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

== Recipes for Parsing \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 Formats}[#label-Source+Formats]
  - {Parsing from a String}[#label-Parsing+from+a+String]
    - {Recipe: Parse from String with Headers}[#label-Recipe-3A+Parse+from+String+with+Headers]
    - {Recipe: Parse from String Without Headers}[#label-Recipe-3A+Parse+from+String+Without+Headers]
  - {Parsing from a File}[#label-Parsing+from+a+File]
    - {Recipe: Parse from File with Headers}[#label-Recipe-3A+Parse+from+File+with+Headers]
    - {Recipe: Parse from File Without Headers}[#label-Recipe-3A+Parse+from+File+Without+Headers]
  - {Parsing from an IO Stream}[#label-Parsing+from+an+IO+Stream]
    - {Recipe: Parse from IO Stream with Headers}[#label-Recipe-3A+Parse+from+IO+Stream+with+Headers]
    - {Recipe: Parse from IO Stream Without Headers}[#label-Recipe-3A+Parse+from+IO+Stream+Without+Headers]
- {RFC 4180 Compliance}[#label-RFC+4180+Compliance]
  - {Row Separator}[#label-Row+Separator]
    - {Recipe: Handle Compliant Row Separator}[#label-Recipe-3A+Handle+Compliant+Row+Separator]
    - {Recipe: Handle Non-Compliant Row Separator}[#label-Recipe-3A+Handle+Non-Compliant+Row+Separator]
  - {Column Separator}[#label-Column+Separator]
    - {Recipe: Handle Compliant Column Separator}[#label-Recipe-3A+Handle+Compliant+Column+Separator]
    - {Recipe: Handle Non-Compliant Column Separator}[#label-Recipe-3A+Handle+Non-Compliant+Column+Separator]
  - {Quote Character}[#label-Quote+Character]
    - {Recipe: Handle Compliant Quote Character}[#label-Recipe-3A+Handle+Compliant+Quote+Character]
    - {Recipe: Handle Non-Compliant Quote Character}[#label-Recipe-3A+Handle+Non-Compliant+Quote+Character]
  - {Recipe: Allow Liberal Parsing}[#label-Recipe-3A+Allow+Liberal+Parsing]
- {Special Handling}[#label-Special+Handling]
  - {Special Line Handling}[#label-Special+Line+Handling]
    - {Recipe: Ignore Blank Lines}[#label-Recipe-3A+Ignore+Blank+Lines]
    - {Recipe: Ignore Selected Lines}[#label-Recipe-3A+Ignore+Selected+Lines]
  - {Special Field Handling}[#label-Special+Field+Handling]
    - {Recipe: Strip Fields}[#label-Recipe-3A+Strip+Fields]
    - {Recipe: Handle Null Fields}[#label-Recipe-3A+Handle+Null+Fields]
    - {Recipe: Handle Empty Fields}[#label-Recipe-3A+Handle+Empty+Fields]
- {Converting Fields}[#label-Converting+Fields]
  - {Converting Fields to Objects}[#label-Converting+Fields+to+Objects]
    - {Recipe: Convert Fields to Integers}[#label-Recipe-3A+Convert+Fields+to+Integers]
    - {Recipe: Convert Fields to Floats}[#label-Recipe-3A+Convert+Fields+to+Floats]
    - {Recipe: Convert Fields to Numerics}[#label-Recipe-3A+Convert+Fields+to+Numerics]
    - {Recipe: Convert Fields to Dates}[#label-Recipe-3A+Convert+Fields+to+Dates]
    - {Recipe: Convert Fields to DateTimes}[#label-Recipe-3A+Convert+Fields+to+DateTimes]
    - {Recipe: Convert Assorted Fields to Objects}[#label-Recipe-3A+Convert+Assorted+Fields+to+Objects]
    - {Recipe: Convert Fields to Other Objects}[#label-Recipe-3A+Convert+Fields+to+Other+Objects]
  - {Recipe: Filter Field Strings}[#label-Recipe-3A+Filter+Field+Strings]
  - {Recipe: Register Field Converters}[#label-Recipe-3A+Register+Field+Converters]
  - {Using Multiple Field Converters}[#label-Using+Multiple+Field+Converters]
    - {Recipe: Specify Multiple Field Converters in Option :converters}[#label-Recipe-3A+Specify+Multiple+Field+Converters+in+Option+-3Aconverters]
    - {Recipe: Specify Multiple Field Converters in a Custom Converter List}[#label-Recipe-3A+Specify+Multiple+Field+Converters+in+a+Custom+Converter+List]
- {Converting Headers}[#label-Converting+Headers]
  - {Recipe: Convert Headers to Lowercase}[#label-Recipe-3A+Convert+Headers+to+Lowercase]
  - {Recipe: Convert Headers to Symbols}[#label-Recipe-3A+Convert+Headers+to+Symbols]
  - {Recipe: Filter Header Strings}[#label-Recipe-3A+Filter+Header+Strings]
  - {Recipe: Register Header Converters}[#label-Recipe-3A+Register+Header+Converters]
  - {Using Multiple Header Converters}[#label-Using+Multiple+Header+Converters]
    - {Recipe: Specify Multiple Header Converters in Option :header_converters}[#label-Recipe-3A+Specify+Multiple+Header+Converters+in+Option+-3Aheader_converters]
    - {Recipe: Specify Multiple Header Converters in a Custom Header Converter List}[#label-Recipe-3A+Specify+Multiple+Header+Converters+in+a+Custom+Header+Converter+List]
- {Diagnostics}[#label-Diagnostics]
  - {Recipe: Capture Unconverted Fields}[#label-Recipe-3A+Capture+Unconverted+Fields]
  - {Recipe: Capture Field Info}[#label-Recipe-3A+Capture+Field+Info]

