Ruby threads and variable

Question

Why the result is not from 1 to 10, but 10s only?

require \'thread\'

def run(i)
  puts i
end

while true
  for i in 0..10
    Thread.new{ run(i)}
  end
  sl         


        

Answer 1

The problem is that you have created 11 threads that are all trying to access the same variable i which was defined by the main thread of your program. One trick to avoid that is to call Thread.new inside a method; then the variable i that the thread has access to is just the particular i that was passed to the method, and it is not shared with other threads. This takes advantage of a closure.

require 'thread'

def run(i)
  puts i
end

def start_thread(i)
  Thread.new { run i }
end

for i in 0..10
  start_thread i
  sleep 0.1
end

Result:

0
1
2
3
4
5
6
7
8
9
10

(I added the sleep just to guarantee that the threads run in numerical order so we can have tidy output, but you could take it out and still have a valid program where each thread gets the correct argument.)