CODEWITHSUNDEEP
rubycsv
Trevor Steen
Trevor Steen

Reputation: 87

Update append to CSV line in-place, Ruby

I am trying to do something similar to this question: Parse a CSV file, update a field, then save

Essentially I have a CSV with each line having a comma separated list of IPs, each row represents a different IP group. I pass this CSV to a function that does some work on each IP per row. After that I want to append a status and timestamp to the line. The solution above requires the creation of a second file, is there a way to do this without creating the additional file, just appending to each row in place?

CSV.open('csv_with_ips.csv', 'r+').each do |row|
  <do some stuff>
  row << 'SCANNED'
  row << Time.now
end

Upvotes: 2

Views: 1405

Answers (1)

the Tin Man
the Tin Man

Reputation: 160551

You don't have to create a new file, but good practice says you should.

Updating a line with a longer line requires the OS to overwrite the following line and possible subsequent ones. If the code crashes before processing the entire file the content will be corrupted.

Also, doing what you want implies you have to read the entire file into memory, which isn't a good idea as your code won't scale well. If you don't read it into memory the line you write will stomp on the next line you want to read, corrupting it.

Safe processing of the data would go something like this:

File.open('output.csv', 'w') do |fo|
  File.foreach('input.csv') do |li|
    # modify the input line "li"
    fo.puts(li)
  end
end

File.mv('input.csv', 'input.csv.bak')
File.mv('output.csv', 'input.csv')

But, of course, you'd want to take advantage of the CSV class that comes with Ruby and let it handle the heavy lifting. CSV file format isn't as simple as people think and it's easy to mess up the data so take advantage of what the CSV class implements.

Upvotes: 4

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