=== Source Formats

You can parse \CSV data from a \String, from a \File (via its path), or from an \IO stream.

==== Parsing from a \String

You can parse \CSV data from a \String, with or without headers.

===== Recipe: Parse from \String with Headers

Use class method CSV.parse with option +headers+ to read a source \String all at once
(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>

Use instance method CSV#each with option +headers+ to read a source \String one row at a time:
  CSV.new(string, headers: true).each do |row|
    p row
  end
Output:
  #<CSV::Row "Name":"foo" "Value":"0">
  #<CSV::Row "Name":"bar" "Value":"1">
  #<CSV::Row "Name":"baz" "Value":"2">

===== Recipe: Parse from \String Without Headers

Use class method CSV.parse without option +headers+ to read a source \String all at once
(may have memory resource implications):
  string = "foo,0\nbar,1\nbaz,2\n"
  CSV.parse(string) # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

Use instance method CSV#each without option +headers+ to 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"]

==== Parsing from a \File

You can parse \CSV data from a \File, with or without headers.

===== Recipe: Parse from \File with Headers

Use instance method CSV#read with option +headers+ to read a 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>

Use class method CSV.foreach with option +headers+ to 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">

===== Recipe: Parse from \File Without Headers

Use class method CSV.read without option +headers+ to 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"]]

Use class method CSV.foreach without option +headers+ to read one row at a time:
  CSV.foreach(path) do |row|
    p row
  end
Output:
  ["foo", "0"]
  ["bar", "1"]
  ["baz", "2"]

==== Parsing from an \IO Stream

You can parse \CSV data from an \IO stream, with or without headers.

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

Use class method CSV.parse with option +headers+ to 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>

Use class method CSV.foreach with option +headers+ to 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">

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

Use class method CSV.parse without option +headers+ to 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"]]

Use class method CSV.foreach without option +headers+ to 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"]

=== RFC 4180 Compliance

By default, \CSV parses data that is compliant with
{RFC 4180}[https://tools.ietf.org/html/rfc4180]
with respect to:
- Row separator.
- Column separator.
- Quote character.

==== Row Separator

RFC 4180 specifies the row separator CRLF (Ruby <tt>"\r\n"</tt>).

Although the \CSV default row separator is <tt>"\n"</tt>,
the parser also by default handles row separator <tt>"\r"</tt> and the RFC-compliant <tt>"\r\n"</tt>.

