ruby--ruby/doc/csv/recipes.rdoc

== Recipes

All code snippets on this page assume that the following has been executed:
  require 'csv'

=== Contents

- {Parsing: Source Formats}[#label-Parsing-3A+Source+Formats]
  - {Parse from String}[#label-Parse+from+String]
    - {Parse from String with Headers}[#label-Parse+from+String+with+Headers]
    - {Parse from String Without Headers}[#label-Parse+from+String+Without+Headers]
  - {Parse from File}[#label-Parse+from+File]
    - {Parse from File with Headers}[#label-Parse+from+File+with+Headers]
    - {Parse from File Without Headers}[#label-Parse+from+File+Without+Headers]
  - {Parse from IO Stream}[#label-Parse+from+IO+Stream]
    - {Parse from IO Stream with Headers}[#label-Parse+from+IO+Stream+with+Headers]
    - {Parse from IO Stream Without Headers}[#label-Parse+from+IO+Stream+Without+Headers]
- {Parsing: Field Converters}[#label-Parsing-3A+Field+Converters]
  - {Convert Fields to Objects}[#label-Convert+Fields+to+Objects]
    - {Convert Fields to Objects Using Built-In Converters}[#label-Convert+Fields+to+Objects+Using+Built-In+Converters]
    - {Convert Fields to Objects Using Custom Converters}[#label-Convert+Fields+to+Objects+Using+Custom+Converters]
  - {Filter Field Strings}[#label-Filter+Field+Strings]
- {Generating: Output Formats}[#label-Generating-3A+Output+Formats]
  - {Generate to String}[#label-Generate+to+String]
    - {Generate to String with Headers}[#label-Generate+to+String+with+Headers]
    - {Generate to String Without Headers}[#label-Generate+to+String+Without+Headers]
  - {Generate to File}[#label-Generate+to+File]
    - {Generate to File with Headers}[#label-Generate+to+File+with+Headers]
    - {Generate to File Without Headers}[#label-Generate+to+File+Without+Headers]
  - {Generate to IO Stream}[#label-Generate+to+IO+Stream]
    - {Generate to IO Stream with Headers}[#label-Generate+to+IO+Stream+with+Headers]
    - {Generate to IO Stream Without Headers}[#label-Generate+to+IO+Stream+Without+Headers]
- {Filtering: Source and Output Formats}[#label-Filtering-3A+Source+and+Output+Formats]
  - {Filter String to String}[#label-Filter+String+to+String]
    - {Filter String to String with Headers}[#label-Filter+String+to+String+with+Headers]
    - {Filter String to String Without Headers}[#label-Filter+String+to+String+Without+Headers]
  - {Filter String to IO Stream}[#label-Filter+String+to+IO+Stream]
    - {Filter String to IO Stream with Headers}[#label-Filter+String+to+IO+Stream+with+Headers]
    - {Filter String to IO Stream Without Headers}[#label-Filter+String+to+IO+Stream+Without+Headers]
  - {Filter IO Stream to String}[#label-Filter+IO+Stream+to+String]
    - {Filter IO Stream to String with Headers}[#label-Filter+IO+Stream+to+String+with+Headers]
    - {Filter IO Stream to String Without Headers}[#label-Filter+IO+Stream+to+String+Without+Headers]
  - {Filter IO Stream to IO Stream}[#label-Filter+IO+Stream+to+IO+Stream]
    - {Filter IO Stream to IO Stream with Headers}[#label-Filter+IO+Stream+to+IO+Stream+with+Headers]
    - {Filter IO Stream to IO Stream Without Headers}[#label-Filter+IO+Stream+to+IO+Stream+Without+Headers]

=== Parsing: Source Formats

==== Parse from \String

===== Parse from \String with Headers

\Class method CSV.parse can read a source \String all at once,
and so may have memory resource implications:
  string = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  CSV.parse(string, headers: true) # => #<CSV::Table mode:col_or_row row_count:4>

