无法修复蒙特卡洛程序中的错误

我正在尝试编写一个程序，模拟在标准曲线上投掷飞镖。每当我接近调试整个东西时，就会弹出其他东西。到目前为止，我收到了很多错误，比如：

错误：变量未在此范围中声明

还有一个错误，我不知道如何修复，它与C++比较指针和整数有关

我是C++的新手，所以任何指针都将不胜感激。

这是我目前为止得到的。

注意：错误出现在第67、70、72和75行。

#include <iostream>
#include <cstdlib>
#include <cmath>
using namespace std;
double seed(int darts, int x);
int main ()
{
    int darts, x_max;
    double area;
    char again = 'y';
    char giveDarts;
    while (again == 'y' || again == 'Y');
        cout << "Run program (y/n)?";
        cin >> giveDarts;
        switch (giveDarts) {
            case 'y':
            case 'Y':
                cout << "Enter the ammount of darts to be thrown: "; //since we are simulating DARTS I will use the varible darts instead of "NumberOfSamples"
                cin >> darts;
                srand(darts);
            default:
                break;
        }
    cout << "Enter maximum value of x: ";
    cin >> x_max;
    while (x_max < 0);
        cout << "Please enter a positive value of x: ";
        cin >> x_max;
        cout << endl;
    srand(time(NULL));
    area = seed(darts, x_max);
    cout << "Estimate of area under curve is: " << area << endl;
    cout << "Go again? ";
    cin >> again;
    return 0;
}
double seed(int darts, int x_max)
{
    int i, num_darts=0; //num_darts instead of SamplesInsideArea.
    double area;
    for(i=1; i<=darts; i++) // for loop
    {    
        double x, y; 
        double pi = 3.14;
        double n (double t);
        return 1/sqrt(2*pi)*exp(-pow(t,2)/2); //error:'t' was not declared in this scope
              x = rand() / static_cast<double>(RAND_MAX);
              y = rand() / static_cast<double>(RAND_MAX);
        n(0) = (x*x_max + y*y_max); //error: y_max was not declared in this scope
        if(num_darts <= n) //error: ISO C++ forbids comparison between pointer and integer
            num_darts++;
            area*n(0)+0.5 = static_cast<double>(num_darts)/darts; //error: invalid Ivalue in assignment.
     }
     return area;
 }