Question

Modify the sum_thread.cpp program to compute sum = 1 + 1/2 + 1/3 + 1/4 + … 1/n.

Let’s estimate natural using n = 20.

sum_thread.cpp

#include <chrono>
#include <iostream>
#include <mutex>
#include <random>
#include <utility>
#include <vector>
#include <thread>

using namespace std;

constexpr long long size= 1000000;   
mutex myMutex;

void sumUp(unsigned long long& sum, const vector<int>& val, 
   unsigned long long beg, unsigned long long end){
   long long localSum = 0;
    for (auto it= beg; it < end; ++it){
        localSum+= val[it];
    }
    lock_guard<mutex> myLock(myMutex);
    sum += localSum;
}

int main(){

  cout << endl;

  vector<int> randValues;
  randValues.reserve(size);

  mt19937 engine (0);
  uniform_int_distribution<> uniformDist(1,10);
  for ( long long i=0 ; i< size ; ++i)
     randValues.push_back(uniformDist(engine));
 
  unsigned long long sum= 0;
  auto start = chrono::system_clock::now();

  int threads = 4;
  thread t[threads];
  long long slice = size / threads;
  int startIdx=0;
  for (int i = 0; i < threads; ++i) {
    cout << "Thread[" << i << "] - slice ["
         << startIdx << ":" << startIdx+slice-1 << "]" << endl;
    t[i] = thread(sumUp, ref(sum), ref(randValues), startIdx, startIdx+slice-1);
    startIdx += slice;
  }

  for (int i = 0; i < threads; ++i)
     t[i].join();

  chrono::duration<double> dur= chrono::system_clock::now() - start;
  cout << "Time for addition " << dur.count() << " seconds" << endl;
  cout << "Result: " << sum << endl;

  cout << endl;

}

Answer 1

Modified code for sum_thread.cpp

#include <chrono>
#include <iostream>
#include <mutex>
#include <random>
#include <utility>
#include <vector>
#include <thread>

using namespace std;

//constexpr long long size = 1000000;
mutex myMutex;

void sumUp(unsigned long long& sum, const vector<int>& val,
   unsigned long long beg, unsigned long long end) {
   long long localSum = 0;
   for (auto it = beg; it < end; ++it) {
       if(val[it])
       {
           //local sum is sum of all elements of vectors that are allocated to this thread
           localSum += 1/val[it];
       }
   }
   lock_guard<mutex> myLock(myMutex);
   sum += localSum;
}

int main() {

   cout << endl;

   //long long size = 1000000;
   //here we are setting the n as 20
   long long size = 20;

   vector<int> randValues;
   randValues.reserve(size);

   mt19937 engine(0);
   uniform_int_distribution<> uniformDist(1, 10);
   for (long long i = 0; i< size; ++i)
       randValues.push_back(uniformDist(engine));

   unsigned long long sum = 0;
   auto start = chrono::system_clock::now();

   int threads = 4;
   //thread t[threads];
   thread t[4];
   long long slice = size / threads;
   int startIdx = 0;
   for (int i = 0; i < threads; ++i) {
       cout << "Thread[" << i << "] - slice ["
           << startIdx << ":" << startIdx + slice - 1 << "]" << endl;
       t[i] = thread(sumUp, ref(sum), ref(randValues), startIdx, startIdx + slice - 1);
       startIdx += slice;
   }

   for (int i = 0; i < threads; ++i)
       t[i].join();

   chrono::duration<double> dur = chrono::system_clock::now() - start;
   cout << "Time for addition " << dur.count() << " seconds" << endl;
   cout << "Result: " << sum << endl;

   cout << endl;

}

Output screenshot