Qt 5.11 | Qt Concurrent | QtConcurrent Word Count Example | main.cpp Example File

Qt 5.11.3 Reference Documentation

# main.cpp Example File

wordcount/main.cpp

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      #include <QList>
      #include <QMap>
      #include <QTextStream>
      #include <QString>
      #include <QStringList>
      #include <QDir>
      #include <QTime>
      #include <QApplication>
      #include <QDebug>

      #include <qtconcurrentmap.h>

      using namespace QtConcurrent;

      /*
          Utility function that recursivily searches for files.
      */
      QStringList findFiles(const QString &startDir, const QStringList &filters)
      {
          QStringList names;
          QDir dir(startDir);

          const auto files = dir.entryList(filters, QDir::Files);
          for (const QString &file : files)
              names += startDir + '/' + file;

          const auto subdirs =  dir.entryList(QDir::AllDirs | QDir::NoDotAndDotDot);
          for (const QString &subdir : subdirs)
              names += findFiles(startDir + '/' + subdir, filters);
          return names;
      }

      typedef QMap<QString, int> WordCount;

      /*
          Single threaded word counter function.
      */
      WordCount singleThreadedWordCount(const QStringList &files)
      {
          WordCount wordCount;
          for (const QString &file : files) {
              QFile f(file);
              f.open(QIODevice::ReadOnly);
              QTextStream textStream(&f);
              while (!textStream.atEnd()) {
                  const auto words =  textStream.readLine().split(' ');
                  for (const QString &word : words)
                      wordCount[word] += 1;
              }
          }
          return wordCount;
      }

      // countWords counts the words in a single file. This function is
      // called in parallel by several threads and must be thread
      // safe.
      WordCount countWords(const QString &file)
      {
          QFile f(file);
          f.open(QIODevice::ReadOnly);
          QTextStream textStream(&f);
          WordCount wordCount;

          while (!textStream.atEnd()) {
              const auto words =  textStream.readLine().split(' ');
              for (const QString &word : words)
                  wordCount[word] += 1;
          }

          return wordCount;
      }

      // reduce adds the results from map to the final
      // result. This functor will only be called by one thread
      // at a time.
      void reduce(WordCount &result, const WordCount &w)
      {
          QMapIterator<QString, int> i(w);
          while (i.hasNext()) {
              i.next();
              result[i.key()] += i.value();
          }
      }

      int main(int argc, char** argv)
      {
          QApplication app(argc, argv);
          qDebug() << "finding files...";
          QStringList files = findFiles("../../", QStringList() << "*.cpp" << "*.h");
          qDebug() << files.count() << "files";

          qDebug() << "warmup";
          {
              WordCount total = singleThreadedWordCount(files);
          }

          qDebug() << "warmup done";

          int singleThreadTime = 0;
          {
              QTime time;
              time.start();
              WordCount total = singleThreadedWordCount(files);
              singleThreadTime = time.elapsed();
              qDebug() << "single thread" << singleThreadTime;
          }

          int mapReduceTime = 0;
          {
              QTime time;
              time.start();
              WordCount total = mappedReduced(files, countWords, reduce);
              mapReduceTime = time.elapsed();
              qDebug() << "MapReduce" << mapReduceTime;
          }
          qDebug() << "MapReduce speedup x" << ((double)singleThreadTime - (double)mapReduceTime) / (double)mapReduceTime + 1;
      }