Instance method CSV#each can read a source \String one row at a time:
  CSV.new(string, headers: true).each do |row|
    p row
  end
Ouput:
  #<CSV::Row "Name":"foo" "Value":"0">
  #<CSV::Row "Name":"bar" "Value":"1">
  #<CSV::Row "Name":"baz" "Value":"2">

===== Parse from \String Without Headers

\Class method CSV.parse can read a source \String all at once,
and so may have memory resource implications:
  string = "foo,0\nbar,1\nbaz,2\n"
  CSV.parse(string) # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Instance method CSV#each can read a source \String one row at a time:
  CSV.new(string).each do |row|
    p row
  end
Output:
  ["foo", "0"]
  ["bar", "1"]
  ["baz", "2"]

==== Parse from \File

===== Parse from \File with Headers

Instance method CSV#read can reada file all at once:
  string = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  path = 't.csv'
  File.write(path, string)
  CSV.read(path, headers: true) # => #<CSV::Table mode:col_or_row row_count:4>

\Class method CSV.foreach can read one row at a time:
  CSV.foreach(path, headers: true) do |row|
    p row
  end
Output:
  #<CSV::Row "Name":"foo" "Value":"0">
  #<CSV::Row "Name":"bar" "Value":"1">
  #<CSV::Row "Name":"baz" "Value":"2">

===== Parse from \File Without Headers

\Class method CSV.read can read a file all at once:
  string = "foo,0\nbar,1\nbaz,2\n"
  path = 't.csv'
  File.write(path, string)
  CSV.read(path) # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

\Class method CSV.foreach can read one row at a time:
  CSV.foreach(path) do |row|
    p row
  end
Output:
  ["foo", "0"]
  ["bar", "1"]
  ["baz", "2"]

==== Parse from \IO Stream

===== Parse from \IO Stream with Headers

\Class method CSV.parse can read an \IO stream all at once:
  string = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  path = 't.csv'
  File.write(path, string)
  File.open(path) do |file|
    CSV.parse(file, headers: true)
  end # => #<CSV::Table mode:col_or_row row_count:4>

\Class method CSV.foreach can read one row at a time:
  File.open(path) do |file|
    CSV.foreach(file, headers: true) do |row|
      p row
    end
  end
Output:
  #<CSV::Row "Name":"foo" "Value":"0">
  #<CSV::Row "Name":"bar" "Value":"1">
  #<CSV::Row "Name":"baz" "Value":"2">

===== Parse from \IO Stream Without Headers

\Class method CSV.parse can read an \IO stream all at once:
  string = "foo,0\nbar,1\nbaz,2\n"
  path = 't.csv'
  File.write(path, string)
  File.open(path) do |file|
    CSV.parse(file)
  end # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

\Class method CSV.foreach can read one row at a time:
  File.open(path) do |file|
    CSV.foreach(file) do |row|
      p row
    end
  end
Output:
  ["foo", "0"]
  ["bar", "1"]
  ["baz", "2"]

=== Parsing: Field Converters

==== Convert Fields to Objects

Use field converters to change parsed Strings into other, more specific, object.

==== Convert Fields to Objects Using Built-In Converters

Without converters (all fields parsed as Strings):
  source = "0,1.1,2020-09-19"
  parsed = CSV.parse(source)
  parsed # => [["0", "1.1", "2020-09-19"]]
  parsed.first.each {|field| p field.class }
Output:
  String
  String
  String

With built-in converters (see {Built-In Field Converters}[../../CSV.html#class-CSV-label-Built-In+Field+Converters]):
  parsed = CSV.parse(source, converters: :all)
  parsed # => [[0, 1.1, #<DateTime: 2020-09-19T00:00:00+00:00 ((2459112j,0s,0n),+0s,2299161j)>]]
  parsed.first.each {|field| p field.class }
Output:
  Integer
  Float
  DateTime

==== Convert Fields to Objects Using Custom Converters

This example defines and uses a custom field converter
that converts each column-1 value to a \Rational object.

