如何清除此队列程序中的键盘缓冲区

Question

我似乎很难清除键盘缓冲区。每当我接受输入并尝试再次接受输入时，它都不会等待用户输入。使用scanf和printf时，有什么方法可以清除缓冲区吗？我不使用cin和cout的唯一原因是因为我的教授不允许我们将其用于组织编程。

int main(int argc, char** argv) 
{
    char userOption, answer;
    int  enqueueNum;
    bool enqueueResult, dequeueResult, ifEmpty;
    answer = 'y';

    QueueClass* queue = NULL;

    queue = new QueueClass();

    printf("This program implements Queues using object oriented programming.\n "
            "Enter a number from 1 to 4 . \n"
            "a. To enqueue a number \n"
            "b. To dequeue a number \n"
            "c. To get the current size of the queue \n"
            "d. To see the contents within the queue \n"
            "e. Quit the program \n" );
    scanf( "%c", &userOption );

    while(answer != 'n')
    {
        switch(userOption)
        {
            case 'a':

                printf("Enter a number to put into the queue \n");
                scanf("%d", &enqueueNum);

                enqueueResult = queue->Enqueue(enqueueNum);

                if(enqueueResult == true)
                    printf( "The number has been put into the queue! \n");
                else
                    printf("The number was not able to be enqueued! \n");
                break;

            case 'b':
                dequeueResult = queue->Dequeue();

                if(dequeueResult == true)
                    printf( "The number has been dequeued! \n");
                else
                    printf("The number wasn't able to be dequeued! \n");
                break;

            case 'c':
                printf( "The current size of the queue is: " + queue->getCurrentSize() );
                break;

            case 'd':
                queue->toString();
        }

        printf("Press y to continue, n to exit \n");
        scanf("%c", &answer);
    }

    system("PAUSE");
    return 0;
}