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--- reduction-openmp [June 11, 2026 at 11:06] – Ivan Janevski
+++ reduction-openmp [Unknown date] (current) – removed - external edit (Unknown date) 127.0.0.1
@@ Line 1: / Line 1: @@
-# 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`.