From: 7stud -- <dolgun@...>
Date: 2007-09-27T14:14:52+09:00
Subject: Re: some file operations

blufur wrote:
> A couple quick questions:
> 
> how can my ruby program tell me how many lines are in a text file?
> 
> how can i have my ruby program edit the last line of a text file (not
> add another line at the end, but get and then change the line at the
> end).
> 


#Read the file and count the lines:

File.open("data.txt", "w") do |file|
  (1..25).each {|num| file.puts("line #{num}")}
end

count = 0
File.open("data.txt") do |file|
  file.each {count += 1}
end

puts count   #25


#To change the last line of a file that isn't
#extremely large:

lines_arr = IO.readlines("data.txt")
lines_arr[-1] = "hello world\n"

File.open("temp.txt", "w") do |file|
    lines_arr.each do |line|
      file.write(line)
    end
end

File.delete("data.txt")
File.rename("temp.txt", "data.txt")


#Display the result:

File.open("data.txt") do |file|
  file.each do |line|
    print line
  end
end

--output--
line 1
line 2
line 3
...
...
line 23
line 24
hello world



For extremely large files on the order of 1-2GB, you can try something 
like this:

last_line = nil
temp_file = File.new("temp.txt", "w")

File.open("data.txt") do |file|
  file.each do |line|
    if last_line
      temp_file.write(last_line)
    end

    last_line = line
  end
end

last_line = "hello world\n"
temp_file.write(last_line)
temp_file.close()

#Delete and rename as above
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