C ++隨機數生成器不是隨機運行的

Question

我有一些C ++代碼正在上面調用另一個函數。 該功能依賴於其選擇之一的隨機性，但並非隨機起作用。 每次都找到相同的數字。 我如何保證它是隨機的？

我嘗試在代碼開頭調用srand ，但沒有幫助。 也許我不了解有關優勝劣汰的重要知識。

這是代碼，我知道很多，但是最重要的東西在最上面：

vector<vector<int> > Successor(vector<int> sudoku_state) {
    srand (time(NULL));
    //Core successor function logic: I choose one of the non-unchangable states at random, and change it to any of its other 3 possible values.  Eg. If a cell with a '1' is chosen at random, then I return the sudoku board with a '2' a '3' and a '4' at that location, in a vector of vectors. 
    vector<vector<int> > list_of_all_successors;
    int random_indice;
    random_indice = rand()%16;
    //make sure it is not one of the unchangable squares
    while((std::find(unchangables.begin(), unchangables.end(), random_indice) != unchangables.end())) 
    {
    random_indice = rand()%16;
    }
    if(sudoku_state[random_indice]==1) {
    sudoku_state[random_indice]=2;
    list_of_all_successors.push_back(sudoku_state);
    sudoku_state[random_indice]=3;
    list_of_all_successors.push_back(sudoku_state);
    sudoku_state[random_indice]=4;
    list_of_all_successors.push_back(sudoku_state);
    }
    else if(sudoku_state[random_indice]==2) {
    sudoku_state[random_indice]=1;
    list_of_all_successors.push_back(sudoku_state);
    sudoku_state[random_indice]=3;
    list_of_all_successors.push_back(sudoku_state);
    sudoku_state[random_indice]=4;
    list_of_all_successors.push_back(sudoku_state);
    }
    else if(sudoku_state[random_indice]==3) {
    sudoku_state[random_indice]=1;
    list_of_all_successors.push_back(sudoku_state);
    sudoku_state[random_indice]=2;
    list_of_all_successors.push_back(sudoku_state);
    sudoku_state[random_indice]=4;
    list_of_all_successors.push_back(sudoku_state);
    }
    else { //then we know == 4
    sudoku_state[random_indice]=2;
    list_of_all_successors.push_back(sudoku_state);
    sudoku_state[random_indice]=3;
    list_of_all_successors.push_back(sudoku_state);
    sudoku_state[random_indice]=1;
    list_of_all_successors.push_back(sudoku_state);
    }
    return list_of_all_successors;
}

這是呼叫者：

//The core hillclimbing functionality.  Performs the logic of the hillclimbing algorithm on the given initial sudoku board, called initial_state.  initial_state is represented as a vector of int of size 16.  The returned value called goal_state, is also a vector of int of size 16.
vector<int> hillClimber(vector<int> initial_state) {
    vector<int> goal_state; // the final goal state
    vector<int> sudoku_config_with_lowest_flaws = initial_state; //the state being worked with in each iteration
    int minimum = 500; //the current minimum amount of flaws found in a state (set to 500 initially as a max)
    int iterations = 0; //if iterations reaches a high amount, random restart.
    while(true) {
        cout << "\nBeginning of hillClimber loop reached.\n";
    vector<vector<int> > all_successors = Successor(sudoku_config_with_lowest_flaws);
    cout << "Potential Successors to beginning state determined. Now analyzing.  \n";
    //****Core hillClimber logic:
    //Loop over the successors and determine one with lowest amount of flaws.
    int j = 0;
    while(j<all_successors.size()) {
        vector<int> next_state = all_successors[j];
        int number_of_flaws = Evaluator(next_state);
        if(number_of_flaws <= minimum) {
        minimum = number_of_flaws;
        sudoku_config_with_lowest_flaws = next_state;
        }
        if(number_of_flaws==0) {
        //print the final solution
        cout << "**********A SOLUTION HAS BEEN DETERMINED**********\n";
        for(int i=0; i < sudoku_config_with_lowest_flaws.size(); i++){
            cout << sudoku_config_with_lowest_flaws[i];
            if(i==3 || i==7 || i==11)
            cout << '\n';
        }
        return sudoku_config_with_lowest_flaws;
        }
        j++;
    }
    //print the next lowest state configuration found:
    cout << "Next configuration (passed into next loop of hillClimber) shown below: (Still has " << minimum << " flaws).  \n";
    for(int i=0; i < sudoku_config_with_lowest_flaws.size(); i++){
        cout << sudoku_config_with_lowest_flaws[i];
        if(i==3 || i==7 || i==11)
        cout << '\n';
    }

    iterations++;
    if(iterations>50) {
        //randomized restart area: re-randomizes the initial_state, and runs again.
        cout << "\n ***Reached a local min -- Had to restart hillClimber with a new initial_state! *** \n";
        int i = 0;
        for(i = 0; i<initial_state.size(); i++) {
        if(!(std::find(unchangables.begin(), unchangables.end(), i) != unchangables.end())) {
            initial_state[i] = rand() % 4 + 1;
        }
        }
        cout << "New initial state: \n";
        for(int i=0; i < initial_state.size(); i++){
        cout << initial_state[i];
        if(i==3 || i==7 || i==11)
            cout << '\n';
        }
        cout << "\n Press Enter to continue \n";
        getchar();
        return hillClimber(initial_state);
    }
    }
}

在稱為后繼函數的函數中生成隨機數時，它將總是一遍又一遍地找到相同的數字，這破壞了它的功能。

Answer 1

在程序中播種一次隨機數生成器，而不是每次都想生成隨機數！

int main()
{
    std::srand( /* some random source */ );

    run_rest_of_program();
}

隨機性的來源可能是像std::time(nullptr)這樣天真的東西（嬰兒的第一顆種子），或更合適的像是std::random_device{}()那樣的東西。 對於后者， #include <random> 。

無論如何，您都應該使用C ++的<random>工具 ，而不是不良的C rand() 。 例如：

#include <iostream>
#include <random>

int main()
{
    std::mt19937 rng(std::random_device{}());   // the PRNG
    std::normal_distribution<double> dist;      // a distribution

    // Print 100 standard-normally distributed numbers.
    for (int i = 0; i != 100; ++i)
    {
        std::cout << dist(rng) << '\n';
    }
}