class CSV

CSV ↑

CSV Data ↑

CSV (comma-separated values) data is a text representation of a table:

This CSV String, with row separator "\n" and column separator ",", has three rows and two columns:

"foo,0\nbar,1\nbaz,2\n"

Despite the name CSV, a CSV representation can use different separators.

For more about tables, see the Wikipedia article “Table (information)”, especially its section “Simple table”

Class CSV ↑

Class CSV provides methods for:

To make CSV available:

require 'csv'

All examples here assume that this has been done.

Keeping It Simple ↑

A CSV object has dozens of instance methods that offer fine-grained control of parsing and generating CSV data. For many needs, though, simpler approaches will do.

This section summarizes the singleton methods in CSV that allow you to parse and generate without explicitly creating CSV objects. For details, follow the links.

Simple Parsing ↑

Parsing methods commonly return either of:

Parsing a String ↑

The input to be parsed can be a string:

string = "foo,0\nbar,1\nbaz,2\n"

Method CSV.parse returns the entire CSV data:

CSV.parse(string) # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Method CSV.parse_line returns only the first row:

CSV.parse_line(string) # => ["foo", "0"]

CSV extends class String with instance method String#parse_csv, which also returns only the first row:

string.parse_csv # => ["foo", "0"]

Parsing Via a File Path ↑

The input to be parsed can be in a file:

string = "foo,0\nbar,1\nbaz,2\n"
path = 't.csv'
File.write(path, string)

Method CSV.read returns the entire CSV data:

CSV.read(path) # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Method CSV.foreach iterates, passing each row to the given block:

CSV.foreach(path) do |row|
  p row
end

Output:

["foo", "0"]
["bar", "1"]
["baz", "2"]

Method CSV.table returns the entire CSV data as a CSV::Table object:

CSV.table(path) # => #<CSV::Table mode:col_or_row row_count:3>

Parsing from an Open IO Stream ↑

The input to be parsed can be in an open IO stream:

Method CSV.read returns the entire CSV data:

File.open(path) do |file|
  CSV.read(file)
end # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

As does method CSV.parse:

File.open(path) do |file|
  CSV.parse(file)
end # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Method CSV.parse_line returns only the first row:

File.open(path) do |file|
 CSV.parse_line(file)
end # => ["foo", "0"]

Method CSV.foreach iterates, passing each row to the given block:

File.open(path) do |file|
  CSV.foreach(file) do |row|
    p row
  end
end

Output:

["foo", "0"]
["bar", "1"]
["baz", "2"]

Method CSV.table returns the entire CSV data as a CSV::Table object:

File.open(path) do |file|
  CSV.table(file)
end # => #<CSV::Table mode:col_or_row row_count:3>

Simple Generating ↑

Method CSV.generate returns 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"

Method CSV.generate_line returns a String containing the single row constructed from an Array:

CSV.generate_line(['foo', '0']) # => "foo,0\n"

CSV extends class Array with instance method Array#to_csv, which forms an Array into a String:

['foo', '0'].to_csv # => "foo,0\n"

“Filtering” CSV ↑

Method CSV.filter provides a Unix-style filter for CSV data. The input data is processed to form the output data:

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"

CSV Objects ↑

There are three ways to create a CSV object:

Instance Methods ↑

CSV has three groups of instance methods:

Delegated Methods ↑

For convenience, a CSV object will delegate to many methods in class IO. (A few have wrapper “guard code” in CSV.) You may call:

Options ↑

The default values for options are:

DEFAULT_OPTIONS = {
  # For both parsing and generating.
  col_sep:            ",",
  row_sep:            :auto,
  quote_char:         '"',
  # For parsing.
  field_size_limit:   nil,
  converters:         nil,
  unconverted_fields: nil,
  headers:            false,
  return_headers:     false,
  header_converters:  nil,
  skip_blanks:        false,
  skip_lines:         nil,
  liberal_parsing:    false,
  nil_value:          nil,
  empty_value:        "",
  strip:              false,
  # For generating.
  write_headers:      nil,
  quote_empty:        true,
  force_quotes:       false,
  write_converters:   nil,
  write_nil_value:    nil,
  write_empty_value:  "",
}

Options for Parsing ↑

Options for parsing, described in detail below, include:

Option row_sep ↑

Specifies the row separator, a String or the Symbol :auto (see below), to be used for both parsing and generating.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:row_sep) # => :auto

When row_sep is a String, that String becomes the row separator. The String will be transcoded into the data’s Encoding before use.

Using "\n":

row_sep = "\n"
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using | (pipe):

row_sep = '|'
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0|bar,1|baz,2|"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using -- (two hyphens):

row_sep = '--'
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0--bar,1--baz,2--"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using '' (empty string):

row_sep = ''
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0bar,1baz,2"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0bar", "1baz", "2"]]

When row_sep is the Symbol :auto (the default), generating uses "\n" as the row separator:

str = CSV.generate do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"

Parsing, on the other hand, invokes auto-discovery of the row separator.

