如何在此for循环中使用openMP进行并行循环？

Question

Is it possible to use openMP for loop parallel here in this code? I tried and it shows 'break' has error. can anyone help me guide how to make this parallel with openMP? The purpose of this code is to generate possible permutations of arithmetic expressions for a value. eg 5+5+1=11 and there may have many more expressions to get 11.
The problem is I want to use openMP to be paralle..but I don't know how can make it coz I am newbie in openMp and C++.

vector<string> solve(vector<int> question, vector<char> operands) 
{
    int targetAnswer = question.back(); //Get the final answer
    question.pop_back();    //remove the final answer from question list
    long int totalLoopingCount_Operands = pow(operands.size(),question.size()); // Calculate total looping numbers(operands)
    bool isRedundantAnswer;
    vector<string> answer;

    sort(question.begin(), question.end());
    do{
        isRedundantAnswer = false;
        vector<int> operationSequence;
        //Fill up the operation sequence with first priority operands (int this case '*')
        for (int i = 0; i < question.size(); i++) {
            operationSequence.push_back(0);
        }
                                        //Start the answer seeking algorithm here
        for (long int i = 0; i < totalLoopingCount_Operands-1; i++) {
            if (operands[operationSequence[0]] == '*' || operands[operationSequence[0]] == '/') {
                operationSequence[0]++;
                continue;
            }
            string checkResult = checkAnswer(targetAnswer, operands, question, operationSequence);  //Check the current equation
            //check redundant answer
            for (vector<string>::iterator it = answer.begin(); it != answer.end();it++) {
                if (*it == checkResult) {
                    isRedundantAnswer = true;
                }
            }
            //if result == -1, means this is not a solution
            if (checkResult != "-1" && !isRedundantAnswer) {
                answer.push_back(checkResult);  //insert the answer into the list
            }
            //increment the operationSequence will change the equation
            operationSequence[0]++;
            for (int j = 0; j < question.size() - 1; j++) {
                if (operationSequence[j] == operands.size()) {

                    operationSequence[j] = 0;
                    operationSequence[j + 1] ++;
                }
            }
            if (operationSequence[i % (question.size() - 1)] == 5) {
                cout << "error" << endl;
                break;
            }
        }



    } while (next_permutation(question.begin(),question.end()));
    return answer;
}

Answer 1

In this solution the error is controlled by the foundError boolean which is modified ASAP the error is found and prevents threads picking other iterations to continue processing. The #pragma omp flush ensures that the modifications to the boolen variable is sent to main memory as soon as it is modified or before reading.

This is based from the solution discussed in http://www.thinkingparallel.com/2007/06/29/breaking-out-of-loops-in-openmp . Notice also, that the solution needs a detailed study of which variables should be privatized or kept shared in the #pragma omp parallel for

    bool foundError = false;

    #pragma omp parallel for
    for (long int i = 0; i < totalLoopingCount_Operands-1; i++) {

        #pragma omp flush(foundError)
        if (!foundError)
        {
            if (operands[operationSequence[0]] == '*' || operands[operationSequence[0]] == '/') {
                operationSequence[0]++;
                continue;
            }
            string checkResult = checkAnswer(targetAnswer, operands, question, operationSequence);  //Check the current equation
            //check redundant answer
            for (vector<string>::iterator it = answer.begin(); it != answer.end();it++) {
                if (*it == checkResult) {
                     isRedundantAnswer = true;
                }
            }
            //if result == -1, means this is not a solution
            if (checkResult != "-1" && !isRedundantAnswer) {
                answer.push_back(checkResult);  //insert the answer into the list
            }
            //increment the operationSequence will change the equation
            operationSequence[0]++;
            for (int j = 0; j < question.size() - 1; j++) {
                if (operationSequence[j] == operands.size()) {
                    operationSequence[j] = 0;
                    operationSequence[j + 1] ++;
                }
            }
            if (operationSequence[i % (question.size() - 1)] == 5) {
               foundError = true;
               #pragma omp flush(foundError)
            }
        }
    }
    if (foundError) {
        cout << "error" << endl;
    }