我的循环查找质数的问题

Question

我需要输出来读取数字，然后判断数字是不是质数。由于某种原因，当它达到11时，它不会输出它是质数，而只是输出11，12，这个数字不是质数。

任何帮助都是非常感谢的。

int main()
{
    bool isPrime = true;

    int sqrtNum;
    int divisor = 3;

    for(int i = 0; i < 100; i++)
    {
        cout << i << "\t";


        if (i == 0 || i == 1)
            cout << "That number is not prime." << endl;
        else if (i == 2)
            cout << "That number is prime." << endl;
        else if (i % 2 == 0)
            cout << "That number is not prime." << endl;
        else
        {
            sqrtNum = static_cast<int> (sqrt(static_cast<double>(i)));

            while (divisor <= sqrtNum)
            {
                if (i % divisor == 0)
                {
                    cout << "That number is not prime." << endl;
                    isPrime = false;
                    break;
                }
                else
                {
                    divisor = divisor + 2;
                }
            }
            if (isPrime)
            {
                cout << "That number is prime" << endl;
            }
        }

    }
    system("pause");
    retu

Answer 1

从注释中可以清楚地看出，您忘记了在循环中重置isPrime和divisor的值。为了避免这样的错误，最好只在使用这些变量的范围内定义这些变量。

int main()
{
    for(int i = 0; i < 100; i++)
    {
        cout << i << "\t";


        if (i == 0 || i == 1)
            cout << "That number is not prime." << endl;
        else if (i == 2)
            cout << "That number is prime." << endl;
        else if (i % 2 == 0)
            cout << "That number is not prime." << endl;
        else
        {
            bool isPrime = true;
            int divisor = 3;
            int sqrtNum = static_cast<int> (sqrt(static_cast<double>(i)));