Auto-discovery reads ahead in the data looking for the next \r\n, \n, or \r sequence. The sequence will be selected even if it occurs in a quoted field, assuming that you would have the same line endings there.

Example:

str = CSV.generate do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

The default $INPUT_RECORD_SEPARATOR ($/) is used if any of the following is true:

Obviously, discovery takes a little time. Set manually if speed is important. Also note that IO objects should be opened in binary mode on Windows if this feature will be used as the line-ending translation can cause problems with resetting the document position to where it was before the read ahead.

Option col_sep ↑

Specifies the String column separator to be used for both parsing and generating. The String will be transcoded into the data’s Encoding before use.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:col_sep) # => "," (comma)

Using the default (comma):

str = CSV.generate do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using : (colon):

col_sep = ':'
str = CSV.generate(col_sep: col_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo:0\nbar:1\nbaz:2\n"
ary = CSV.parse(str, col_sep: col_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using :: (two colons):

col_sep = '::'
str = CSV.generate(col_sep: col_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo::0\nbar::1\nbaz::2\n"
ary = CSV.parse(str, col_sep: col_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using '' (empty string):

col_sep = ''
str = CSV.generate(col_sep: col_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo0\nbar1\nbaz2\n"

Raises an exception if parsing with the empty String:

col_sep = ''
# Raises ArgumentError (:col_sep must be 1 or more characters: "")
CSV.parse("foo0\nbar1\nbaz2\n", col_sep: col_sep)
Option quote_char ↑

Specifies the character (String of length 1) used used to quote fields in both parsing and generating. This String will be transcoded into the data’s Encoding before use.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:quote_char) # => "\"" (double quote)

This is useful for an application that incorrectly uses ' (single-quote) to quote fields, instead of the correct " (double-quote).

Using the default (double quote):

str = CSV.generate do |csv|
  csv << ['foo', 0]
  csv << ["'bar'", 1]
  csv << ['"baz"', 2]
end
str # => "foo,0\n'bar',1\n\"\"\"baz\"\"\",2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["'bar'", "1"], ["\"baz\"", "2"]]

Using ' (single-quote):

quote_char = "'"
str = CSV.generate(quote_char: quote_char) do |csv|
  csv << ['foo', 0]
  csv << ["'bar'", 1]
  csv << ['"baz"', 2]
end
str # => "foo,0\n'''bar''',1\n\"baz\",2\n"
ary = CSV.parse(str, quote_char: quote_char)
ary # => [["foo", "0"], ["'bar'", "1"], ["\"baz\"", "2"]]

Raises an exception if the String length is greater than 1:

# Raises ArgumentError (:quote_char has to be nil or a single character String)
CSV.new('', quote_char: 'xx')

Raises an exception if the value is not a String:

# Raises ArgumentError (:quote_char has to be nil or a single character String)
CSV.new('', quote_char: :foo)
Option field_size_limit ↑

Specifies the Integer field size limit.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:field_size_limit) # => nil

This is a maximum size CSV will read ahead looking for the closing quote for a field. (In truth, it reads to the first line ending beyond this size.) If a quote cannot be found within the limit CSV will raise a MalformedCSVError, assuming the data is faulty. You can use this limit to prevent what are effectively DoS attacks on the parser. However, this limit can cause a legitimate parse to fail; therefore the default value is nil (no limit).

For the examples in this section:

str = <<~EOT
  "a","b"
  "
  2345
  ",""
EOT
str # => "\"a\",\"b\"\n\"\n2345\n\",\"\"\n"

Using the default nil:

ary = CSV.parse(str)
ary # => [["a", "b"], ["\n2345\n", ""]]

Using 50:

field_size_limit = 50
ary = CSV.parse(str, field_size_limit: field_size_limit)
ary # => [["a", "b"], ["\n2345\n", ""]]

Raises an exception if a field is too long:

big_str = "123456789\n" * 1024
# Raises CSV::MalformedCSVError (Field size exceeded in line 1.)
CSV.parse('valid,fields,"' + big_str + '"', field_size_limit: 2048)
Option converters ↑

Specifies converters to be used in parsing fields. See Field Converters

Default value:

CSV::DEFAULT_OPTIONS.fetch(:converters) # => nil

The value may be a field converter name (see Stored Converters):

str = '1,2,3'
# Without a converter
array = CSV.parse_line(str)
array # => ["1", "2", "3"]
# With built-in converter :integer
array = CSV.parse_line(str, converters: :integer)
array # => [1, 2, 3]

