Binary Search Using Pthread in C Program

We know that the binary search approach is one of the most suitable and effective sorting algorithm. This works on sorted sequence. The algorithm is simple, it simply finds the element from middle, then divide the list by two parts, and moves either towards the left sublist, or right sublist.

We know the algorithm of it. Now we will see how to use binary search technique in multithreading environment. The number of threads depends on number of cores are present in the system. Let us see the code to get the idea.

Example

#include <iostream>
#define MAX 16
#define MAX_THREAD 4
using namespace std;
//place arr, key and other variables as global to access from different thread
int arr[] = { 1, 6, 8, 11, 13, 14, 15, 19, 21, 23, 26, 28, 31, 65, 108, 220 };
int key = 31;
bool found = false;
int part = 0;
void* binary_search(void* arg) {
   // There are four threads, each will take 1/4th part of the list
   int thread_part = part++;
   int mid;
   int start = thread_part * (MAX / 4); //set start and end using the thread part
   int end = (thread_part + 1) * (MAX / 4);
   // search for the key until low < high
   // or key is found in any portion of array
   while (start < end && !found) { //if some other thread has got the element, it will stop
      mid = (end - start) / 2 + start;
      if (arr[mid] == key) {
         found = true;
         break;
      }
      else if (arr[mid] > key)
         end = mid - 1;
      else
         start = mid + 1;
   }
}
main() {
   pthread_t threads[MAX_THREAD];
   for (int i = 0; i < MAX_THREAD; i++)
      pthread_create(&threads[i], NULL, binary_search, (void*)NULL);
   for (int i = 0; i < MAX_THREAD; i++)
      pthread_join(threads[i], NULL); //wait, to join with the main thread
   if (found)
      cout << key << " found in array" << endl;
   else
      cout << key << " not found in array" << endl;
}

Output

31 found in array