OpenMP increment a count and assign the value to an array - c

Question

I am new to the OpenMP, not sure what was wrong with this code, the results are not making sense.
Thanks.
#include <omp.h>
#include <stdio.h>
#define N 20
int cnt = 0;
int A[N];
int main (int argc, char *argv[]) {
#pragma omp parallel for
for (int i = 0; i <= N; i++) {
if ((i%2)==0) cnt++;
A[i] = cnt;
printf("i=%d, cnt=%d\n", i, cnt);
}
printf("outside the parallel cnt=%d\n", cnt);
for (int i = 0; i <= N; i++)
printf("A[%d]=%d\n", i, A[i]);
}
Edit:
the cnt outside the parallel region should be 11, most time it was correct, but sometime it gave me 10. For array A I understand why the values do not match with the indices, but I would hope the array A be like this following, is it possible ?
A[0]=1 A[1]=1 A[2]=2 A[3]=2 A[4]=3 A[5]=3 A[6]=4 A[7]=4 A[8]=5 A[9]=5 A[10]=6
A[11]=6 A[12]=7 A[13]=7 A[14]=8 A[15]=8 A[16]=9 A[17]=9 A[18]=10 A[19]=10
A[20]=11

Answer 1

Your code has multiple bugs. Let's address the silly one first. You write to N+1 elements but only allocate N elements. Change N to 21 and then change
for (int i = 0; i <= N; i++)
to
for (int i = 0; i < N; i++)
But your code has another more subtle bug. You're using an induction variable. I don't know an easy way to use induction variables with OpenMP.
In your case one easy fix is not use an induction variable and instead do
#pragma omp parallel for
for (int i = 0; i < N; i++) {
int j = i / 2 + 1;
A[i] = j;
}
cnt = N/2;
You can also use a reduction for the final value of cnt but it's redundant and less efficient.
#pragma omp parallel for reduction(+:cnt)
for (int i = 0; i < N; i++) {
if ((i % 2) == 0) cnt++;
int j = i / 2 + 1;
A[i] = j;
}
If you really want to use an induction variable then you have to do something like this:
#pragma omp parallel
{
int ithread = omp_get_thread_num();
int nthreads = omp_get_num_threads();
int start = ithread*N/nthreads;
int finish = (ithread + 1)*N/nthreads;
int j = start / 2;
if (start % 2) j++;
for (int i = start; i < finish; i++) {
if ((i % 2) == 0) j++;
A[i] = j;
}
}
cnt = N/2;
You can also use a reduction for the final value of cnt but as is clear in the code below it's redundant.
#pragma omp parallel reduction(+:cnt)
{
int ithread = omp_get_thread_num();
int nthreads = omp_get_num_threads();
int start = ithread*N/nthreads;
int finish = (ithread + 1)*N/nthreads;
int j = start / 2;
if (start % 2) j++;
for (int i = start; i <finish; i++) {
if ((i % 2) == 0) {
j++; cnt++;
}
A[i] = j;
}
}