The value may be a converter list (see Converter Lists):

str = '1,3.14159'
# Without converters
array = CSV.parse_line(str)
array # => ["1", "3.14159"]
# With built-in converters
array = CSV.parse_line(str, converters: [:integer, :float])
array # => [1, 3.14159]

The value may be a Proc custom converter: (see Custom Field Converters):

str = ' foo  ,  bar  ,  baz  '
# Without a converter
array = CSV.parse_line(str)
array # => [" foo  ", "  bar  ", "  baz  "]
# With a custom converter
array = CSV.parse_line(str, converters: proc {|field| field.strip })
array # => ["foo", "bar", "baz"]

See also Custom Field Converters

Raises an exception if the converter is not a converter name or a Proc:

str = 'foo,0'
# Raises NoMethodError (undefined method `arity' for nil:NilClass)
CSV.parse(str, converters: :foo)
Option unconverted_fields ↑

Specifies the boolean that determines whether unconverted field values are to be available.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:unconverted_fields) # => nil

The unconverted field values are those found in the source data, prior to any conversions performed via option converters.

When option unconverted_fields is true, each returned row (Array or CSV::Row) has an added method, unconverted_fields, that returns the unconverted field values:

str = <<-EOT
foo,0
bar,1
baz,2
EOT
# Without unconverted_fields
csv = CSV.parse(str, converters: :integer)
csv # => [["foo", 0], ["bar", 1], ["baz", 2]]
csv.first.respond_to?(:unconverted_fields) # => false
# With unconverted_fields
csv = CSV.parse(str, converters: :integer, unconverted_fields: true)
csv # => [["foo", 0], ["bar", 1], ["baz", 2]]
csv.first.respond_to?(:unconverted_fields) # => true
csv.first.unconverted_fields # => ["foo", "0"]
Option headers ↑

Specifies a boolean, Symbol, Array, or String to be used to define column headers.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:headers) # => false

Without headers:

str = <<-EOT
Name,Count
foo,0
bar,1
bax,2
EOT
csv = CSV.new(str)
csv # => #<CSV io_type:StringIO encoding:UTF-8 lineno:0 col_sep:"," row_sep:"\n" quote_char:"\"">
csv.headers # => nil
csv.shift # => ["Name", "Count"]

If set to true or the Symbol :first_row, the first row of the data is treated as a row of headers:

str = <<-EOT
Name,Count
foo,0
bar,1
bax,2
EOT
csv = CSV.new(str, headers: true)
csv # => #<CSV io_type:StringIO encoding:UTF-8 lineno:2 col_sep:"," row_sep:"\n" quote_char:"\"" headers:["Name", "Count"]>
csv.headers # => ["Name", "Count"]
csv.shift # => #<CSV::Row "Name":"bar" "Count":"1">

If set to an Array, the Array elements are treated as headers:

str = <<-EOT
foo,0
bar,1
bax,2
EOT
csv = CSV.new(str, headers: ['Name', 'Count'])
csv
csv.headers # => ["Name", "Count"]
csv.shift # => #<CSV::Row "Name":"bar" "Count":"1">

If set to a String str, method CSV::parse_line(str, options) is called with the current options, and the returned Array is treated as headers:

str = <<-EOT
foo,0
bar,1
bax,2
EOT
csv = CSV.new(str, headers: 'Name,Count')
csv
csv.headers # => ["Name", "Count"]
csv.shift # => #<CSV::Row "Name":"bar" "Count":"1">
Option return_headers ↑

Specifies the boolean that determines whether method shift returns or ignores the header row.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:return_headers) # => false

Examples:

str = <<-EOT
Name,Count
foo,0
bar,1
bax,2
EOT
# Without return_headers first row is str.
csv = CSV.new(str, headers: true)
csv.shift # => #<CSV::Row "Name":"foo" "Count":"0">
# With return_headers first row is headers.
csv = CSV.new(str, headers: true, return_headers: true)
csv.shift # => #<CSV::Row "Name":"Name" "Count":"Count">
Option header_converters ↑

Specifies converters to be used in parsing headers. See Header Converters

Default value:

CSV::DEFAULT_OPTIONS.fetch(:header_converters) # => nil

Identical in functionality to option converters except that:

This section assumes prior execution of:

str = <<-EOT
Name,Value
foo,0
bar,1
baz,2
EOT
# With no header converter
table = CSV.parse(str, headers: true)
table.headers # => ["Name", "Value"]

The value may be a header converter name (see Stored Converters):

table = CSV.parse(str, headers: true, header_converters: :downcase)
table.headers # => ["name", "value"]

The value may be a converter list (see Converter Lists):

header_converters = [:downcase, :symbol]
table = CSV.parse(str, headers: true, header_converters: header_converters)
table.headers # => [:name, :value]

The value may be a Proc custom converter (see Custom Header Converters):

upcase_converter = proc {|field| field.upcase }
table = CSV.parse(str, headers: true, header_converters: upcase_converter)
table.headers # => ["NAME", "VALUE"]

See also Custom Header Converters

Option skip_blanks ↑

Specifies a boolean that determines whether blank lines in the input will be ignored; a line that contains a column separator is not considered to be blank.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:skip_blanks) # => false

See also option skiplines.

