srand（time（0））不作随机数？

Question

这里是代码，但是输出不是随机输出的吗？可能是因为程序在运行时与所有循环都具有相同的时间？

#include <iostream>

using namespace std;

#include <string>
#include <cmath>
#include <ctime>
#include <cstdlib>


int main()
{
    long int x = 0;
    bool repeat = true;
    srand( time(0));

    int r1 = 2 + rand() % (11 - 2);     //has a range of 2-10
    int r3 = rand();

    for (int i = 0; i <5; i++)
    {
        cout << r1 << endl;          //loops 5 times but the numbers are all the same
        cout << r3 << endl;          // is it cause when the program is run all the times are 
    }                                // the same?
}

Answer 1

您需要将对rand()的呼叫转移到循环内：

#include <iostream>

using namespace std;

#include <string>
#include <cmath>
#include <ctime>
#include <cstdlib>


int main()
{
    long int x = 0;
    bool repeat = true;
    srand( time(0));


    for (int i = 0; i <5; i++)
    {
        int r1 = 2 + rand() % (11 - 2);     //has a range of 2-10
        int r3 = rand();

        cout << r1 << endl;          //loops 5 times but the numbers are all the same
        cout << r3 << endl;          // is it cause when the program is run all the times are 
    }                                // the same?
}

就是说：由于您正在编写C ++，因此您确实想使用C ++ 11中添加的新随机数生成类，而不是完全使用srand / rand。

Answer 2

您必须在每个循环迭代中调用rand()。

for (int i = 0; i <5; i++)
{
    cout << 2 + rand() % (11 - 2) << endl;    
    cout << rand() << endl;          
}

之前发生的事情是您两次调用rand()，一次调用r1，一次调用r3，然后简单地将结果打印5次。