# Reduction (OpenMP)
Summing an array in serial C is a straightforward accumulation loop:

```c
double sum = 0.0;
for (int i = 0; i < N; i++)
    sum += a[i];
```

Adding `#pragma omp parallel for` to this loop creates a race condition: multiple threads read `sum`, add to it, and write back simultaneously, so some updates are lost. The `reduction` clause fixes this.

**Reduction** in OpenMP gives each thread a private copy of the accumulation variable, lets it accumulate locally without contention, then merges all copies at the end of the loop using the specified operator.

```c
double sum = 0.0;

#pragma omp parallel for reduction(+:sum)
for (int i = 0; i < N; i++) {
    sum += a[i];
}
```

Built-in reduction operators include `+`, `*`, `-`, `min`, `max`, and the bitwise operators `&`, `|`, `^`. Custom reducers are possible in C++ via `declare reduction`.