For examples in this section:

str = <<-EOT
foo,0

bar,1
baz,2

,
EOT

Using the default, false:

ary = CSV.parse(str)
ary # => [["foo", "0"], [], ["bar", "1"], ["baz", "2"], [], [nil, nil]]

Using true:

ary = CSV.parse(str, skip_blanks: true)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"], [nil, nil]]

Using a truthy value:

ary = CSV.parse(str, skip_blanks: :foo)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"], [nil, nil]]
Option skip_lines ↑

Specifies an object to use in identifying comment lines in the input that are to be ignored:

Default value:

CSV::DEFAULT_OPTIONS.fetch(:skip_lines) # => nil

For examples in this section:

str = <<-EOT
# Comment
foo,0
bar,1
baz,2
# Another comment
EOT
str # => "# Comment\nfoo,0\nbar,1\nbaz,2\n# Another comment\n"

Using the default, nil:

ary = CSV.parse(str)
ary # => [["# Comment"], ["foo", "0"], ["bar", "1"], ["baz", "2"], ["# Another comment"]]

Using a Regexp:

ary = CSV.parse(str, skip_lines: /^#/)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using a String:

ary = CSV.parse(str, skip_lines: '#')
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Raises an exception if given an object that is not a Regexp, a String, or nil:

# Raises ArgumentError (:skip_lines has to respond to #match: 0)
CSV.parse(str, skip_lines: 0)
Option strip ↑

Specifies the boolean value that determines whether whitespace is stripped from each input field.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:strip) # => false

With default value false:

ary = CSV.parse_line(' a , b ')
ary # => [" a ", " b "]

With value true:

ary = CSV.parse_line(' a , b ', strip: true)
ary # => ["a", "b"]
Option liberal_parsing ↑

Specifies the boolean or hash value that determines whether CSV will attempt to parse input not conformant with RFC 4180, such as double quotes in unquoted fields.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:liberal_parsing) # => false

For the next two examples:

str = 'is,this "three, or four",fields'

Without liberal_parsing:

# Raises CSV::MalformedCSVError (Illegal quoting in str 1.)
CSV.parse_line(str)

With liberal_parsing:

ary = CSV.parse_line(str, liberal_parsing: true)
ary # => ["is", "this \"three", " or four\"", "fields"]

Use the backslash_quote sub-option to parse values that use a backslash to escape a double-quote character. This causes the parser to treat \" as if it were "".

For the next two examples:

str = 'Show,"Harry \"Handcuff\" Houdini, the one and only","Tampa Theater"'

With liberal_parsing, but without the backslash_quote sub-option:

# Incorrect interpretation of backslash; incorrectly interprets the quoted comma as a field separator.
ary = CSV.parse_line(str, liberal_parsing: true)
ary # => ["Show", "\"Harry \\\"Handcuff\\\" Houdini", " the one and only\"", "Tampa Theater"]
puts ary[1] # => "Harry \"Handcuff\" Houdini

With liberal_parsing and its backslash_quote sub-option:

ary = CSV.parse_line(str, liberal_parsing: { backslash_quote: true })
ary # => ["Show", "Harry \"Handcuff\" Houdini, the one and only", "Tampa Theater"]
puts ary[1] # => Harry "Handcuff" Houdini, the one and only
Option nil_value ↑

Specifies the object that is to be substituted for each null (no-text) field.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:nil_value) # => nil

With the default, nil:

CSV.parse_line('a,,b,,c') # => ["a", nil, "b", nil, "c"]

With a different object:

CSV.parse_line('a,,b,,c', nil_value: 0) # => ["a", 0, "b", 0, "c"]
Option empty_value ↑

Specifies the object that is to be substituted for each field that has an empty String.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:empty_value) # => "" (empty string)

With the default, "":

CSV.parse_line('a,"",b,"",c') # => ["a", "", "b", "", "c"]

With a different object:

CSV.parse_line('a,"",b,"",c', empty_value: 'x') # => ["a", "x", "b", "x", "c"]

Options for Generating ↑

Options for generating, described in detail below, include:

Option row_sep ↑

Specifies the row separator, a String or the Symbol :auto (see below), to be used for both parsing and generating.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:row_sep) # => :auto

When row_sep is a String, that String becomes the row separator. The String will be transcoded into the data’s Encoding before use.