===== Recipe: Handle Compliant Row Separator

For strict compliance, use option +:row_sep+ to specify row separator <tt>"\r\n"</tt>,
which allows the compliant row separator:
  source = "foo,1\r\nbar,1\r\nbaz,2\r\n"
  CSV.parse(source, row_sep: "\r\n") # => [["foo", "1"], ["bar", "1"], ["baz", "2"]]
But rejects other row separators:
  source = "foo,1\nbar,1\nbaz,2\n"
  CSV.parse(source, row_sep: "\r\n") # Raised MalformedCSVError
  source = "foo,1\rbar,1\rbaz,2\r"
  CSV.parse(source, row_sep: "\r\n") # Raised MalformedCSVError
  source = "foo,1\n\rbar,1\n\rbaz,2\n\r"
  CSV.parse(source, row_sep: "\r\n") # Raised MalformedCSVError

===== Recipe: Handle Non-Compliant Row Separator

For data with non-compliant row separators, use option +:row_sep+.
This example source uses semicolon (<tt>";"</tt>) as its row separator:
  source = "foo,1;bar,1;baz,2;"
  CSV.parse(source, row_sep: ';') # => [["foo", "1"], ["bar", "1"], ["baz", "2"]]

==== Column Separator

RFC 4180 specifies column separator COMMA (Ruby <tt>","</tt>).

===== Recipe: Handle Compliant Column Separator

Because the \CSV default comma separator is ',',
you need not specify option +:col_sep+ for compliant data:
  source = "foo,1\nbar,1\nbaz,2\n"
  CSV.parse(source) # => [["foo", "1"], ["bar", "1"], ["baz", "2"]]

===== Recipe: Handle Non-Compliant Column Separator

For data with non-compliant column separators, use option +:col_sep+.
This example source uses TAB (<tt>"\t"</tt>) as its column separator:
  source = "foo,1\tbar,1\tbaz,2"
  CSV.parse(source, col_sep: "\t") # => [["foo", "1"], ["bar", "1"], ["baz", "2"]]

==== Quote Character

RFC 4180 specifies quote character DQUOTE (Ruby <tt>"\""</tt>).

===== Recipe: Handle Compliant Quote Character

Because the \CSV default quote character is <tt>"\""</tt>,
you need not specify option +:quote_char+ for compliant data:
  source = "\"foo\",\"1\"\n\"bar\",\"1\"\n\"baz\",\"2\"\n"
  CSV.parse(source) # => [["foo", "1"], ["bar", "1"], ["baz", "2"]]

===== Recipe: Handle Non-Compliant Quote Character

For data with non-compliant quote characters, use option +:quote_char+.
This example source uses SQUOTE (<tt>"'"</tt>) as its quote character:
  source = "'foo','1'\n'bar','1'\n'baz','2'\n"
  CSV.parse(source, quote_char: "'") # => [["foo", "1"], ["bar", "1"], ["baz", "2"]]

==== Recipe: Allow Liberal Parsing

Use option +:liberal_parsing+ to specify that \CSV should
attempt to parse input not conformant with RFC 4180, such as double quotes in unquoted fields:
  source = 'is,this "three, or four",fields'
  CSV.parse(source) # Raises MalformedCSVError
  CSV.parse(source, liberal_parsing: true) # => [["is", "this \"three", " or four\"", "fields"]]

=== Special Handling

You can use parsing options to specify special handling for certain lines and fields.

==== Special Line Handling

Use parsing options to specify special handling for blank lines, or for other selected lines.

===== Recipe: Ignore Blank Lines

Use option +:skip_blanks+ to ignore blank lines:
  source = <<-EOT
  foo,0

  bar,1
  baz,2

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

===== Recipe: Ignore Selected Lines

Use option +:skip_lines+ to ignore selected lines.
  source = <<-EOT
  # Comment
  foo,0
  bar,1
  baz,2
  # Another comment
  EOT
  parsed = CSV.parse(source, skip_lines: /^#/)
  parsed # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]

==== Special Field Handling

Use parsing options to specify special handling for certain field values.

===== Recipe: Strip Fields

Use option +:strip+ to strip parsed field values:
  CSV.parse_line(' a , b ', strip: true) # => ["a", "b"]

===== Recipe: Handle Null Fields

Use option +:nil_value+ to specify a value that will replace each field
that is null (no text):
  CSV.parse_line('a,,b,,c', nil_value: 0) # => ["a", 0, "b", 0, "c"]

===== Recipe: Handle Empty Fields

Use option +:empty_value+ to specify a value that will replace each field
that is empty (\String of length 0);
  CSV.parse_line('a,"",b,"",c', empty_value: 'x') # => ["a", "x", "b", "x", "c"]

=== Converting Fields

You can use field converters to change parsed \String fields into other objects,
or to otherwise modify the \String fields.

==== Converting Fields to Objects

Use field converters to change parsed \String objects into other, more specific, objects.

