/* 

sum3.c 

compute sum = atan(1) + atan(2) + ... + atan(n)

*/

#include <stdio.h> 
#include <assert.h> 
#include <math.h> 

#include <omp.h> 

#define NUM_THREADS 4

int main(int argc, char** argv) {

  long n  = 100000000; 
  double  totalsum; 
  long  i; 
  double t1, t2; 
  double sum[NUM_THREADS]; //one for each thread 

  t1 = omp_get_wtime(); 

  //initialize all sums to 0 
  for (i=0; i< NUM_THREADS; i++) sum[i] = 0; 

  omp_set_num_threads(NUM_THREADS); 
#pragma omp parallel  private (i)
  { 
    int thread_id = omp_get_thread_num(); 
    printf("Start(%d)\n", thread_id); 
    
    for (i=thread_id; i<n; i+= NUM_THREADS) {
      sum[thread_id]  = sum[thread_id] + atan(i); 
    }
    printf("Done(%d) sum = %.1f\n", thread_id, sum[thread_id]); 
  } //end omp 

  //add all local sums 
  totalsum = 0; 
  for (i=0; i< NUM_THREADS; i++)  
    totalsum += sum[i]; 
				      
  t2 = omp_get_wtime();   
 
  printf("sum=%.1f ", totalsum);
  printf("Done. time = %.1f\n",  t2-t1);
  
  return 0; 
}