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How do I create a new CSV file in Ruby?

Posted by: admin November 30, 2017 Leave a comment

Questions:

I have a CSV file called “A.csv”. I need to generate a new CSV file called “B.csv” with data from “A.csv”.

I will be using a subset of columns from “A.csv” and will have to update one column’s value to new value in “B.csv”. Then I use this data from B.csv to validate at database.

  1. How do I create a new CSV file?
  2. How do I copy the required column’s data from A.csv to “B.csv”?
  3. How do I append values for a particular column?

I am able to read CSV, get an array, or hash.

Answers:

As mikeb pointed out, there are the docs – http://ruby-doc.org/stdlib-1.9.3/libdoc/csv/rdoc/CSV.html – Or you can follow along with the examples below (all are tested and working):

To create a new file:

In this file we’ll have two rows, a header row and data row, very simple CSV:

require "csv"
CSV.open("file.csv", "wb") do |csv|
  csv << ["animal", "count", "price"]
  csv << ["fox", "1", "$90.00"]
end

result, a file called “file.csv” with the following:

animal,count,price
fox,1,$90.00

How to append data to a CSV

Almost the same forumla as above only instead of using “wb” mode, we’ll use “a+” mode. For more information on these see this stack overflow answer: What are the Ruby File.open modes and options?

CSV.open("file.csv", "a+") do |csv|
  csv << ["cow", "3","2500"]
end

Now when we open our file.csv we have:

animal,count,price
fox,1,$90.00
cow,3,2500

Read from our CSV file

Now you know how to copy and to write to a file, to read a CSV and therefore grab the data for manipulation you just do:

CSV.foreach("file.csv") do |row|
  puts row #first row would be ["animal", "count", "price"] - etc.
end

Of course this is like one of like a hundred different ways you can pull info from a CSV using this gem. For more info I suggest visiting the docs now that you have a primer: http://ruby-doc.org/stdlib-1.9.3/libdoc/csv/rdoc/CSV.html

Questions:
Answers:

Have you seen Ruby’s CSV class? It seems pretty comprehensive. Check it out here:
http://ruby-doc.org/stdlib-1.9.3/libdoc/csv/rdoc/CSV.html