There are built-in field converters for converting to objects of certain classes:
- \Float
- \Integer
- \Date
- \DateTime

Other built-in field converters include:
- +:numeric+: converts to \Integer and \Float.
- +:all+: converts to \DateTime, \Integer, \Float.

You can also define field converters to convert to objects of other classes.

===== Recipe: Convert Fields to Integers

Convert fields to \Integer objects using built-in converter +:integer+:
  source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  parsed = CSV.parse(source, headers: true, converters: :integer)
  parsed.map {|row| row['Value'].class} # => [Integer, Integer, Integer]

===== Recipe: Convert Fields to Floats

Convert fields to \Float objects using built-in converter +:float+:
  source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  parsed = CSV.parse(source, headers: true, converters: :float)
  parsed.map {|row| row['Value'].class} # => [Float, Float, Float]

===== Recipe: Convert Fields to Numerics

Convert fields to \Integer and \Float objects using built-in converter +:numeric+:
  source = "Name,Value\nfoo,0\nbar,1.1\nbaz,2.2\n"
  parsed = CSV.parse(source, headers: true, converters: :numeric)
  parsed.map {|row| row['Value'].class} # => [Integer, Float, Float]

===== Recipe: Convert Fields to Dates

Convert fields to \Date objects using built-in converter +:date+:
  source = "Name,Date\nfoo,2001-02-03\nbar,2001-02-04\nbaz,2001-02-03\n"
  parsed = CSV.parse(source, headers: true, converters: :date)
  parsed.map {|row| row['Date'].class} # => [Date, Date, Date]

===== Recipe: Convert Fields to DateTimes

Convert fields to \DateTime objects using built-in converter +:date_time+:
  source = "Name,DateTime\nfoo,2001-02-03\nbar,2001-02-04\nbaz,2020-05-07T14:59:00-05:00\n"
  parsed = CSV.parse(source, headers: true, converters: :date_time)
  parsed.map {|row| row['DateTime'].class} # => [DateTime, DateTime, DateTime]

===== Recipe: Convert Assorted Fields to Objects

Convert assorted fields to objects using built-in converter +:all+:
  source = "Type,Value\nInteger,0\nFloat,1.0\nDateTime,2001-02-04\n"
  parsed = CSV.parse(source, headers: true, converters: :all)
  parsed.map {|row| row['Value'].class} # => [Integer, Float, DateTime]

===== Recipe: Convert Fields to Other Objects

Define a custom field converter to convert \String fields into other objects.
This example defines and uses a custom field converter
that converts each column-1 value to a \Rational object:
  rational_converter = proc do |field, field_context|
    field_context.index == 1 ? field.to_r : field
  end
  source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  parsed = CSV.parse(source, headers: true, converters: rational_converter)
  parsed.map {|row| row['Value'].class} # => [Rational, Rational, Rational]

==== Recipe: Filter Field Strings

Define a custom field converter to modify \String fields.
This example defines and uses a custom field converter
that strips whitespace from each field value:
  strip_converter = proc {|field| field.strip }
  source = "Name,Value\n foo , 0 \n bar , 1 \n baz , 2 \n"
  parsed = CSV.parse(source, headers: true, converters: strip_converter)
  parsed['Name'] # => ["foo", "bar", "baz"]
  parsed['Value'] # => ["0", "1", "2"]

==== Recipe: Register Field Converters

Register a custom field converter, assigning it a name;
then refer to the converter by its name:
  rational_converter = proc do |field, field_context|
    field_context.index == 1 ? field.to_r : field
  end
  CSV::Converters[:rational] = rational_converter
  source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  parsed = CSV.parse(source, headers: true, converters: :rational)
  parsed['Value'] # => [(0/1), (1/1), (2/1)]

==== Using Multiple Field Converters

You can use multiple field converters in either of these ways:
- Specify converters in option +:converters+.
- Specify converters in a custom converter list.

===== Recipe: Specify Multiple Field Converters in Option +:converters+

Apply multiple field converters by specifying them in option +:conveters+:
  source = "Name,Value\nfoo,0\nbar,1.0\nbaz,2.0\n"
  parsed = CSV.parse(source, headers: true, converters: [:integer, :float])
  parsed['Value'] # => [0, 1.0, 2.0]

===== Recipe: Specify Multiple Field Converters in a Custom Converter List

Apply multiple field converters by defining and registering a custom converter list:
  strip_converter = proc {|field| field.strip }
  CSV::Converters[:strip] = strip_converter
  CSV::Converters[:my_converters] = [:integer, :float, :strip]
  source = "Name,Value\n foo , 0 \n bar , 1.0 \n baz , 2.0 \n"
  parsed = CSV.parse(source, headers: true, converters: :my_converters)
  parsed['Name'] # => ["foo", "bar", "baz"]
  parsed['Value'] # => [0, 1.0, 2.0]

=== Converting Headers

You can use header converters to modify parsed \String headers.

