How to add/read rows from Ruby CSV instance

Question

Though it seems far more common for people to use the Ruby CSV class methods, I have an occasion to use a CSV instance, but it seams completely uncooperative.

What I'd like to do is create a CSV instance, add some rows to it, then be able to retrieve all those rows and write them to a file. Sadly, the following code doesn't work as I would like at all.

require 'csv'

csv = CSV.new('', headers: ['name', 'age'])
csv.read # Apparently I need to do this so that the headers are actually read in.

csv.add_row(['john', '22'])
csv.add_row(['jane', '24'])
csv.read

csv.to_a
csv.to_s

All I want to be able to retrieve the information I put into the csv and then write that to a file, but I can't seem to do that :/

What am I doing wrong?

Wand Maker · Accepted Answer · 2015-09-25 19:10:18Z

3

You need to use CSV#rewind

Here is the sample:

require 'csv'

csv = CSV.new(File.new("data1.csv", "r+"), headers: ['name', 'age'], write_headers: true)

csv.add_row(['john', '22'])
csv.add_row(['jane', '24'])

p csv.to_a  # Empty array

csv.rewind

p csv.to_a  # Array with three CSV::Row objects (including header)

answered Sep 25, 2015 at 19:10

Wand Maker

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2 Comments

the Tin Man Over a year ago

Be careful with rewind, because it moves CSV's concept of the file placeholder back to the top, and any subsequent add_row statements would be inserted at the top of the file, not at the bottom where we'd expect.

Krishna_Kulkarni Over a year ago

Rewind seems like a bit of hack to achieve what I want (which seems weird to me since I feel like my use case is not abnormal), but it does work -- thank you! Follow up: After csv.rewind, is this a better way to then write this to a file (in CSV format)than this: csv.map(&:to_s).join(',')

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