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Parse CSV file with header fields as attributes for each row

Posted by: admin November 30, 2017 Leave a comment

Questions:

I would like to parse a CSV file so that each row is treated like an object with the header-row being the names of the attributes in the object. I could write this, but I’m sure its already out there.

Here is my CSV input:

"foo","bar","baz"
1,2,3
"blah",7,"blam"
4,5,6

The code would look something like this:

CSV.open('my_file.csv','r') do |csv_obj|
  puts csv_obj.foo   #prints 1 the 1st time, "blah" 2nd time, etc
  puts csv.bar       #prints 2 the first time, 7 the 2nd time, etc
end

With Ruby’s CSV module I believe I can only access the fields by index. I think the above code would be a bit more readable. Any ideas?

Answers:

Using Ruby 1.9 and above, you can get a an indexable object:

CSV.foreach('my_file.csv', :headers => true) do |row|
  puts row['foo'] # prints 1 the 1st time, "blah" 2nd time, etc
  puts row['bar'] # prints 2 the first time, 7 the 2nd time, etc
end

It’s not dot syntax but it is much nicer to work with than numeric indexes.

As an aside, for Ruby 1.8.x FasterCSV is what you need to use the above syntax.

Questions:
Answers:

Here is an example of the symbolic syntax using Ruby 1.9. In the examples below, the code reads a CSV file named data.csv from Rails db directory.

:headers => true treats the first row as a header instead of a data row. :header_converters => :symbolize parameter then converts each cell in the header row into Ruby symbol.

CSV.foreach("#{Rails.root}/db/data.csv", {:headers => true, :header_converters => :symbol}) do |row|
  puts "#{row[:foo]},#{row[:bar]},#{row[:baz]}"
end

In Ruby 1.8:

require 'fastercsv'
CSV.foreach("#{Rails.root}/db/data.csv", {:headers => true, :header_converters => :symbol}) do |row|
  puts "#{row[:foo]},#{row[:bar]},#{row[:baz]}"
end

Based on the CSV provided by the Poul (the StackOverflow asker), the output from the example code above will be:

1,2,3
blah,7,blam
4,5,6

Depending on the characters used in the headers of the CSV file, it may be necessary to output the headers in order to see how CSV (FasterCSV) converted the string headers to symbols. You can output the array of headers from within the CSV.foreach.

row.headers

Questions:
Answers:

Although I am pretty late to the discussion, a few months ago I started a “CSV to object mapper” at https://github.com/vicentereig/virgola.

Given your CSV contents, mapping them to an array of FooBar objects is pretty straightforward:

"foo","bar","baz"
1,2,3
"blah",7,"blam"
4,5,6
require 'virgola'

class FooBar
  include Virgola

  attribute :foo
  attribute :bar
  attribute :baz
end

csv = <<CSV
"foo","bar","baz"
1,2,3
"blah",7,"blam"
4,5,6
CSV

foo_bars = FooBar.parse(csv).all
foo_bars.each { |foo_bar| puts foo_bar.foo, foo_bar.bar, foo_bar.baz }

Questions:
Answers:

Easy to get a hash in Ruby 2.3:

CSV.foreach('my_file.csv', headers: true, header_converters: :symbol) do |row|
  puts row.to_h[:foo]
  puts row.to_h[:bar]
end

Questions:
Answers:

Since I hit this question with some frequency:

array_of_hashmaps = CSV.read("path/to/file.csv", headers: true)
puts array_of_hashmaps.first["foo"] # 1

This is the non-block version, when you want to slurp the whole file.