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]
- {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]

=== 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
Ouput:
  #<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"]

=== 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:
- <tt>:numeric</tt>: converts to \Integer and \Float.
- <tt>:all</tt>: 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 <tt>:integer</tt>:
  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 <tt>:float</tt>:
  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 <tt>:numeric</tt>:
  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 <tt>:date</tt>:
  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 <tt>:date_time</tt>:
  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 <tt>:all</tt>:
  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 <tt>:converters</tt>.
- Specify converters in a custom converter list.

===== Recipe: Specify Multiple Field Converters in Option <tt>:converters</tt>

Apply multiple field converters by specifying them in option <tt>:conveters</tt>:
  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:
- <tt>:symbol</tt>: converts \String header to \Symbol.
- <tt>:downcase</tt>: 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 <tt>:downcase</tt>:
  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 <tt>:symbol</tt>:
  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 <tt>:header_converters</tt>.
- 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 <tt>:header_conveters</tt>:
  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]