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Using Polyline() to draw line segments from a loop.

rocapp (1)
Hi all,
I'm trying to use Polyline() to draw lines to the screen. So far I have done my best to do the following:

1) Create a loop that simulates basic Euler integration.
2) Store values from each step in a POINT struct.
3) Assign those POINT structs to a vector<POINT>.
4) Use a pointer to store the vector.
5) Draw the lines using the points contained in the array.

I'm using 2 CPP files.

First CPP
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#include <iomanip>
#include <iostream>
#include <windows.h>
#include <atltypes.h>
#include <vector>

typedef long F(long,long); // defines the function F as having type "double" and gives it two parameters, both of type "double"
/*
Approximates y(t) in y'(t)=f(t,y) with y(a)=y0 and
t=a..b and the step size h.
*/
long t;
long y_n;

using std::vector;
vector<POINT> Pt;

POINT pointvalues;
const POINT * pointpointer;

int counter;

long euler(F f, long y0, long a, long b, long h) // defines a class of type "void" (returns nothing); gives it parameters: function F, doubles: y0, a, b, h
{
	long y_n = y0;
	long t = a;

    for (long 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.
		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.
		pointvalues.x = t*100;
		pointvalues.y = y_n*100;
		Pt.push_back(pointvalues);
		counter++;
	}
    std::cout << "done\n"; // Print "done"
	pointpointer = &pointvalues;
}



// Example: Newton's cooling law
long newtonCoolingLaw(long, long t) // Creates a function newtonCoolingLaw of type double with two input arguments of type double, one being the variable t.
{
    return 0.07 * (t - 20); // return statement ends the function; here, it gives the time derivative y' = -0.07 * (t - 20)
}

long main() // Calls the main function with type int
{
    euler(newtonCoolingLaw, 10, 0, 5, 4); // instantiation of class euler with arguments (newtonCoolingLaw, 100, 0, 100, 2)
    euler(newtonCoolingLaw, 100, 0, 100, 4 ); // etc
    euler(newtonCoolingLaw, 100, 0, 100, 10 ); // etc
}


Important part of Second CPP
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case WM_PAINT:
		hdc = BeginPaint(hWnd, &ps);
      //Draw lines to screen.
		hPen = CreatePen(PS_SOLID, 1, RGB(255, 25, 5));
        SelectObject(hdc, hPen);
		Polyline(hdc,pointpointer,counter);
		// the below for loop doesn't work, but I have successfully produced a line this way.
		//for (std::vector<POINT>::iterator it = Pt.begin(); it != Pt.end(); ++it)
		//{
		//	MoveToEx(hdc,10*pointvalues.x++,10*pointvalues.y++,NULL);
		//	LineTo(hdc,10*pointvalues.x++,10*pointvalues.y++);
		//}
		EndPaint(hWnd, &ps);
		break;
	case WM_DESTROY:
		PostQuitMessage(0);
		break;
	default:
		return DefWindowProc(hWnd, message, wParam, lParam);
	}
	return 0;


I also used this in the second CPP file to call the necessary externals:
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	extern std::vector<POINT> Pt;
	extern POINT pointvalues;
	extern const POINT * pointpointer;
	extern int counter;



Currently I get no errors but a blank screen when I build. Suggestions?
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