srand(time(0)) not making random numbers?

问题

Here is the code, but the outputs aren't coming out random? Maybe cause when the program runs it has the same time as all the loops?

#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?
}

回答1:

You need to move your calls to rand() to inside your loop:

#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?
}

That said: since you're writing C++, you really want to use the new random number generation classes added in C++ 11 rather than using srand/rand at all.

回答2:

You must call rand() each loop iteration.

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

What was happening before was that you were calling rand() twice, one for r1 and once for r3, and then simply printing the result 5 times.

来源：https://stackoverflow.com/questions/22749167/srandtime0-not-making-random-numbers

标签

c++

random

srand