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Recipes for Filtering CSV ↑

For other recipes, see Recipes for CSV.

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

require 'csv'

Contents ↑

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"