for (OpenMP)

 

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Causes the work done in a for loop inside a parallel region to be divided among threads.

Syntax

#pragma omp [parallel] for [clauses]  
   for_statement  

Remarks

where,

clause (optional)
Zero or more clauses. See the Remarks section for a list of the clauses supported by for.

for_statement
A for loop. Undefined behavior will result if user code in the for loop changes the index variable.

Remarks

The for directive supports the following OpenMP clauses:

• firstprivate

• lastprivate

• nowait

• ordered

• private

• reduction

• schedule

If parallel is also specified, clause can be any clause accepted by the parallel or for directives, except nowait.

For more information, see 2.4.1 for Construct.

Example

// omp_for.cpp  
// compile with: /openmp   
#include <stdio.h>  
#include <math.h>  
#include <omp.h>  
  
#define NUM_THREADS 4  
#define NUM_START 1  
#define NUM_END 10  
  
int main() {  
   int i, nRet = 0, nSum = 0, nStart = NUM_START, nEnd = NUM_END;  
   int nThreads = 0, nTmp = nStart + nEnd;  
   unsigned uTmp = (unsigned((abs(nStart - nEnd) + 1)) *   
                               unsigned(abs(nTmp))) / 2;  
   int nSumCalc = uTmp;  
  
   if (nTmp < 0)  
      nSumCalc = -nSumCalc;  
  
   omp_set_num_threads(NUM_THREADS);  
  
   #pragma omp parallel default(none) private(i) shared(nSum, nThreads, nStart, nEnd)  
   {  
      #pragma omp master  
      nThreads = omp_get_num_threads();  
  
      #pragma omp for  
      for (i=nStart; i<=nEnd; ++i) {  
            #pragma omp atomic  
            nSum += i;  
      }  
   }  
  
   if  (nThreads == NUM_THREADS) {  
      printf_s("%d OpenMP threads were used.\n", NUM_THREADS);  
      nRet = 0;  
   }  
   else {  
      printf_s("Expected %d OpenMP threads, but %d were used.\n",  
               NUM_THREADS, nThreads);  
      nRet = 1;  
   }  
  
   if (nSum != nSumCalc) {  
      printf_s("The sum of %d through %d should be %d, "  
               "but %d was reported!\n",  
               NUM_START, NUM_END, nSumCalc, nSum);  
      nRet = 1;  
   }  
   else  
      printf_s("The sum of %d through %d is %d\n",  
               NUM_START, NUM_END, nSum);  
}  

4 OpenMP threads were used.  
The sum of 1 through 10 is 55  

See Also

Directives