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c++parallel-processingopenmp
Specyk
Specyk

Reputation: 203

Why this C++ code using OpenMP library throws an error of bad condition in for loop while compilation?

I tried to parallelize Sieve of Eratosthenes algoritm using OpenMP library.

My code:

#pragma omp parallel for num_threads(4)
for (uint p = 2; (p * p) <= n; ++p)
{
    if (prime[p] == true)
    {
        for (uint i = p * p; i <= n; i += p)
            prime[i] = false;
    }
}

When I try to compile, I get error message: condition of OpenMP for loop must be a relational comparison ('<', '<=', '>', or '>=') of loop variable 'p'

How to resolve this issue?

Upvotes: 0

Views: 173

Answers (1)

1201ProgramAlarm
1201ProgramAlarm

Reputation: 32742

OpenMP has restrictions on the condition in a for loop. The condition must be of the form p relop var or var relop p. Expressions using p are not allowed. (There are also restrictions on what var can be.)

In your case, you'll need to utilize the square root (p <= sqrt(n)) in your condition. However, this sqrt call needs to be made ahead of time and saved in another local variable (declared before the #pragma omp directive).

uint sqrtn = uint(sqrt(n));
#pragma omp parallel for num_threads(4)
for (uint p = 2; p <= sqrtn; ++p)

With these restrictions on the condition, the OpenMP system can calculate how many times the loop will execute before entering it. This allows the work to be distributed appropriately.

Upvotes: 2

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