# Binary Search using pthread

**Binary search** is a popular method of searching in a sorted array or list. It simply divides the list into two halves and discard the half which has zero probability of having the key. On dividing we check the mid point for the key and uses the lower half if key is less than mid point and upper half if key is greater than mid point. Binary search has time complexity of O(log(n)).

Binary search can also be implemented using **multi-threading** where we utilizes the cores of processor by providing each thread a portion of list to search for the key.

Number of threads depends upon the number of cores your processor has and its better to create one thread for each core.

Examples:

Input : list = 1, 5, 7, 10, 12, 14, 15, 18, 20, 22, 25, 27, 30, 64, 110, 220 key = 7 Output : 7 found in list Input : list = 1, 5, 7, 10, 12, 14, 15, 18, 20, 22, 25, 27, 30, 64, 110, 220 key = 111 Output : 111 not found in list

**Note –** It is advised to execute the program in Linux based system.

Compile in linux using following code:

g++ -pthread program_name.cpp

`// CPP Program to perform binary search using pthreads ` `#include <iostream> ` `using` `namespace` `std; ` ` ` `// size of array ` `#define MAX 16 ` ` ` `// maximum number of threads ` `#define MAX_THREAD 4 ` ` ` `// array to be searched ` `int` `a[] = { 1, 5, 7, 10, 12, 14, 15, 18, 20, 22, 25, 27, 30, 64, 110, 220 }; ` ` ` `// key that needs to be searched ` `int` `key = 110; ` `bool` `found = ` `false` `; ` `int` `part = 0; ` ` ` `void` `* binary_search(` `void` `* arg) ` `{ ` ` ` ` ` `// Each thread checks 1/4 of the array for the key ` ` ` `int` `thread_part = part++; ` ` ` `int` `mid; ` ` ` ` ` `int` `low = thread_part * (MAX / 4); ` ` ` `int` `high = (thread_part + 1) * (MAX / 4); ` ` ` ` ` `// search for the key until low < high ` ` ` `// or key is found in any portion of array ` ` ` `while` `(low < high && !found) { ` ` ` ` ` `// normal iterative binary search algorithm ` ` ` `mid = (high - low) / 2 + low; ` ` ` ` ` `if` `(a[mid] == key) { ` ` ` `found = ` `true` `; ` ` ` `break` `; ` ` ` `} ` ` ` ` ` `else` `if` `(a[mid] > key) ` ` ` `high = mid - 1; ` ` ` `else` ` ` `low = mid + 1; ` ` ` `} ` `} ` ` ` ` ` `// Driver Code ` `int` `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); ` ` ` ` ` `// key found in array ` ` ` `if` `(found) ` ` ` `cout << key << ` `" found in array"` `<< endl; ` ` ` ` ` `// key not found in array ` ` ` `else` ` ` `cout << key << ` `" not found in array"` `<< endl; ` ` ` ` ` `return` `0; ` `} ` |

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Output:

110 found in array

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