c++ - 即使调用 srand() 也没有得到不同的随机值

Question

我刚开始学习 C++，我尝试制作一个小程序，随机选择要为体育博彩投注的游戏和球队。我有一个比较两个随机数的函数，较大的值是游戏的选择结果。

程序编译并运行，但由于某种原因，它总是最终选择一个结果，即 HOME 结果。有人可以查看我的代码并让我知道我做错了什么吗？我已经看了一段时间了，看不出问题所在。我认为这与 double 到 int 的转换或其他事情有关。

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <ctime>

using namespace std;
inline void keep_window_open() { char ch; cin>>ch; }

int a;
int b;
int z;
int games_in_total (int, int);
double vector_home(int, int);
double vector_away(int, int);
string selection(double, double);

//takes user input with regard to how many games are playing and how many games you'd like to bet on

int main()
{ 

    cout << "How many games are there this week? " << endl;
    cin >> a;
    cout << "How many games do you want to bet on this week? " << endl;
    cin >> b;

    cout << " " << endl;

    //calls two functions in order to randomly pick which games to bet on and which team to choose within those games.

    z = 0;
    while (z < b){

    cout << "Pick game No." << games_in_total(a,b) << '\t' << "and choose" << selection((vector_home(a,b)), (vector_away(a,b))) << endl; 
    cout << " " << endl;
    ++z;

    }

    keep_window_open(); 
    return 0;

}

//randomly chooses games within the users input range
int games_in_total(int, int) {
    vector <int> games(0);
    srand (time(NULL));

    int i = 0;
    while(i<b){
    games.push_back(rand()% a+1);
    ++i;
        }

    return games[z];

}

//randomly assigns double values to the home team vector. Also adds 1.75 to the random number to give slight advantage to home teams.
double vector_home(int, int) {

    vector<double>home(0);
    srand (time(NULL));

    int i = 0;
    while(i<b){

    home.push_back((rand()% a+1) + 1.75);
    ++i;
    }

    return home[z];    
}

//randomly assigns double values to the away team vector
double vector_away(int, int) {
    vector<double>away(0);
    srand (time(NULL));

    int i = 0;
    while(i<b){

    away.push_back((rand()% a+1));
    ++i;
    }

    return away[z];
}

//compares the home team and away team vector values and assigns the larger values to the randomly chosen games to bet on.

string selection(double, double ){
    string pick_home;
    string pick_away;

    pick_home = " HOME.";
    pick_away = " AWAY.";

    if ((vector_home(a, b)) > (vector_away(a, b))){ 
         return pick_home;         
         }
    else 
         return pick_away;                  
}

Answer 1

1. srand() 初始化随机数生成器。
2. time(NULL) 返回自 1970 年 1 月 1 日起的秒数。
3. 因为您在每次调用 rand() 之前调用了 srand(time(NULL)) 并且您的程序可能会在不到一秒内执行，即 99,99 ...% 你最终会在每次调用 rand() 之前使用相同的种子初始化随机数生成器，因此 rand() 的结果将是相同的在程序的整个运行过程中。
4. 从那时起，您将 1.75 添加到主场值，它的值将始终大于客场值。

你必须做的:

• 从您的代码中删除对 srand() 的所有现有调用
• 在main()中只调用一次srand()