Define a custom field converter:
  rational_converter = proc do |field, field_context|
    field_context.index == 1 ? field.to_r : field
  end

Without the new converter:
  string = "foo,0\nbar,1\nbaz,2\n"
  array = CSV.parse(string)
  array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

With the new converter:
  array = CSV.parse(string, converters: rational_converter)
  array # => [["foo", (0/1)], ["bar", (1/1)], ["baz", (2/1)]]

You can also register a custom field converter, then refer to it by name:
  CSV::Converters[:rational] = rational_converter
  array = CSV.parse(string, converters: :rational)
  array # => [["foo", (0/1)], ["bar", (1/1)], ["baz", (2/1)]]

==== Filter Field Strings

This example defines and uses a custom field converter
that strips whitespace from each field value.

Define a custom field converter:
  strip_converter = proc {|field| field.strip }

Without the new converter:
  string = " foo , 0 \n bar , 1 \n baz , 2 \n"
  array = CSV.parse(string)
  array # => [[" foo ", " 0 "], [" bar ", " 1 "], [" baz ", " 2 "]]

With the new converter:
  array = CSV.parse(string, converters: strip_converter)
  array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

You can also register a custom field converter, then refer to it by name:
  CSV::Converters[:strip] = strip_converter
  array = CSV.parse(string, converters: :strip)
  array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

=== Generating: Output Formats

==== Generate to \String

===== Generate to \String with Headers

\Class method CSV.generate can generate to a \String.

This example uses method CSV#<< to append the rows
that are to be generated:
  output_string = CSV.generate('', headers: ['Name', 'Value'], write_headers: true) do |csv|
    csv << ['Foo', 0]
    csv << ['Bar', 1]
    csv << ['Baz', 2]
  end
  output_string # => "Name,Value\nFoo,0\nBar,1\nBaz,2\n"

===== Generate to \String Without Headers

\Class method CSV.generate can generate to a \String.

This example uses method CSV#<< to append the rows
that are to be generated:
  output_string = CSV.generate do |csv|
    csv << ['Foo', 0]
    csv << ['Bar', 1]
    csv << ['Baz', 2]
  end
  output_string # => "Foo,0\nBar,1\nBaz,2\n"

==== Generate to \File

===== Generate to \File with Headers

\Class method CSV.open can generate to a \File.

This example uses method CSV#<< to append the rows
that are to be generated:
  path = 't.csv'
  CSV.open(path, 'w', headers: ['Name', 'Value'], write_headers: true) do |csv|
    csv << ['Foo', 0]
    csv << ['Bar', 1]
    csv << ['Baz', 2]
  end
  p File.read(path) # => "Name,Value\nFoo,0\nBar,1\nBaz,2\n"

===== Generate to \File Without Headers

\Class method CSV.open can generate to a \File.

This example uses method CSV#<< to append the rows
that are to be generated:
  path = 't.csv'
  CSV.open(path, 'w') do |csv|
    csv << ['Foo', 0]
    csv << ['Bar', 1]
    csv << ['Baz', 2]
  end
  p File.read(path) # => "Foo,0\nBar,1\nBaz,2\n"

==== Generate to \IO Stream

==== Generate to \IO Stream with Headers

\\Classs method CSV.new can generate \CSV data to an \IO stream:
  path = 't.csv'
  File.open(path, 'w') do |file|
    csv = CSV.new(file, headers: ['Name', 'Value'], write_headers: true)
    csv << ['Foo', 0]
    csv << ['Bar', 1]
    csv << ['Baz', 2]
  end
  p File.read(path) # => "Name,Value\nFoo,0\nBar,1\nBaz,2\n"

===== Generate to \IO Stream Without Headers

\Class method CSV.new can generate \CSV data to an \IO stream:
  path = 't.csv'
  File.open(path, 'w') do |file|
    csv = CSV.new(file)
    csv << ['Foo', 0]
    csv << ['Bar', 1]
    csv << ['Baz', 2]
  end
  p File.read(path) # => "Foo,0\nBar,1\nBaz,2\n"

=== Filtering: Source and Output Formats

\Class method CSV.filter provides a Unix-style filter for \CSV data.
The source \CSV data is processed to form output \CSV data.

