Draw Lines Between Points Contained in a Vector (VC++)

Question

I want to draw line segments between points contained in a vector and display them in a window.

Currently, this is what I have:

case WM_PAINT:
    hdc = BeginPaint(hWnd, &ps);
  //Draw lines to screen.
    using std::vector;
    using std::iterator;
    extern vector<int> euler_time;
    extern vector<int> euler_soln;
    hPen = CreatePen(PS_SOLID, 1, RGB(255, 25, 5));
    SelectObject(hdc, hPen);
    for(std::vector<int>::size_type i = 0; i != euler_time.size(); i++){
    MoveToEx(hdc,euler_time[i],euler_soln[i],NULL);
    LineTo(hdc,euler_time[i],euler_soln[i]);
    }
    EndPaint(hWnd, &ps);

This is contained in a larger .CPP source file generated as when creating a standard Win32 Application.

As you can see, my idea was to iterate through my vector using a for loop and then use LineTo and MoveToEx to go to the next point and draw a line form the previous one.

Currently I get a completely blank window with no errors. Suggestions?

EDIT:

So I'm guessing the break point message mentioned in the comments below was caused by my loading the external vector. The vector is an output of a for loop in another .CPP file.

    using std::vector;
    using std::iterator;
    extern vector<int> euler_time;
    extern vector<int> euler_soln;

And the loop in the other .CPP file:

for (double t = a; t < b; t += h )
{
    std::cout << std::fixed << std::setprecision(3) << t << " " << y << "\n";
    euler_time.insert (euler_time.begin(),t); // Insert the values of t and y into the respective vectors.
    euler_soln.insert (euler_soln.begin(),y);
    y += h * f(t, y);
}

EDIT 2:

So I started with the Polyline API. I created an array of type const POINT* called Pt and attempted to assign the values t and y_n to it. I then called Polyline and told it to draw from the array.

I end up with no errors and a blank window again.

From the Window CPP file:

    extern const POINT* Pt;
  //Draw lines to screen.
    hPen = CreatePen(PS_SOLID, 1, RGB(255, 25, 5));
    SelectObject(hdc, hPen);
    Polyline(hdc,Pt,10);

From the other CPP file:

const POINT * Pt;
void euler(F f, int y0, int a, int b, int h ) // defines a class of type     "void" (returns nothing); gives it parameters: function F, doubles: y0, a, b, h
{
int y_n = y0;
int t = a;
for (int t = a; t < b; t += h ) // creates a for loop beginning with time t = a and ending with t ~= b with stepsize h.
{
    std::cout << std::fixed << std::setprecision(3) << t << " " << y_n << "\n"; // Calls the standard output from std, with floating-point numbers with precision 3; assigns the variable t, then a space, then variable y, then a new line.
    LONG x = t;
    LONG y = y_n;
    y_n += h * f(t, y_n); // y increases by h * f(t,y) where f is the derivative y' until the condition is met y ~= b.
}
std::cout << "done\n"; // Print "done"
}

EDIT 3:

I'm now trying to use vector<POINT> to create a vector with the point values. However, my attempt results in the following error:

6   IntelliSense: no instance of overloaded function "std::vector<_Ty, _Alloc>::insert [with _Ty=POINT, _Alloc=std::allocator<POINT>]" matches the argument list
        argument types are: (std::_Vector_iterator<std::_Vector_val<std::_Simple_types<POINT>>>, double)
        object type is: std::vector<POINT, std::allocator<POINT>>   c:\Users\ahlroth\Documents\Visual Studio 2012\Projects\Euler\Euler\eulerclass.cpp   22

My code is below:

using std::vector;
vector<POINT> Pt;
POINT euler(F f, double y0, double a, double b, double h, vector<POINT> Pt) // defines a class of type "void" (returns nothing); gives it parameters: function F, doubles: y0, a, b, h
{
    double y_n = y0;
    double t = a;
    for (double t = a; t < b; t += h ) // creates a for loop beginning with time t = a and ending with t ~= b with stepsize h.
{
    std::cout << std::fixed << std::setprecision(3) << t << " " << y_n << "\n"; // Calls the standard output from std, with floating-point numbers with precision 3; assigns the variable t, then a space, then variable y, then a new line.
    Pt.insert(Pt.begin(),t);
    y_n += h * f(t, y_n); // y increases by h * f(t,y) where f is the derivative y' until the condition is met y ~= b.
}
return Pt;
std::cout << "done\n"; // Print "done"

}

The error is for this line: Pt.insert(Pt.begin(),t);

EDIT: See this post for an answer.

Answer 1

This is because you are drawing line segments of length zero.

Only do a MoveTo on the first point, then do LineTo on the rest.