspect => string
+    #
+    # Returns a <tt>US-ASCII</tt>-encoded \String showing table:
+    # - Class: <tt>CSV::Table</tt>.
+    # - Access mode: <tt>:row</tt>, <tt>:col</tt>, or <tt>:col_or_row</tt>.
+    # - Size:  Row count, including the header row.
+    #
+    # Example:
+    #   source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #   table = CSV.parse(source, headers: true)
+    #   table.inspect # => "#<CSV::Table mode:col_or_row row_count:4>\nName,Value\nfoo,0\nbar,1\nbaz,2\n"
+    #
     def inspect
-      "#<#{self.class} mode:#{@mode} row_count:#{to_a.size}>".encode("US-ASCII")
+      inspected = +"#<#{self.class} mode:#{@mode} row_count:#{to_a.size}>"
+      summary = to_csv(limit: 5)
+      inspected << "\n" << summary if summary.encoding.ascii_compatible?
+      inspected
     end
   end
 end