兰德（）：怪异的行为

Question

#include <iostream>
#include <ctime>    
#include <cstdlib>  
#include <fstream>      
#include <algorithm> 
#include <array>

using namespace std;

//generate random integer between 0 and MAX
int randomn(int MAX);

int main()
{
    const int N = 10000;
    const int T = 100;     //Number of trials

    srand((unsigned)time(0));   

    ofstream writefile ("Observation.out");

    for (int indexT = 0; indexT < T; indexT++) {

        //initializing myArray
        array<bool, N> myArray;
        array<bool, N> newArray;

        auto middle = myArray.begin() + myArray.size() / 2;
        fill(myArray.begin(), middle, false);
        fill(middle, myArray.end(), true);

        int counterIt = 0;  //desired Iteration number

        for (;;) {
            int counterF = 0;
            int randompos = 0;
            bool savedata = true;

            //suffling myArray using Fisher–Yates shuffle
            for (int indexN = N-1; indexN > 0; indexN--) {
                randompos = randomn(indexN);
                savedata = myArray[randompos];
                myArray[randompos] = myArray[indexN] ;
                myArray[indexN] = savedata;
            }           
            //next Iteration
            for (int indexN = 0; indexN < N; indexN++) {
                randompos = randomn(N-1);
                savedata = myArray[randompos];
                newArray[indexN] = savedata;
                if (savedata == false){
                    counterF += 1;
                }
            }

            copy(begin(newArray), end(newArray), begin(myArray));

            //updating Iteration number
            counterIt += 1;

            if ((counterF == 0)|| (counterF == N)) {
                break;
            }

        }

        writefile << indexT+1 <<"\t"<<counterIt <<endl;
    }

    writefile.close();

    return 0;
}

int randomn (int MAX){
    int temp;
    for (;;){
        temp = rand();
        if ( temp < RAND_MAX - RAND_MAX%(MAX+1) ) 
            return temp%(MAX+1);
    }
}

1st run            2nd run
1  28278          1    13583  
2  7754           2    7308   
3  11308          3    22580  
4  5093           4    6307    ** oscillation starts
5  4952           5    42060   
6  5017           6    10485   
7  10400          7    8525    
8  6307   **      8    31061   
9  42060          9    6307   ** 1st period 
10  10485         10   42060  
11  8525          11   10485 
12  31061         12   8525   
13  6307   **     13   31061  
14  42060         14   6307   ** 2nd period 
15  10485         15   42060 

Answer 1

rand()不应用于任何严重的操作。它的质量可能很差。

例如，我用它做了一个模拟，我知道确切的答案。使用rand()，模拟收敛到一个与确切答案略有不同的数字。我用更好的东西替换了rand()，并且：

• 模拟速度提高3倍
• 模拟收敛到精确解。

uint64_t s[2]; // seed this

uint64_t next(void) {
    uint64_t s1 = s[0];
    const uint64_t s0 = s[1];
    const uint64_t result = s0 + s1;
    s[0] = s0;
    s1 ^= s1 << 23; // a
    s[1] = s1 ^ s0 ^ (s1 >> 18) ^ (s0 >> 5); // b, c
    return result; 
}