Using "\n":

row_sep = "\n"
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using | (pipe):

row_sep = '|'
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0|bar,1|baz,2|"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using -- (two hyphens):

row_sep = '--'
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0--bar,1--baz,2--"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using '' (empty string):

row_sep = ''
str = CSV.generate(row_sep: row_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0bar,1baz,2"
ary = CSV.parse(str, row_sep: row_sep)
ary # => [["foo", "0bar", "1baz", "2"]]

When row_sep is the Symbol :auto (the default), generating uses "\n" as the row separator:

str = CSV.generate do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"

Parsing, on the other hand, invokes auto-discovery of the row separator.

Auto-discovery reads ahead in the data looking for the next \r\n, \n, or \r sequence. The sequence will be selected even if it occurs in a quoted field, assuming that you would have the same line endings there.

Example:

str = CSV.generate do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

The default $INPUT_RECORD_SEPARATOR ($/) is used if any of the following is true:

Obviously, discovery takes a little time. Set manually if speed is important. Also note that IO objects should be opened in binary mode on Windows if this feature will be used as the line-ending translation can cause problems with resetting the document position to where it was before the read ahead.

Option col_sep ↑

Specifies the String column separator to be used for both parsing and generating. The String will be transcoded into the data’s Encoding before use.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:col_sep) # => "," (comma)

Using the default (comma):

str = CSV.generate do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using : (colon):

col_sep = ':'
str = CSV.generate(col_sep: col_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo:0\nbar:1\nbaz:2\n"
ary = CSV.parse(str, col_sep: col_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using :: (two colons):

col_sep = '::'
str = CSV.generate(col_sep: col_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo::0\nbar::1\nbaz::2\n"
ary = CSV.parse(str, col_sep: col_sep)
ary # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Using '' (empty string):

col_sep = ''
str = CSV.generate(col_sep: col_sep) do |csv|
  csv << [:foo, 0]
  csv << [:bar, 1]
  csv << [:baz, 2]
end
str # => "foo0\nbar1\nbaz2\n"

Raises an exception if parsing with the empty String:

col_sep = ''
# Raises ArgumentError (:col_sep must be 1 or more characters: "")
CSV.parse("foo0\nbar1\nbaz2\n", col_sep: col_sep)
Option quote_char ↑

Specifies the character (String of length 1) used used to quote fields in both parsing and generating. This String will be transcoded into the data’s Encoding before use.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:quote_char) # => "\"" (double quote)

This is useful for an application that incorrectly uses ' (single-quote) to quote fields, instead of the correct " (double-quote).

Using the default (double quote):

str = CSV.generate do |csv|
  csv << ['foo', 0]
  csv << ["'bar'", 1]
  csv << ['"baz"', 2]
end
str # => "foo,0\n'bar',1\n\"\"\"baz\"\"\",2\n"
ary = CSV.parse(str)
ary # => [["foo", "0"], ["'bar'", "1"], ["\"baz\"", "2"]]

Using ' (single-quote):

quote_char = "'"
str = CSV.generate(quote_char: quote_char) do |csv|
  csv << ['foo', 0]
  csv << ["'bar'", 1]
  csv << ['"baz"', 2]
end
str # => "foo,0\n'''bar''',1\n\"baz\",2\n"
ary = CSV.parse(str, quote_char: quote_char)
ary # => [["foo", "0"], ["'bar'", "1"], ["\"baz\"", "2"]]

Raises an exception if the String length is greater than 1:

# Raises ArgumentError (:quote_char has to be nil or a single character String)
CSV.new('', quote_char: 'xx')

Raises an exception if the value is not a String:

# Raises ArgumentError (:quote_char has to be nil or a single character String)
CSV.new('', quote_char: :foo)
Option write_headers ↑

Specifies the boolean that determines whether a header row is included in the output; ignored if there are no headers.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:write_headers) # => nil

Without write_headers:

file_path = 't.csv'
CSV.open(file_path,'w',
    :headers => ['Name','Value']
  ) do |csv|
    csv << ['foo', '0']
end
CSV.open(file_path) do |csv|
  csv.shift
end # => ["foo", "0"]

With write_headers“:

CSV.open(file_path,'w',
    :write_headers => true,
    :headers => ['Name','Value']
  ) do |csv|
    csv << ['foo', '0']
end
CSV.open(file_path) do |csv|
  csv.shift
end # => ["Name", "Value"]
Option force_quotes ↑

Specifies the boolean that determines whether each output field is to be double-quoted.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:force_quotes) # => false

For examples in this section:

ary = ['foo', 0, nil]

Using the default, false:

str = CSV.generate_line(ary)
str # => "foo,0,\n"

Using true:

str = CSV.generate_line(ary, force_quotes: true)
str # => "\"foo\",\"0\",\"\"\n"
Option quote_empty ↑

Specifies the boolean that determines whether an empty value is to be double-quoted.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:quote_empty) # => true

With the default true:

