scala - What is the purpose of an empty loop in this volatile var example? -

-

code:

  class page(val txt: string, var position: int)    val rand = new random()   object volatile {     val pages = for(i <- 1 5) yield new page("na" * rand.nextint(1000) + " batman!", -1)     @volatile var found = false     (p <- pages) yield thread {       var = 0       while (i < p.txt.length && !found) if (p.txt(i) == '!') {         p.position =         found = true       } else += 1       while(!found) {}       log(s"results: ${pages.map(_.position)}")     }   }

what purpose of empty loop while(!found) {}?

the first loop: while (i < p.txt.length && !found) has 2 conditions stopping: when search space exhausted, or when match found. note found variable used in multiple threads: thread {...} presumably spawns new thread.

so there several possibilities how threads' loops can play out:

  • the thread finds '!' , sets found. first loop breaks, second doesn't execute.
  • another thread sets found. first loop breaks, second doesn't execute.
  • all characters searched. first loop breaks, second executes until thread finds '!'.

the third 1 1 forgot. consequence results logged once loops finished.

granted, can't tell why code looks does: each thread, separately prints results pages, i.e. if there n pages, n^2 results logged. also, threads access shared object without synchronization. if goal find 1 exclamation mark, code fails. anyway, these seem separate issues.


Comments

Post a Comment

Popular posts from this blog

C++: Boost interprocess memory mapped file error -

-
i'm trying create memory mapped file using this answer, i'm getting compile errors. code: namespace bi = boost::interprocess; std::string vecfile = "vector.dat"; bi::managed_mapped_file file_vec(bi::open_or_create,vecfile.c_str(), sizeof(struct rectangle) * data_size); typedef bi::allocator<struct rectangle, bi::managed_mapped_file::segment_manager> rect_alloc; typedef std::vector<struct rectangle, rect_alloc> myvec; myvec * vecptr = file_vec.find_or_construct<myvec>("myvector")(file_vec.get_segment_manager()); vecptr->push_back(random_rectangle); the struct this: struct rectangle{ rectangle(float *minarr, float *maxarr, int arr, int exp, int id){ this->arrival = arr; this->expiry = exp; this->id = id; for(int i=0; < 2; i++){ min[i] = minarr[i]; max[i] = maxarr[i]; } int arrival, expiry, id; float min[2]; float max[2]; } the error is: compiler not deduce template argu...
Read more

python - IO.UnsupportedOperation: Not Writable -

-
i have made program stores score attached name in .txt file. however, want print alphabetical order of name attached score. when run program come error io.unsupportedoperation: not writable here code: file = open(class_name , 'r') #opens file in 'append' mode don't delete information name = (name) file.write(str(name + " : " )) #writes information file file.write(str(score)) file.write('\n') linelist = file.readlines() line in sorted(linelist): print(line.rstrip()); file.close() you have opened file read only, attempted write it. file left open, attempt read it, if in append mode, file pointer @ end of file. try instead: class_name = "class.txt" name = "joe" score = 1.5 file = open(class_name , 'a') #opens file in 'append' mode don't delete information name = (n...
Read more

python - malformed header from script index.py Bad header -

-
i want run python code in apache2(ubuntu 14.04) server. have followed these steps , getting error: step 1: configuration 1: have created directory , file under /var/www/cgi-bin configuration 2 : have edited /etc/apache2/sites-available/000-default.conf alias /cgi-bin /var/www/cgi-bin <directory "/var/www/cgi-bin"> options indexes followsymlinks execcgi addhandler cgi-script .cgi .py allow </directory> <directory /var/www/cgi-bin> options </directory> step 2: and python script is: index.py #!/usr/bin/env python import cgi; import cgitb;cgitb.enable() print "content-type: text/plain\n" print "<b>hello python</b>" step 3: when ran through chrome browser using: url : http://localhost/cgi-bin/index.py step 4: i getting error in error-log malformed header script 'index.py': bad header: hello python you should end header \r\n, must print out yet \r\n signal body coming. (i...
Read more