How to add new line in a file - ruby

Question

I want to add newline character below.
But the result is wrong.
Teach me what is wrong.
test.txt(before)
------------------
2014-09
2014-10
2014-11
------------------
test.txt(after)
------------------
2014-09
2014-10
2014-11
------------------
I make a ruby script below, but the result is wrong.
f = File.open("test.txt","r+")
f.each{|line|
if line.include?("2014-10")
f.puts nil
end
}
f.close
the result
------------------
2014-09
2014-10
014-11
------------------

Answer 1

To solve your problem, the easiest way is to create a new file to output your new text into. To do you'll need to open the input file and the output file and iterate each line of the file check the condition and put desired line into the output file.
Example
require 'fileutils'
File.open("text-output.txt", "w") do |output|
File.foreach("text.txt") do |line|
if line.include?("2014-10")
output.puts line + "\n"
else
output.puts line
end
end
end
FileUtils.mv("text-output.txt", "text.txt")
Easy way
File.write(f = "text.txt", File.read(f).gsub(/2014-10/,"2014-10\n"))

Answer 2

Reading and writing a file at the same time can get messy, same thing with other data structures like arrays. You should build a new file as you go along.
Some notes:
you should use the block form of File.open because it will stop you from forgetting to call f.close
puts nil is the same as puts without arguments
single quotes are preferred over double quotes when you don’t need string interpolation
you should use do ... end instead of { ... } for multi-line blocks
File.open(...).each can be replaced with File.foreach
the intermediate result can be stored in a StringIO object which will respond to puts etc.
Example:
require 'stringio'
file = 'test.txt'
output = StringIO.new
File.foreach(file) do |line|
if line.include? '2014-10'
output.puts
else
output << line
end
end
output.rewind
File.open(file, 'w') do |f|
f.write output.read
end