CSV.generate_line(['"', ""]) # => "\"\"\"\",\"\"\n"

With false:

CSV.generate_line(['"', ""], quote_empty: false) # => "\"\"\"\",\n"
Option write_converters ↑

Specifies converters to be used in generating fields. See Write Converters

Default value:

CSV::DEFAULT_OPTIONS.fetch(:write_converters) # => nil

With no write converter:

str = CSV.generate_line(["\na\n", "\tb\t", " c "])
str # => "\"\na\n\",\tb\t, c \n"

With a write converter:

strip_converter = proc {|field| field.strip }
str = CSV.generate_line(["\na\n", "\tb\t", " c "], write_converters: strip_converter)
str # => "a,b,c\n"

With two write converters (called in order):

upcase_converter = proc {|field| field.upcase }
downcase_converter = proc {|field| field.downcase }
write_converters = [upcase_converter, downcase_converter]
str = CSV.generate_line(['a', 'b', 'c'], write_converters: write_converters)
str # => "a,b,c\n"

See also Write Converters

Option write_nil_value ↑

Specifies the object that is to be substituted for each nil-valued field.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:write_nil_value) # => nil

Without the option:

str = CSV.generate_line(['a', nil, 'c', nil])
str # => "a,,c,\n"

With the option:

str = CSV.generate_line(['a', nil, 'c', nil], write_nil_value: "x")
str # => "a,x,c,x\n"
Option write_empty_value ↑

Specifies the object that is to be substituted for each field that has an empty String.

Default value:

CSV::DEFAULT_OPTIONS.fetch(:write_empty_value) # => ""

Without the option:

str = CSV.generate_line(['a', '', 'c', ''])
str # => "a,\"\",c,\"\"\n"

With the option:

str = CSV.generate_line(['a', '', 'c', ''], write_empty_value: "x")
str # => "a,x,c,x\n"

CSV with Headers ↑

CSV allows to specify column names of CSV file, whether they are in data, or provided separately. If headers are specified, reading methods return an instance of CSV::Table, consisting of CSV::Row.

# Headers are part of data
data = CSV.parse(<<~ROWS, headers: true)
  Name,Department,Salary
  Bob,Engineering,1000
  Jane,Sales,2000
  John,Management,5000
ROWS

data.class      #=> CSV::Table
data.first      #=> #<CSV::Row "Name":"Bob" "Department":"Engineering" "Salary":"1000">
data.first.to_h #=> {"Name"=>"Bob", "Department"=>"Engineering", "Salary"=>"1000"}

# Headers provided by developer
data = CSV.parse('Bob,Engineering,1000', headers: %i[name department salary])
data.first      #=> #<CSV::Row name:"Bob" department:"Engineering" salary:"1000">

Converters ↑

By default, each value (field or header) parsed by CSV is formed into a String. You can use a field converter or header converter to intercept and modify the parsed values:

Also by default, each value to be written during generation is written ‘as-is’. You can use a write converter to modify values before writing.

Specifying Converters ↑

You can specify converters for parsing or generating in the options argument to various CSV methods:

There are three forms for specifying converters:

Converter Procs ↑

This converter proc, strip_converter, accepts a value field and returns field.strip:

strip_converter = proc {|field| field.strip }

In this call to CSV.parse, the keyword argument converters: string_converter specifies that:

Example:

string = " foo , 0 \n bar , 1 \n baz , 2 \n"
array = CSV.parse(string, converters: strip_converter)
array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

A converter proc can receive a second argument, field_info, that contains details about the field. This modified strip_converter displays its arguments:

strip_converter = proc do |field, field_info|
  p [field, field_info]
  field.strip
end
string = " foo , 0 \n bar , 1 \n baz , 2 \n"
array = CSV.parse(string, converters: strip_converter)
array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Output:

[" foo ", #<struct CSV::FieldInfo index=0, line=1, header=nil>]
[" 0 ", #<struct CSV::FieldInfo index=1, line=1, header=nil>]
[" bar ", #<struct CSV::FieldInfo index=0, line=2, header=nil>]
[" 1 ", #<struct CSV::FieldInfo index=1, line=2, header=nil>]
[" baz ", #<struct CSV::FieldInfo index=0, line=3, header=nil>]
[" 2 ", #<struct CSV::FieldInfo index=1, line=3, header=nil>]

Each CSV::FieldInfo object shows:

Stored Converters ↑

A converter may be given a name and stored in a structure where the parsing methods can find it by name.

The storage structure for field converters is the Hash CSV::Converters. It has several built-in converter procs:

. This example creates a converter proc, then stores it:

strip_converter = proc {|field| field.strip }
CSV::Converters[:strip] = strip_converter

Then the parsing method call can refer to the converter by its name, :strip:

string = " foo , 0 \n bar , 1 \n baz , 2 \n"
array = CSV.parse(string, converters: :strip)
array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

The storage structure for header converters is the Hash CSV::HeaderConverters, which works in the same way. It also has built-in converter procs:

There is no such storage structure for write headers.

