一个关于骰子概率计算的C++问题

Question

写一个程序，找出每一个的概率 "total value" 当几个无偏的不规则骰子（可能有不同数量的 faces) 被同时抛出。

当掷出一个无偏的骰子时，出现不同面值的概率应该等于。例如，一个典型的立方骰子应该给出 1/6 的概率为面值 1、2、3、4、5 和 6。如果掷出两个立方骰子，则两个骰子的面值总和在范围内 [2..12]。然而，每个“总值”的概率并不相等。例如，总数为 4 的概率为 3/36（对于组合 1+3、2+2 和 3+3），而总共 2 的概率只有 1/36（当两个骰子都给出 1 时）。

Sample output as follow: (the one with * are the input from user)
Input the number of dice(s): *2
Input the number of faces for the 1st dice: *6
Input the number of faces for the 2nd dice: *6
Probability of 2 = 1/36
Probability of 3 = 2/36
Probability of 4 = 3/36
Probability of 5 = 4/36
Probability of 6 = 5/36
Probability of 7 = 6/36
Probability of 8 = 5/36
Probability of 9 = 4/36
Probability of 10 = 3/36
Probability of 11 = 2/36
Probability of 12 = 1/36

Input the number of dice(s): *5
Input the number of faces for the 1st dice: *1
Input the number of faces for the 2nd dice: *2
Input the number of faces for the 3rd dice: *3
Input the number of faces for the 4th dice: *4
Input the number of faces for the 5th dice: *5
Probability of 5 = 1/120
Probability of 6 = 4/120
Probability of 7 = 9/120
Probability of 8 = 15/120
Probability of 9 = 20/120
Probability of 10 = 22/120
Probability of 11 = 20/120
Probability of 12 = 15/120
Probability of 13 = 9/120
Probability of 14 = 4/120
Probability of 15 = 1/120

我其实不知道概率部分是怎么完成的。我想对计算问题的方法有一些提示。

#include <iostream>
#include <string>
using namespace std;

//Initialise output function
string output(int num){
  case 1:
    return "st";
    break;
  case 2:
    return "nd";
    break;
  case 3:
    return "rd";
    break;
  default:
    return "th";
}



//Roll function
int roll(int num, int result, int value[20]){
    int dice[num][20];
    for (int i=0; i<num;i++){
        for (int j=1; j<=value[i];j++){
            for (int k=0; k<value[i];k++)
                dice[i][k]=j;
            }
        }
    }
}

int main(){
    int number;
    //Initialise the number variable
    cout <<"Input the number of dice(s): ";
    cin >> number;
    cout<<endl;

    //Initialise the face of the dice using std::array
    int value[11];
    for (int i=0; i<number; i++){
        cout << "Input the number of faces for the "<< i+1 << output(i+1) 
    <<" dice: ";
        cin>>value[i];
    }

    //set the base of the probability (multiply), the maxrange (sum) for 
    the dice probability
    int base=1;
    int sum;
    for (int i=0; i<number; i++){
        base = base*value[i];
        sum = sum +value[i];
    }

    //Output statements
    if (sum >9){
        for (int i=number; i<10; i++){
            cout << "Probability of  "<<i<<" = "<<roll(number, i, value);
        }
        for (int i=10; i<=sum;i++){
            cout << "Probability of "<<i<<" = "<<roll(number, i, value);
        }
    } else {
        for (int i=number; i<=sum; i++){
            cout << "Probability of "<<i<<" = "<<roll(number, i, value);
        }
    }

    return 0;
}

Answer 1

您可以使用暴力破解并使用递归函数计算所有组合，并使用映射来计算每个结果出现的次数。

此外，使用 C++ 容器而不是 C 风格的数组。

喜欢：

#include <iostream>
#include <vector>
#include <map>

void calcAll(const uint32_t value,
             const uint32_t index, 
             const std::vector<uint32_t>& dices, 
             std::map<uint32_t, uint32_t>& count, 
             uint32_t& total)
{
    if (index == dices.size())
    {
        // No more dices -> save result and stop recursion
        auto it = count.find(value);
        if (it == count.end())
        {
            count[value]=1;
        }
        else
        {
            count[value]++;
        }
        ++total;
        return;
    }

    // Iterate over all dice values
    for (uint32_t i = 0; i < dices[index]; ++i)
    {
        calcAll(value + i + 1, index + 1, dices, count, total);
    }
}

int main() {
    std::vector<uint32_t> dices {6, 6, 6}; // 3 dices, 6 sides each
    std::map<uint32_t, uint32_t> count;
    uint32_t total = 0;   

    calcAll(0, 0, dices, count, total);

    for (const auto& v : count)
    {
        std::cout << v.first << " seen " << v.second << " times out of " << total << std::endl;
    }

    return 0;
}

输出：

3 seen 1 times out of 216
4 seen 3 times out of 216
5 seen 6 times out of 216
6 seen 10 times out of 216
7 seen 15 times out of 216
8 seen 21 times out of 216
9 seen 25 times out of 216
10 seen 27 times out of 216
11 seen 27 times out of 216
12 seen 25 times out of 216
13 seen 21 times out of 216
14 seen 15 times out of 216
15 seen 10 times out of 216
16 seen 6 times out of 216
17 seen 3 times out of 216
18 seen 1 times out of 216

Answer 2

对于 6 面骰子，您有一个 "probability vector" [1/6, 1/6, 1/6, 1/6, 1/6, 1/6]，然后对于每个额外的骰子，您 convolve 向量与下一个骰子的 "probability vector" 得到更长更 "bell shaped" 向量。

[1/6, 1/6, 1/6, 1/6, 1/6, 1/6] * [1/6, 1/6, 1/6, 1/6, 1/6, 1/6] =
  [1/36, 2/36, 3/36, 4/36, 5/36, 6/36, 5/36, 4/36, 3/36, 2/36, 1/36]

您可以这样编码：（请注意，我从卷积中分解出了分母）。

static std::vector<int> conv(const std::vector<int>& f, const std::vector<int>& g) {
  const int nf = f.size();
  const int ng = g.size();
  const int n  = nf + ng - 1;

  std::vector<int> out(n);

  for(int i = 0; i < n; ++i) {
    const int jmn = (i >= ng - 1) ? i - (ng - 1) : 0;
    const int jmx = (i <  nf - 1) ? i            : nf - 1;

    for(int j = jmn; j <= jmx; ++j) {
      out[i] += (f[j] * g[i - j]);
    }
  }

  return out; 
}

static void rollDice(const std::vector<int>& dice) {
    std::vector<int> firstDie(dice[0], 1);
    std::vector<int> a = firstDie;
    int denominator = dice[0];

    for (int i = 1; i < dice.size(); ++i) {
        a = conv(a, std::vector<int>(dice[i], 1));
        denominator *= dice[i];
    }

    for (auto aa : a) {
        std::cout << aa << '/' << denominator << '\n';
    }
}

int main() {
    rollDice({6, 6});
    rollDice({1, 2, 3, 4, 5});
}

输出为：

一个关于骰子概率计算的C++问题

A C++ question about dice probability calculation

c++

probability

dice