Built-in header converters include:
- +:symbol+: converts \String header to \Symbol.
- +:downcase+: converts \String header to lowercase.

You can also define header converters to otherwise modify header \Strings.

==== Recipe: Convert Headers to Lowercase

Convert headers to lowercase using built-in converter +:downcase+:
  source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  parsed = CSV.parse(source, headers: true, header_converters: :downcase)
  parsed.headers # => ["name", "value"]

==== Recipe: Convert Headers to Symbols

Convert headers to downcased Symbols using built-in converter +:symbol+:
  source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  parsed = CSV.parse(source, headers: true, header_converters: :symbol)
  parsed.headers # => [:name, :value]
  parsed.headers.map {|header| header.class} # => [Symbol, Symbol]

==== Recipe: Filter Header Strings

Define a custom header converter to modify \String fields.
This example defines and uses a custom header converter
that capitalizes each header \String:
  capitalize_converter = proc {|header| header.capitalize }
  source = "NAME,VALUE\nfoo,0\nbar,1\nbaz,2\n"
  parsed = CSV.parse(source, headers: true, header_converters: capitalize_converter)
  parsed.headers # => ["Name", "Value"]

==== Recipe: Register Header Converters

Register a custom header converter, assigning it a name;
then refer to the converter by its name:
  capitalize_converter = proc {|header| header.capitalize }
  CSV::HeaderConverters[:capitalize] = capitalize_converter
  source = "NAME,VALUE\nfoo,0\nbar,1\nbaz,2\n"
  parsed = CSV.parse(source, headers: true, header_converters: :capitalize)
  parsed.headers # => ["Name", "Value"]

==== Using Multiple Header Converters

You can use multiple header converters in either of these ways:
- Specify header converters in option +:header_converters+.
- Specify header converters in a custom header converter list.

===== Recipe: Specify Multiple Header Converters in Option :header_converters

Apply multiple header converters by specifying them in option +:header_conveters+:
  source = "Name,Value\nfoo,0\nbar,1.0\nbaz,2.0\n"
  parsed = CSV.parse(source, headers: true, header_converters: [:downcase, :symbol])
  parsed.headers # => [:name, :value]

===== Recipe: Specify Multiple Header Converters in a Custom Header Converter List

Apply multiple header converters by defining and registering a custom header converter list:
  CSV::HeaderConverters[:my_header_converters] = [:symbol, :downcase]
  source = "NAME,VALUE\nfoo,0\nbar,1.0\nbaz,2.0\n"
  parsed = CSV.parse(source, headers: true, header_converters: :my_header_converters)
  parsed.headers # => [:name, :value]

=== Diagnostics

==== Recipe: Capture Unconverted Fields

To capture unconverted field values, use option +:unconverted_fields+:
  source = "Name,Value\nfoo,0\nbar,1\nbaz,2\n"
  parsed = CSV.parse(source, converters: :integer, unconverted_fields: true)
  parsed # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]
  parsed.each {|row| p row.unconverted_fields }
Output:
  ["Name", "Value"]
  ["foo", "0"]
  ["bar", "1"]
  ["baz", "2"]

==== Recipe: Capture Field Info

To capture field info in a custom converter, accept two block arguments.
The first is the field value; the second is a +CSV::FieldInfo+ object:
  strip_converter = proc {|field, field_info| p field_info; field.strip }
  source = " foo , 0 \n bar , 1 \n baz , 2 \n"
  parsed = CSV.parse(source, converters: strip_converter)
  parsed # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]
Output:
  #<struct CSV::FieldInfo index=0, line=1, header=nil>
  #<struct CSV::FieldInfo index=1, line=1, header=nil>
  #<struct CSV::FieldInfo index=0, line=2, header=nil>
  #<struct CSV::FieldInfo index=1, line=2, header=nil>
  #<struct CSV::FieldInfo index=0, line=3, header=nil>
  #<struct CSV::FieldInfo index=1, line=3, header=nil>