Distance Between a point to a line formed by two points

I am trying to find the perpendicular distance from point 3 (newPoint) to a line formed by point 1 (prevPoint) and point 2 (curPoint) in C++.

I was using this formula previously. But now I am questioning the correctness of my program upon some cross-checking.

EDIT:
all the points(prevPoint, curPoint and newPoint) is of Point type

struct Point {
    string name;
    int x;
    int y;
    bool visited;
};


double d = distance(prevPoint, curPoint);
double dx = (curPoint.x - prevPoint.x) / d;
double dy = (curPoint.y - prevPoint.y) / d;
double distToLine = abs((newPoint.x - prevPoint.x) * dy - (newPoint.y - prevPoint.y) * dx);

The distance function:

double distance(Point a, Point b) {
double dx = a.x - b.x;
double dy = a.y - b.y;
return sqrt(dx * dx + dy * dy);}

Is my code correct? I need some correct formula if its not. Thanks!

The formula would be as follows in your case:

double d = distance(prevPoint, curPoint);
double area = abs((curPoint.x - prevPoint.x) * (newPoint.y - prevPoint.y) - (curPoint.y - prevPoint.y) * (newPoint.x - prevPoint.x));
double distToLine = area / d;

In the following test code, your code had the same results as mine.

struct Point
{
	int x,y;
	Point( int x=0, int y=0 ) : x(x),y(y) {}
};

double MyMethod( Point prevPoint, Point curPoint, Point newPoint )
{
	//For simplicity, solve the problem in a coordinate system with the origin at prevPoint.
	Point B( curPoint.x-prevPoint.x, curPoint.y-prevPoint.y );
	Point C( newPoint.x-prevPoint.x, newPoint.y-prevPoint.y );

	// Considering the Vector P as
	//	P = t*B - C
	// where, t is scalar.
	//
	// What we need to do is find the value of t that minimizes the length of this P.
	// Objective Function is :
	//	Err(t) = (P dot P) = (t*Bx - Cx)^2 + (t*By - Cy)^2
	// And Then
	//	dErr(t)/dt  =  2*(t*Bx - Cx)Bx + 2*(t*By - Cy)By = 0
	// So
	//	t = (CxBx + CyBy) / (BxBx + ByBy)
	double t = ( C.x*B.x + C.y*B.y ) / double(B.x*B.x + B.y*B.y);

	// Now the distance we want to obtain is length of P (with the calculated t value).
	double Px = t*B.x - C.x;
	double Py = t*B.y - C.y;
	return sqrt( Px*Px + Py*Py );
}

namespace YourCode
{
	double distance(Point a, Point b)
	{
		double dx = a.x - b.x;
		double dy = a.y - b.y;
		return sqrt(dx * dx + dy * dy);
	}

	double YourMethod( Point prevPoint, Point curPoint, Point newPoint )
	{
		double d = distance(prevPoint, curPoint);
		double dx = (curPoint.x - prevPoint.x) / d;
		double dy = (curPoint.y - prevPoint.y) / d;
		double distToLine = abs((newPoint.x - prevPoint.x) * dy - (newPoint.y - prevPoint.y) * dx);
		return distToLine;
	}
}

int main(int argc, char *argv[])
{
	Point A( 3,10 );
	Point B( -5, 22 );
	Point C( 1,4 );

	std::cout << YourCode::YourMethod(A,B,C) << std::endl;
	std::cout << MyMethod(A,B,C) << std::endl;
	
	return 0;
}