How to edit every x amount of lines in txtfile in Ruby? - ruby

Question

I'm trying to change something in every other line in a text file using Ruby (and some text files I need to change something every third line and so on.)
I found this question helpful for iterating over every line, but I specifically need help making changes every x amount of lines.
The ### is the part I'm having trouble with (the iterating over x amount of lines.)
text = File.open('fr.txt').read
clean = ### .sub("\n", " ");
new = File.new("edit_fr.txt", "w")
new.puts clean
new.close

Answer 1

You can use modulus division as below where n refers to the nth line you want to process and i refers to the 0-based index for the file lines. Using those two values, modulo math provides the remainder from integer division which will be 0 whenever the 1-based index (i+1) is multiple of n.
n = 3 # modify every 3rd line
File.open('edit_fr.txt','w') do |f| # Open the output file
File.open('fr.txt').each_with_index do |line,i| # Open the input file
if (i+1) % n == 0 # Every nth line
f.print line.chomp # Remove newline
else # Every non-nth line
f.puts line # Print line
end
end
end
More info is available on Wikipedia: http://en.wikipedia.org/wiki/Modulo_operation
In computing, the modulo operation finds the remainder after division of one number by another (sometimes called modulus).
Given two positive numbers, a (the dividend) and n (the divisor), a modulo n (abbreviated as a mod n) is the remainder of the Euclidean division of a by n. For instance, the expression "5 mod 2" would evaluate to 1 because 5 divided by 2 leaves a quotient of 2 and a remainder of 1, while "9 mod 3" would evaluate to 0 because the division of 9 by 3 has a quotient of 3 and leaves a remainder of 0; there is nothing to subtract from 9 after multiplying 3 times 3. (Note that doing the division with a calculator will not show the result referred to here by this operation; the quotient will be expressed as a decimal fraction.)

Answer 2

every_other = 2
File.open('data.txt') do |f|
e = f.each
target_line = nil
loop do
every_other.times do
target_line = e.next
end
puts target_line
end
end

Answer 3

You wish to write each line of an input file to an output file, but you want to modify each nth line of the input file before writing it, beginning with the first line of the file.
Suppose we have defined a method modify, which accepts a line of text as its argument and returns a modified string. Then you can do it like this:
def modify_and_write(in_fname, out_fname, n)
enum = Array.new(n) { |i| i.zero? ? :process : :skip }.cycle
f = File.open(out_fname, 'w')
IO.foreach(in_fname) do |line|
(line = process(line)) if enum.next == :process
f.puts(line)
end
f.close
end
I'm reading one line at a time (rather using IO#readlines) to read the entire file into an array) so that it will work with files of any size.
Suppose:
n = 3
The key here is the enumerator:
enum = Array.new(n) { |i| i.zero? ? :process : :skip }.cycle
#=> #<Enumerator: [:process, :skip, :skip]:cycle>
enum.next #=> :process
enum.next #=> :skip
enum.next #=> :skip
enum.next #=> :process
enum.next #=> :skip
enum.next #=> :skip
enum.next #=> :process
enum.next #=> :skip
...
Edit: after answering I noticed the OP's comment: I need to combine every two lines: line1 /n line2 /n line3 /n line would become line1 space line2 /n line3 space line4, which is not consistent with "I'm trying to change something in every other line in a text file". To address the specific requirement, my solution could be modified as follows:
def combine_lines(in_fname, out_fname, n)
enum = Array.new(n) { |i| (i==n-1) ? :write : :read }.cycle
f = File.open(out_fname, 'w')
combined = []
IO.foreach(in_fname) do |line|
combined << line.chomp
if enum.next == :write
f.puts(combined.join(' '))
combined.clear
end
end
f.puts(combined.join(' ')) if combined.any?
f.close
end
Let's try it:
text =<<_
Now is
the time
for all
good
Rubyists
to do
something
other
than
code.
_
File.write('in',text)
combine_lines('in', 'out', 3)
puts File.read('out')
# Now is the time for all
# good Rubyists to do
# something other than
# code.
You could also use a regex, as #Stefan has done, which would be my preference for less-than-humongous files. Here's another regex implementation:
def combine_lines(in_fname, out_fname, n)
IO.write(out_fname,
IO.read(in_fname)
.scan(/(?:.*?\n){1,#{n}}/)
.map { |s| s.split.join(' ') }
)
end
combine_lines('in', 'out', 3)
puts File.read('out')
# Now is the time for all
# good Rubyists to do
# something other than
# code.
We could write the above regex with the final / changed to /x to include comments:
r = /
(?: # begin a non-capture group
.*? # match any number of any character, non-greedily
\n # match (the first, because of non-greedily) end-of-line
) # end the non-capture group
{1,#{n}} # match between 1 and n of the preceding non-capture group
/x
{1,#{n}} is "greedy" in the sense that it will match as many lines as possible, up to n. If the number of lines were always a multiple of n, we could instead write {{#n}}, meaning match n non-capture groups (i.e., n lines). However, if the number of lines is not a multiple of n (as in my example above), we need {1,#{n}} to match the last few lines in the last non-capture group.

Answer 4

I think you could do it with just a regex:
EDIT
OK, I knew I could do this with each_slice and a simple regex:
def chop_it(file,num)
#file name and the number of lines to join
arr = []
#create an empty array to hold the lines we create
File.open(file) do |f|
#open your file into a `do..end` block, it closes automatically for you
f.each_slice(num) do |slice|
#get an array of lines equal to num
arr << slice.join(' ').gsub!(/\n/, '') + "\n"
#join the lines with ' ', then remove all the newlines and tack one
# on the end, adding the resulting line to the array.
end
end
arr.join
#join all of the lines back into one string that can be sent to a file.
end
And there you have it, simple and flexible. Just enter file name and the number of lines you want reduced down to one line. i.e. if you want every two lines joined, chop_it('data.txt',2). Every three? chop_it('data.txt,3).
** old answer **
old_text = File.read(data.txt)
new_text = old_text.gsub(/(?:(^.*)\n(^.*\n))/i,'\1 \2')
The regex matches the first line up to "\n" and the second line up to and including "\n". The substitution returns the two matches with a space between them.
"this is line one\nthis is line two\n this is line three\nthis is line four]n"
\1 = "this is line one"
\2 = "this is line two\n"
'\1 \2' = "this is line one this is line two\n"
This regex will also handle removing every other blank line in successive blank lines

Answer 5

new = File.new("edit_fr.txt", "w")
File.readlines("test.txt").each_slice(2) do |batch| # or each_slice(3) etc
new.puts batch.map(&:chomp).join(" ")
end
new.close

Answer 6

I need to combine every two lines: line1 /n line2 /n line3 /n line would become line1 space line2 /n line3 space line4
You could read the entire file into a string, use gsub! and with_index to replace every nth newline with space and write the replaced content to a new file:
content = IO.read('fr.txt')
content.gsub!("\n").with_index(1) { |m, i| (i % 2).zero? ? m : ' ' }
IO.write('edit-fr.txt', content)
Input fr.txt:
line1
line2
line3
line4
Output edit-fr.txt:
line1 line2
line3 line4