Ruby getting a particular line from a file?

Question

Is it possible to extract a particular line from a file knowing it's line number? e.g just get the contents of line N as a string from file text.txt

Answer 1

Try one of these two solutions:

f = File.open('file.txt')

#1 solution
p [*f][n-1]

#2 solution
n.times{f.gets}
p $_

f.close

But I don't sure about how much memory will be used in #1.

Answer 2

You could get it by index from readlines.

line = IO.readlines("file.txt")[42]

Only use this if it's a small file.

Answer 3

def get_line_from_file(path, line)
  result = nil

  File.open(path, "r") do |f|
    while line > 0
      line -= 1
      result = f.gets
    end
  end

  return result
end

get_line_from_file("/tmp/foo.txt", 20)

This is a good solution because:

• You don't use File.read, thus you don't read the entire file into memory. Doing so could become a problem if the file is 20MB large and you read often enough so GC doesn't keep up.
• You only read from the file until the line you want. If your file has 1000 lines, getting line 20 will only read the 20 first lines into Ruby.

You can replace gets with readline if you want to raise an error (EOFError) instead of returning nil when passing an out-of-bounds line.

Answer 4

linenumber=5
open("file").each_with_index{|line,ind|
  if  ind+1==linenumber
    save=line
    # break or exit if needed.
  end
}

or

linenumber=5
f=open("file")
while line=f.gets
  if $. == linenumber # $. is line number
    print "#{f.lineno} #{line}" # another way
    # break  # break or exit if needed
  end
end
f.close

If you just want to get the line and do nothing else, you can use this one liner

ruby -ne '(print $_ and exit) if $.==5' file

Answer 5

File has a nice #lineno method.

def get_line(filename, lineno)
  File.open(filename,'r') do |f|
     f.gets until f.lineno == lineno - 1
     f.gets
  end
end

Answer 6

Here's a very small method (probably too small, it sacrifices some readability)

def get_line(file, n) 
  open(file) do |f|; n.times {f.gets}; end; $_
end

EDIT: Here's another way:

class File
  def get_line(n)
    n.times{self.gets}
    $_
  end
end

Then you can do:

open(file).get_line(20)

Answer 7

If you want one liner and do not care about memory usage, use (assuming lines are numbered from 1)

lineN = IO.readlines('text.txt')[n-1]

or

lineN = f.readlines[n-1]

if you already have file opened.

Otherwise it would be better to do like this:

lineN = File.open('text.txt') do |f|
          (n-1).times { f.gets } # skip lines preceeding line N
          f.gets                 # read line N contents
        end