In order for the parsing methods to access stored converters in non-main-Ractors, the storage structure must be made shareable first. Therefore, Ractor.make_shareable(CSV::Converters) and Ractor.make_shareable(CSV::HeaderConverters) must be called before the creation of Ractors that use the converters stored in these structures. (Since making the storage structures shareable involves freezing them, any custom converters that are to be used must be added first.)

Converter Lists ↑

A converter list is an Array that may include any assortment of:

Examples:

numeric_converters = [:integer, :float]
date_converters = [:date, :date_time]
[numeric_converters, strip_converter]
[strip_converter, date_converters, :float]

Like a converter proc, a converter list may be named and stored in either CSV::Converters or CSV::HeaderConverters:

CSV::Converters[:custom] = [strip_converter, date_converters, :float]
CSV::HeaderConverters[:custom] = [:downcase, :symbol]

There are two built-in converter lists:

CSV::Converters[:numeric] # => [:integer, :float]
CSV::Converters[:all] # => [:date_time, :numeric]

Field Converters ↑

With no conversion, all parsed fields in all rows become Strings:

string = "foo,0\nbar,1\nbaz,2\n"
ary = CSV.parse(string)
ary # => # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

When you specify a field converter, each parsed field is passed to the converter; its return value becomes the stored value for the field. A converter might, for example, convert an integer embedded in a String into a true Integer. (In fact, that’s what built-in field converter :integer does.)

There are three ways to use field converters.

Installing a field converter does not affect already-read rows:

csv = CSV.new(string)
csv.shift # => ["foo", "0"]
# Add a converter.
csv.convert(:integer)
csv.converters # => [:integer]
csv.read # => [["bar", 1], ["baz", 2]]

There are additional built-in converters, and custom converters are also supported.

Built-In Field Converters ↑

The built-in field converters are in Hash CSV::Converters:

Display:

CSV::Converters.each_pair do |name, value|
  if value.kind_of?(Proc)
    p [name, value.class]
  else
    p [name, value]
  end
end

Output:

[:integer, Proc]
[:float, Proc]
[:numeric, [:integer, :float]]
[:date, Proc]
[:date_time, Proc]
[:time, Proc]
[:all, [:date_time, :numeric]]

Each of these converters transcodes values to UTF-8 before attempting conversion. If a value cannot be transcoded to UTF-8 the conversion will fail and the value will remain unconverted.

Converter :integer converts each field that Integer() accepts:

data = '0,1,2,x'
# Without the converter
csv = CSV.parse_line(data)
csv # => ["0", "1", "2", "x"]
# With the converter
csv = CSV.parse_line(data, converters: :integer)
csv # => [0, 1, 2, "x"]

Converter :float converts each field that Float() accepts:

data = '1.0,3.14159,x'
# Without the converter
csv = CSV.parse_line(data)
csv # => ["1.0", "3.14159", "x"]
# With the converter
csv = CSV.parse_line(data, converters: :float)
csv # => [1.0, 3.14159, "x"]

Converter :numeric converts with both :integer and :float..

Converter :date converts each field that Date::parse accepts:

data = '2001-02-03,x'
# Without the converter
csv = CSV.parse_line(data)
csv # => ["2001-02-03", "x"]
# With the converter
csv = CSV.parse_line(data, converters: :date)
csv # => [#<Date: 2001-02-03 ((2451944j,0s,0n),+0s,2299161j)>, "x"]

Converter :date_time converts each field that DateTime::parse accepts:

data = '2020-05-07T14:59:00-05:00,x'
# Without the converter
csv = CSV.parse_line(data)
csv # => ["2020-05-07T14:59:00-05:00", "x"]
# With the converter
csv = CSV.parse_line(data, converters: :date_time)
csv # => [#<DateTime: 2020-05-07T14:59:00-05:00 ((2458977j,71940s,0n),-18000s,2299161j)>, "x"]

Converter time converts each field that Time::parse accepts:

data = '2020-05-07T14:59:00-05:00,x'
# Without the converter
csv = CSV.parse_line(data)
csv # => ["2020-05-07T14:59:00-05:00", "x"]
# With the converter
csv = CSV.parse_line(data, converters: :time)
csv # => [2020-05-07 14:59:00 -0500, "x"]

Converter :numeric converts with both :date_time and :numeric..

