From: Bob Bobrov <bob.bobrov@...>
Date: 2008-06-19T06:23:13+09:00
Subject: Threads and synchronized access to an array

Hi everyone!
A n00b question - I have a following ruby code:
===============================================
@arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]

def iterate(thread_number)
  @arr.each do |line|
    puts "#{line} - I am thread number #{thread_number}"
  end
end

number_of_threads = 4

threads = []
number_of_threads.times do |t|
  threads[t] = Thread.new do
    iterate(t)
  end
end

threads.each {|t| t.join}
================================================



The output for this code is:
================================================
1 - I am thread number 0
2 - I am thread number 0
3 - I am thread number 0
4 - I am thread number 0
5 - I am thread number 0
6 - I am thread number 0
7 - I am thread number 0
8 - I am thread number 0
1 - I am thread number 1
2 - I am thread number 1
3 - I am thread number 1
4 - I am thread number 1
5 - I am thread number 1
6 - I am thread number 1
7 - I am thread number 1
8 - I am thread number 1
1 - I am thread number 2
2 - I am thread number 2
3 - I am thread number 2
4 - I am thread number 2
5 - I am thread number 2
6 - I am thread number 2
7 - I am thread number 2
8 - I am thread number 2
1 - I am thread number 3
2 - I am thread number 3
3 - I am thread number 3
4 - I am thread number 3
5 - I am thread number 3
6 - I am thread number 3
7 - I am thread number 3
8 - I am thread number 3
================================================


What should be changed in this code to make the output looks like this?
================================================
1 - I am thread number #{thread_number}
2 - I am thread number #{thread_number}
3 - I am thread number #{thread_number}
4 - I am thread number #{thread_number}
5 - I am thread number #{thread_number}
6 - I am thread number #{thread_number}
7 - I am thread number #{thread_number}
8 - I am thread number #{thread_number}
================================================

I want to iterate the array values only one time by some number of
threads, how should I control the access to array values? Each thread
must be prohobited from using the array values, which have been already
used by other threads.

I was trying to change the code like this:
@mutex = Mutex.new
@arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]

@mutex.synchronize {
  def iterate(thread_number)
    @arr.each do |line|
      puts "#{line} - I am thread number #{thread_number}"
    end
  end
}

number_of_threads = 4

threads = []
number_of_threads.times do |t|
  threads[t] = Thread.new do
    iterate(t)
  end
end

threads.each {|t| t.join}

and this:
@mutex = Mutex.new
@arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]


def iterate(thread_number)
  @arr.each do |line|
    puts "#{line} - I am thread number #{thread_number}"
  end
end


number_of_threads = 4

threads = []
number_of_threads.times do |t|
  threads[t] = Thread.new do
    @mutex.synchronize {
      iterate(t)
    }
  end
end

threads.each {|t| t.join}
-- 
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