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# Odd Even Transposition Sort / Brick Sort using pthreads

• Difficulty Level : Hard
• Last Updated : 06 May, 2019

Odd-Even Transposition Sort is a parallel sorting algorithm. It is based on the Bubble Sort technique, which compares every 2 consecutive numbers in the array and swap them if first is greater than the second to get an ascending order array. It consists of 2 phases – the odd phase and even phase:

• Odd phase: Every odd indexed element is compared with the next even indexed element(considering 1-based indexing).
• Even phase: Every even indexed element is compared with the next odd indexed element.

This article uses the concept of multi-threading, specifically pthread. In each iteration, every pair of 2 consecutive elements is compared using individual threads executing in parallel as illustrated below. Examples:

```Input: { 2, 1, 4, 9, 5, 3, 6, 10 }
Output: 1, 2, 3, 4, 5, 6, 9, 10

Input: { 11, 19, 4, 20, 1, 22, 25, 8}
Output: 1, 4, 8, 11, 19, 20, 22, 25
```

Note: Compile the program using following command on your Linux based system.

```g++ program_name.cpp -pthread
```

Below is the implementation of the above topic:

 `// CPP Program for Odd-Even Transpostion sort``// using pthreads`` ` `#include ``#include `` ` `using` `namespace` `std;`` ` `// size of array``#define n 8`` ` `// maximum number of threads``int` `max_threads = (n + 1) / 2;`` ` `int` `a[] = { 2, 1, 4, 9, 5, 3, 6, 10 };``int` `tmp;`` ` `// Function to compare and exchange``// the consecutive elements of the array``void``* compare(``void``* arg)``{`` ` `    ``// Each thread compares``    ``// two consecutive elements of the array``    ``int` `index = tmp;``    ``tmp = tmp + 2;`` ` `    ``if` `((a[index] > a[index + 1]) && (index + 1 < n)) {``        ``swap(a[index], a[index + 1]);``    ``}``}`` ` `void` `oddEven(pthread_t threads[])``{``    ``int` `i, j;`` ` `    ``for` `(i = 1; i <= n; i++) {``        ``// Odd step``        ``if` `(i % 2 == 1) {``            ``tmp = 0;`` ` `            ``// Creating threads``            ``for` `(j = 0; j < max_threads; j++)``                ``pthread_create(&threads[j], NULL, compare, NULL);`` ` `            ``// joining threads i.e. waiting``            ``// for all the threads to complete``            ``for` `(j = 0; j < max_threads; j++)``                ``pthread_join(threads[j], NULL);``        ``}`` ` `        ``// Even step``        ``else` `{``            ``tmp = 1;`` ` `            ``// Creating threads``            ``for` `(j = 0; j < max_threads - 1; j++)``                ``pthread_create(&threads[j], NULL, compare, NULL);`` ` `            ``// joining threads i.e. waiting``            ``// for all the threads to complete``            ``for` `(j = 0; j < max_threads - 1; j++)``                ``pthread_join(threads[j], NULL);``        ``}``    ``}``}`` ` `// Function to print an array``void` `printArray()``{``    ``int` `i;``    ``for` `(i = 0; i < n; i++)``        ``cout << a[i] << ``" "``;``    ``cout << endl;``}`` ` `// Driver Code``int` `main()``{`` ` `    ``pthread_t threads[max_threads];`` ` `    ``cout << ``"Given array is: "``;``    ``printArray();`` ` `    ``oddEven(threads);`` ` `    ``cout << ``"\nSorted array is: "``;``    ``printArray();`` ` `    ``return` `0;``}`

Output:

```Given array is:  2 1 4 9 5 3 6 10
Sorted array is: 1 2 3 4 5 6 9 10
```

Time complexity: The time complexity is reduced to O(N) due to parallel computation using threads.
Work complexity: The work complexity of this program is O(N) as N/2 number of threads(resources) are being used to sort the array. So, the work-time complexity of the program is O(N^2).

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