# OpenMP Parallel loops
**[Parallel loops](https://hpc-tutorials.llnl.gov/openmp/do_for_directive/)** in OpenMP parallelise loops over independent iterations by distributing the iteration range across all threads, each thread writing to its own slice of the array. Adding `#pragma omp parallel for` above the loop enables this pattern, which is the most common in C when each iteration writes to a different output location and doesn't read results that other iterations produce. No synchronisation is needed because the iterations are independent:

```c
#pragma omp parallel for
for (int i = 0; i < N; i++) {
    a[i] = i * 0.5;
}
```

The directive requires the loop to have a countable iteration space and no data dependencies between iterations. Writing to `a[i]` must not depend on `a[i-1]` or any other `a[j]`. Loops with such dependencies require a different strategy (e.g. a scan) or explicit synchronisation.