As seen above, method convert adds converters to a CSV instance, and method converters returns an Array of the converters in effect:

csv = CSV.new('0,1,2')
csv.converters # => []
csv.convert(:integer)
csv.converters # => [:integer]
csv.convert(:date)
csv.converters # => [:integer, :date]
Custom Field Converters ↑

You can define a custom field converter:

strip_converter = proc {|field| field.strip }
string = " foo , 0 \n bar , 1 \n baz , 2 \n"
array = CSV.parse(string, converters: strip_converter)
array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

You can register the converter in Converters Hash, which allows you to refer to it by name:

CSV::Converters[:strip] = strip_converter
string = " foo , 0 \n bar , 1 \n baz , 2 \n"
array = CSV.parse(string, converters: :strip)
array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Header Converters ↑

Header converters operate only on headers (and not on other rows).

There are three ways to use header converters; these examples use built-in header converter :downcase, which downcases each parsed header.

Built-In Header Converters ↑

The built-in header converters are in Hash CSV::HeaderConverters. The keys there are the names of the converters:

CSV::HeaderConverters.keys # => [:downcase, :symbol]

Converter :downcase converts each header by downcasing it:

string = "Name,Count\nFoo,0\n,Bar,1\nBaz,2"
tbl = CSV.parse(string, headers: true, header_converters: :downcase)
tbl.class # => CSV::Table
tbl.headers # => ["name", "count"]

Converter :symbol converts each header by making it into a Symbol:

string = "Name,Count\nFoo,0\n,Bar,1\nBaz,2"
tbl = CSV.parse(string, headers: true, header_converters: :symbol)
tbl.headers # => [:name, :count]

Details:

Custom Header Converters ↑

You can define a custom header converter:

upcase_converter = proc {|header| header.upcase }
string = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
table = CSV.parse(string, headers: true, header_converters: upcase_converter)
table # => #<CSV::Table mode:col_or_row row_count:4>
table.headers # => ["NAME", "VALUE"]

You can register the converter in HeaderConverters Hash, which allows you to refer to it by name:

CSV::HeaderConverters[:upcase] = upcase_converter
table = CSV.parse(string, headers: true, header_converters: :upcase)
table # => #<CSV::Table mode:col_or_row row_count:4>
table.headers # => ["NAME", "VALUE"]
Write Converters ↑

When you specify a write converter for generating CSV, each field to be written is passed to the converter; its return value becomes the new value for the field. A converter might, for example, strip whitespace from a field.

Using no write converter (all fields unmodified):

output_string = CSV.generate do |csv|
  csv << [' foo ', 0]
  csv << [' bar ', 1]
  csv << [' baz ', 2]
end
output_string # => " foo ,0\n bar ,1\n baz ,2\n"

Using option write_converters with two custom write converters:

strip_converter = proc {|field| field.respond_to?(:strip) ? field.strip : field }
upcase_converter = proc {|field| field.respond_to?(:upcase) ? field.upcase : field }
write_converters = [strip_converter, upcase_converter]
output_string = CSV.generate(write_converters: write_converters) do |csv|
  csv << [' foo ', 0]
  csv << [' bar ', 1]
  csv << [' baz ', 2]
end
output_string # => "FOO,0\nBAR,1\nBAZ,2\n"

Character Encodings (M17n or Multilingualization) ↑

This new CSV parser is m17n savvy. The parser works in the Encoding of the IO or String object being read from or written to. Your data is never transcoded (unless you ask Ruby to transcode it for you) and will literally be parsed in the Encoding it is in. Thus CSV will return Arrays or Rows of Strings in the Encoding of your data. This is accomplished by transcoding the parser itself into your Encoding.

Some transcoding must take place, of course, to accomplish this multiencoding support. For example, :col_sep, :row_sep, and :quote_char must be transcoded to match your data. Hopefully this makes the entire process feel transparent, since CSV’s defaults should just magically work for your data. However, you can set these values manually in the target Encoding to avoid the translation.

It’s also important to note that while all of CSV’s core parser is now Encoding agnostic, some features are not. For example, the built-in converters will try to transcode data to UTF-8 before making conversions. Again, you can provide custom converters that are aware of your Encodings to avoid this translation. It’s just too hard for me to support native conversions in all of Ruby’s Encodings.

Anyway, the practical side of this is simple: make sure IO and String objects passed into CSV have the proper Encoding set and everything should just work. CSV methods that allow you to open IO objects (CSV::foreach(), CSV::open(), CSV::read(), and CSV::readlines()) do allow you to specify the Encoding.

One minor exception comes when generating CSV into a String with an Encoding that is not ASCII compatible. There’s no existing data for CSV to use to prepare itself and thus you will probably need to manually specify the desired Encoding for most of those cases. It will try to guess using the fields in a row of output though, when using CSV::generate_line() or Array#to_csv().

I try to point out any other Encoding issues in the documentation of methods as they come up.

This has been tested to the best of my ability with all non-“dummy” Encodings Ruby ships with. However, it is brave new code and may have some bugs. Please feel free to report any issues you find with it.