Delete first two lines and add two lines to file - ruby

Question

I have a text file that starts with:
Title
aaa
bbb
ccc
I don't know what the line would include, but I know that the structure of the file will be Title, then an empty line, then the actual lines. I want to modify it to:
New Title
fff
aaa
bbb
ccc
I had this in mind:
lineArray = File.readlines(destinationFile).drop(2)
lineArray.insert(0, 'fff\n')
lineArray.insert(0, '\n')
lineArray.insert(0, 'new Title\n')
File.writelines(destinationFile, lineArray)
but writelines doesn't exist.
`writelines' for File:Class (NoMethodError)
Is there a way to delete the first two lines of the file an add three new lines?

Answer 1

I'd start with something like this:
NEWLINES = {
0 => "New Title",
1 => "\nfff"
}
File.open('test.txt.new', 'w') do |fo|
File.foreach('test.txt').with_index do |li, ln|
fo.puts (NEWLINES[ln] || li)
end
end
Here's the contents of test.txt.new after running:
New Title
fff
aaa
bbb
ccc
The idea is to provide a list of replacement lines in the NEWLINES hash. As each line is read from the original file the line number is checked in the hash, and if the line exists then the corresponding value is used, otherwise the original line is used.
If you want to read the entire file then substitute, it reduces the code a little, but the code will have scalability issues:
NEWLINES = [
"New Title",
"",
"fff"
]
file = File.readlines('test.txt')
File.open('test.txt.new', 'w') do |fo|
fo.puts NEWLINES
fo.puts file[(NEWLINES.size - 1) .. -1]
end
It's not very smart but it'll work for simple replacements.
If you really want to do it right, learn how diff works, create a diff file, then let it do the heavy lifting, as it's designed for this sort of task, runs extremely fast, and is used millions of times every day on *nix systems around the world.

Answer 2

Use put with the whole array:
File.open("destinationFile", "w+") do |f|
f.puts(lineArray)
end

Answer 3

If your files are big, the performance and memory implications of reading them into memory in their entirety are worth thinking about. If that's a concern, then your best bet is to treat the files as streams. Here's how I would do it.
First, define your replacement text:
require "stringio"
replacement = StringOI.new <<END
New Title
fff
END
I've made this a StringIO object, but it could also be a File object if your replacement text is in a file.
Now, open your destination file (a new file) and write each line from the replacement text into it.
dest = File.open(dest_fn, 'wb') do |dest|
replacement.each_line {|ln| dest << ln }
We could have done this more efficiently, but there's a good reason to do it this way: Now we can call replacement.lineno to get the number of lines read, instead of iterating over it a second time to count the lines.
Next, open the original file and seek ahead by calling gets replacement.lineno times:
orig = File.open(orig_fn, 'r')
replacement.lineno.times { orig.gets }
Finally, write the remaining lines from the original file to the new file. We'll do it more efficiently this time with File.copy_stream:
File.copy_stream(orig, dest)
orig.close
dest.close
That's it. Of course, it's a drag closing those files manually (and when we do we should do it in an ensure block), so it's better to use the block form of File.open to automatically close them. Also, we can move the orig.gets calls into the replacement.each_line loop:
File.open(dest_fn, 'wb') do |dest|
File.open(orig_fn, 'r') do |orig|
replacement.each_line {|ln| dest << ln; orig.gets }
File.copy_stream(orig, dest)
end
end

Answer 4

First create an input test file.
FNameIn = "test_in"
text = <<_
Title
How now,
brown cow?
_
#=> "Title\n\nHow now,\nbrown cow?\n"
File.write(FNameIn, text)
#=> 27
Now read and write line-by-line.
FNameOut = "test_out"
File.open(FNameIn) do |fin|
fin.gets; fin.gets
File.open(FNameOut, 'w') do |fout|
fout.puts "New Title"
fout.puts
fout.puts "fff"
until fin.eof?
fout.puts fin.gets
end
end
end
Check the result:
puts File.read(FNameOut)
# New Title
#
# fff
# How now,
# brown cow?
Ruby will close each of the two files when its block terminates.
If the files are not large, you could instead write:
File.write(FNameOut,
["New Title\n", "\n", "fff\n"].concat(File.readlines(FNameIn).drop(2)).join)