C++ OpenMP each thread print from private struct object

Question

I'm new at OpenMP and I want to make multithread program. So I have txt file

London 2
A
B
Miami 3
C
D
E

And when I read my file I put all my data into struct called LetterStruct

struct LetterStruct{
    string city;
    int nr;
    string letters[ARRAY_SIZE];
};

I want to print my data something like like that(I know that order will be different each time I run my program)

thread_0 A
thread_0 B
thread_1 C
thread_1 D
thread_1 E

so each thread should print one of the city's letters(for example thread 0 should print London and thread 1 should print Miami letters)

so here what I did

void setUpThreads(int arraySize) {
    LetterStruct letter;
    omp_set_num_threads(2); // number of threads 2 (because 2 Miami and London)
    for (int j = 0; j < 1; j++) {
        #pragma omp parallel private(letter)
        {
            int id = omp_get_thread_num();
            letter = letterArray[j]; // get struct info
            for (int i = 0; i < ARRAY_SIZE; i++) {
                cout << "thread_" << id << " " << letter.letters[i] << endl;
            }
        }
    }
}

And here's my result

thread_0 thread_1 A
A
thread_0 thread_1 B
B
thread_0 thread_1 C
thread_1 C
thread_0 thread_1 D
thread_1 D
thread_0 thread_1 E
thread_1 E

it seems that both threads have Miami and London letter information (but I made this private(letter) ) and for some reason everything prints incorrectly... So what I'm missing?

Answer 1

Currently, your threads are duplicating work. That is, they are both doing exactly the same thing. What the #pragma omp parallel does is tell the code to do everything in the enclosed brackets for each thread. This is not what you want it to do. Instead, replace your #pragma omp parallel private(letter) with a #pragma omp parallel for private(letter) right above the for loop. This will tell your code to split up each iteration of the loop to different threads.