== Recipes for Filtering \CSV

For other recipes, see {Recipes for CSV}[./recipes_rdoc.html].

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

=== Contents

- {Source and Output Formats}[#label-Source+and+Output+Formats]
  - {Filtering String to String}[#label-Filtering+String+to+String]
    - {Recipe: Filter String to String with Headers}[#label-Recipe-3A+Filter+String+to+String+with+Headers]
    - {Recipe: Filter String to String Without Headers}[#label-Recipe-3A+Filter+String+to+String+Without+Headers]
  - {Filtering String to IO Stream}[#label-Filtering+String+to+IO+Stream]
    - {Recipe: Filter String to IO Stream with Headers}[#label-Recipe-3A+Filter+String+to+IO+Stream+with+Headers]
    - {Recipe: Filter String to IO Stream Without Headers}[#label-Recipe-3A+Filter+String+to+IO+Stream+Without+Headers]
  - {Filtering IO Stream to String}[#label-Filtering+IO+Stream+to+String]
    - {Recipe: Filter IO Stream to String with Headers}[#label-Recipe-3A+Filter+IO+Stream+to+String+with+Headers]
    - {Recipe: Filter IO Stream to String Without Headers}[#label-Recipe-3A+Filter+IO+Stream+to+String+Without+Headers]
  - {Filtering IO Stream to IO Stream}[#label-Filtering+IO+Stream+to+IO+Stream]
    - {Recipe: Filter IO Stream to IO Stream with Headers}[#label-Recipe-3A+Filter+IO+Stream+to+IO+Stream+with+Headers]
    - {Recipe: Filter IO Stream to IO Stream Without Headers}[#label-Recipe-3A+Filter+IO+Stream+to+IO+Stream+Without+Headers]

=== Source and Output Formats

You can use a Unix-style "filter" for \CSV data.
The filter reads source \CSV data and writes output \CSV data as modified by the filter.
The input and output \CSV data may be any mixture of \Strings and \IO streams.

==== Filtering \String to \String

You can filter one \String to another, with or without headers.

===== Recipe: Filter \String to \String with Headers

Use class method CSV.filter with option +headers+ to filter a \String to another \String:
  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"

===== Recipe: Filter \String to \String Without Headers

Use class method CSV.filter without option +headers+ to filter a \String to another \String:
  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"

==== Filtering \String to \IO Stream

You can filter a \String to an \IO stream, with or without headers.

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

Use class method CSV.filter with option +headers+ to filter a \String to an \IO stream:
  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"

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

Use class method CSV.filter without option +headers+ to filter a \String to an \IO stream:
  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"

==== Filtering \IO Stream to \String

You can filter an \IO stream to a \String, with or without headers.

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

Use class method CSV.filter with option +headers+ to filter an \IO stream to a \String:
  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"

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

Use class method CSV.filter without option +headers+ to filter an \IO stream to a \String:
  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"

==== Filtering \IO Stream to \IO Stream

You can filter an \IO stream to another \IO stream, with or without headers.

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

Use class method CSV.filter with option +headers+ to filter an \IO stream to another \IO stream:
  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"

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

Use class method CSV.filter without option +headers+ to filter an \IO stream to another \IO stream:
  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"