==== Filter \String to \String

===== Filter \String to \String with Headers

  in_string = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  out_string = ''
  CSV.filter(in_string, out_string, headers: true) do |row|
    row[0] = row[0].upcase
    row[1] *= 4
  end
  out_string # => "Name,Value\nFOO,0000\nBAR,1111\nBAZ,2222\n"

===== Filter \String to \String Without Headers

  in_string = "foo,0\nbar,1\nbaz,2\n"
  out_string = ''
  CSV.filter(in_string, out_string) do |row|
    row[0] = row[0].upcase
    row[1] *= 4
  end
  out_string # => "FOO,0000\nBAR,1111\nBAZ,2222\n"

==== Filter \String to \IO Stream

===== Filter \String to \IO Stream with Headers

  in_string = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  path = 't.csv'
  File.open(path, 'w') do |out_io|
    CSV.filter(in_string, out_io, headers: true) do |row|
      row[0] = row[0].upcase
      row[1] *= 4
    end
  end
  p File.read(path) # => "Name,Value\nFOO,0000\nBAR,1111\nBAZ,2222\n"

===== Filter \String to \IO Stream Without Headers

  in_string = "foo,0\nbar,1\nbaz,2\n"
  path = 't.csv'
  File.open(path, 'w') do |out_io|
    CSV.filter(in_string, out_io) do |row|
      row[0] = row[0].upcase
      row[1] *= 4
    end
  end
  p File.read(path) # => "FOO,0000\nBAR,1111\nBAZ,2222\n"

==== Filter \IO Stream to \String

===== Filter \IO Stream to \String with Headers

  in_string = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  path = 't.csv'
  File.write(path, in_string)
  out_string = ''
  File.open(path, headers: true) do |in_io|
    CSV.filter(in_io, out_string, headers: true) do |row|
      row[0] = row[0].upcase
      row[1] *= 4
    end
  end
  out_string # => "Name,Value\nFOO,0000\nBAR,1111\nBAZ,2222\n"

===== Filter \IO Stream to \String Without Headers

  in_string = "foo,0\nbar,1\nbaz,2\n"
  path = 't.csv'
  File.write(path, in_string)
  out_string = ''
  File.open(path) do |in_io|
    CSV.filter(in_io, out_string) do |row|
      row[0] = row[0].upcase
      row[1] *= 4
    end
  end
  out_string # => "FOO,0000\nBAR,1111\nBAZ,2222\n"

==== Filter \IO Stream to \IO Stream

===== Filter \IO Stream to \IO Stream with Headers

  in_path = 't.csv'
  in_string = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  File.write(in_path, in_string)
  out_path = 'u.csv'
  File.open(in_path) do |in_io|
    File.open(out_path, 'w') do |out_io|
      CSV.filter(in_io, out_io, headers: true) do |row|
        row[0] = row[0].upcase
        row[1] *= 4
      end
    end
  end
  p File.read(out_path) # => "Name,Value\nFOO,0000\nBAR,1111\nBAZ,2222\n"

===== Filter \IO Stream to \IO Stream Without Headers

  in_path = 't.csv'
  in_string = "foo,0\nbar,1\nbaz,2\n"
  File.write(in_path, in_string)
  out_path = 'u.csv'
  File.open(in_path) do |in_io|
    File.open(out_path, 'w') do |out_io|
      CSV.filter(in_io, out_io) do |row|
        row[0] = row[0].upcase
        row[1] *= 4
      end
    end
  end
  p File.read(out_path) # => "FOO,0000\nBAR,1111\nBAZ,2222\n"