Merge pull request #24 from asrivast28/FixScan

Fix in global_scan and global_scan_inplace.

include/mxx/reduction.hpp

@@ -741,14 +741,14 @@ void global_scan(InIterator begin, InIterator end, OutIterator out, Func func, c
  local_scan(begin, end, out, func);
  // mxx::scan
  typedef typename std::iterator_traits<OutIterator>::value_type T;
- T sum = T();
- if (n > 0)
- sum = *(out+(n-1));
+ T sum = *(out+(n-1));
  T presum = exscan(sum, func, nonzero_comm, commutative);
- // accumulate previous sum on all local elements
- for (size_t i = 0; i < n; ++i) {
- *o = func(presum, *o);
- ++o;
+ if (nonzero_comm.rank() != 0) {
+ // accumulate previous sum on all local elements
+ for (size_t i = 0; i < n; ++i) {
+ *o = func(presum, *o);
+ ++o;
+ }
  }
  }
 }
@@ -773,10 +773,12 @@ inline void global_scan_inplace(Iterator begin, Iterator end, Func func, const b
  T sum = *(begin + (n-1));
  T presum = exscan(sum, func, nonzero_comm, commutative);
 
- // accumulate previous sum on all local elements
- for (size_t i = 0; i < n; ++i) {
- *o = func(presum, *o);
- ++o;
+ if (nonzero_comm.rank() != 0) {
+ // accumulate previous sum on all local elements
+ for (size_t i = 0; i < n; ++i) {
+ *o = func(presum, *o);
+ ++o;
+ }
  }
  }
 }

test/test_reductions.cpp

@@ -247,7 +247,7 @@ TEST(MxxReduce, GlobalReduce) {
 
 TEST(MxxReduce, GlobalScan) {
  mxx::comm c;
- // test reduce with zero elements for some processes
+ // test scan with zero elements for some processes
  size_t n = 0;
  int presize = 0;
  if (c.rank() % 2 == 0) {
@@ -273,6 +273,31 @@ TEST(MxxReduce, GlobalScan) {
  }
 }
 
+TEST(MxxReduce, GlobalScanMin) {
+ mxx::comm c;
+ // test scan with min and an array of positive
+ // elements sorted in ascending order
+ size_t n = c.size();
+ std::vector<int> local(n);
+ for (size_t i = 0; i < n; ++i) {
+ local[i] = (c.rank()*n)+i+1;
+ }
+ // test inplace scan
+ std::vector<int> local_cpy(local);
+ mxx::global_scan_inplace(local_cpy.begin(), local_cpy.end(), mxx::min<int>(), c);
+ for (size_t i = 0; i < n; ++i) {
+ // 1 is both the first as well as the minimum element
+ ASSERT_EQ(1, local_cpy[i]);
+ }
+ // test scan
+ std::vector<int> result = mxx::global_scan(local, mxx::min<int>(), c);
+ ASSERT_EQ(local.size(), result.size());
+ for (size_t i = 0; i < n; ++i) {
+ // 1 is both the first as well as the minimum element
+ ASSERT_EQ(1, result[i]);
+ }
+}
+
 TEST(MxxReduce, GlobalExScan) {
  mxx::comm c;
  // test reduce with zero elements for some processes