Sum of an array using pthreads


Sum of array is a small problem where we have to add each element in the array by traversing through the entire array. But when the number of elements are too large, it could take a lot of time. But this could solved by dividing the array into parts and finding sum of each part simultaneously i.e. by finding sum of each portion in parallel.

This could be done by using multi-threading where each core of the processor is used. In our case, each core will evaluate sum of one portion and finally we will add the sum of all the portion to get the final sum. In this way we could improve the performance of a program as well as utilize the cores of processor.

It is better to use one thread for each core. Although you can create as many thread as you want for better understanding of multi-threading.

Examples:

Input :  1, 5, 7, 10, 12, 14, 15, 18, 20, 22, 25, 27, 30, 64, 110, 220
Output : sum is 600

Input :  10, 50, 70, 100, 120, 140, 150, 180, 200, 220, 250, 270, 300, 640, 110, 220
Output : sum is 3030

Note – It is advised to execute the program in Linux based system.
Compile in linux using following code:

g++ -pthread program_name.cpp

Code –

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// CPP Program to find sum of array
#include <iostream>
#include <pthread.h>
  
// size of array
#define MAX 16
  
// maximum number of threads
#define MAX_THREAD 4
  
using namespace std;
  
int a[] = { 1, 5, 7, 10, 12, 14, 15, 18, 20, 22, 25, 27, 30, 64, 110, 220 };
int sum[4] = { 0 };
int part = 0;
  
void* sum_array(void* arg)
{
  
    // Each thread computes sum of 1/4th of array
    int thread_part = part++;
  
    for (int i = thread_part * (MAX / 4); i < (thread_part + 1) * (MAX / 4); i++)
        sum[thread_part] += a[i];
}
  
// Driver Code
int main()
{
  
    pthread_t threads[MAX_THREAD];
  
    // Creating 4 threads
    for (int i = 0; i < MAX_THREAD; i++)
        pthread_create(&threads[i], NULL, sum_array, (void*)NULL);
  
    // joining 4 threads i.e. waiting for all 4 threads to complete
    for (int i = 0; i < MAX_THREAD; i++)
        pthread_join(threads[i], NULL);
  
    // adding sum of all 4 parts
    int total_sum = 0;
    for (int i = 0; i < MAX_THREAD; i++)
        total_sum += sum[i];
  
    cout << "sum is " << total_sum << endl;
  
    return 0;
}

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Output:

sum is 600


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