Generate random 10% of file to be used in testing with Ruby

Question

I'm new to Ruby, but not to programming. I just need a simple script that, given a text file, will pull out around 10% of the lines at random. Below is what I came up with based upon a Python script I wrote. What do I need to do differently to make it more Ruby-like?

prng = Random.new

File.open('english-words-partial.txt', 'w') {  |f| 
  File.readlines('english-words-full.txt').each do |line| 
    if prng.rand >= 0.9
      f.write(line) 
    end
  end
}

Answer 1

Here's my shot at it:

File.open('english-words-partial.txt', 'w') do |file|
    File.foreach('english-words-full.txt') do |line|
        file.puts(line) if rand(10) == 0
    end
end

In Ruby, you can add an if statement to the end of a line, to conditionally execute that line. This also works with the unless statement.

Also, the rand function can take a number, and produce that many possible numbers from it. So rand(10) will give you a random number from 0 to 9

In Ruby, you usually want to use { } for 1 line blocks, and do/end for multiline blocks, but they both act slightly different. Theres more on that here: https://stackoverflow.com/questions/5587264/do-end-vs-curly-braces-for-blocks-in-ruby

File.foreach('...') is just a bit shorter and neater then File.readlines('...').each (thanks to @Flambino for this)

And lastly, you can use IO methods on a file object like print and puts, so I liked that better but file.write works too. Thats really personal preference.

Answer 2

This is even more random (since the order will change):

File.open('english-words-partial.txt', 'w') {  |f| 
  full = File.readlines('english-words-full.txt')
  f.puts full.sample(full.size/10)
end

Answer 3

Here's a way that uses the method Array#sample and several class methods for reading and writing text files that are defined in the IO class. I have made the following assumptions:

• a specified number of lines are to be chosen at random
• the lines chosen at random are to be kept in order
• the entire file is not to be read into memory
• lines to be written to file are not accumulated in memory before being written

Code

def doit(fname_in, fname_out, nbr)
  n_lines = File.foreach(fname_in).reduce(0) { |c, _| c+1 }
  return nil if n_lines.zero? || nbr.zero? 
  ndx = [*(0...n_lines)].sample(nbr).sort
  f_out = File.open(FNAME_OUT, 'w')
  offset = ndx.shift
  File.foreach(fname_in).with_index do |line, i|
    if i == offset
      f_out.write(line)
      offset = ndx.shift
    end     
    break unless offset
  end
  f_out.close
end

Example

FNAME_IN, FNAME_OUT = 'infile', 'outfile'

Let's first create an input file:

lines =<<_
Now
is
the
time
for
all
good
Rubyists
to
put
aside
work
and
watch
the
World
Cup
final
between
Germany
and
Argentina
_

File.write(FNAME_IN, lines) #=> 116

We can confirm the contents of this file with:

File.foreach(FNAME_IN) { |l| puts l }

Now try it:

doit(FNAME_IN, FNAME_OUT, 10)

Let's see what was written. (Drum roll.)

File.foreach(FNAME_OUT) { |l| puts l }
Now
is
the
for
to
aside
watch
World
Cup
final