c++ - Can I use OpenMP reduction when the variable is an array element? -

the openmp manual says

a type name in declare reduction directive cannot function type, array type, reference type, or type qualified const, volatile or restrict.

what can produce results array elements? started with:

int main() {     // create input array     static const int snum = 5000;     int input[snum];     for(int i=0; i<snum; ++i){         input[i] = i+1;     }      // shared output variables reduction     int sum[2];     sum[0] = 0;     sum[1] = 0;  #pragma omp parallel #pragma omp declare reduction(+:sum[0]) #pragma omp declare reduction(+:sum[1])     for(int i=0; i<snum; ++i) {             int* p = input+i;             if(i%2==0)                 sum[0] += *p;             else                 sum[1] += *p;         } } 

this gives compiler error:

27013152.cpp:16:9: error: ‘#pragma’ not allowed here  #pragma omp declare reduction(+:sum[0])          ^~~ 27013152.cpp:17:33: error: ‘sum’ not name type  #pragma omp declare reduction(+:sum[1])                                  ^~~ 27013152.cpp:17:36: error: expected ‘:’ before ‘[’ token  #pragma omp declare reduction(+:sum[1])                                     ^ 

you're mistaking error you're getting. doesn't mean can't reduction on alternating elements. means can't reduction element of array.

that is, can't reduction(+:sum[0]). can reduction scalar variable , copy element of array afterwards:

void sum_int(const int input[], int num, int *sum) {     int sum_even = 0, sum_odd = 0;      #pragma omp parallel reduction(+:sum_even) reduction(+:sum_odd)     (int = 0; < num; i++) {             if (i % 2 == 0)                     sum_even += input[i];             else                     sum_odd += input[i];     }      sum[0] = sum_even;     sum[1] = sum_odd; }