public void Parabola100ReverseNeg28()
{
// Test Parabola with A=-2, B=8, and negative, non-unity step size
ParabolaRk4 rk4 = new ParabolaRk4()
{
A = -2d, B = 8d
};
// Initial x0=100, y0=-19200; h=-2, stop after fourth step
List <List <double> > result = rk4.Rk4_solve(100d, -19200d, 100d - 7.9d, -2d);
Assert.AreEqual(2, result.Count); // Check lists' sizes
Assert.AreEqual(5, result[1].Count);
Assert.AreEqual(5, result[0].Count);
// Check values
double val = -19200d;
for (int i = 0; i < 5; ++i)
{
double x = 100d - (i << 1);
Assert.IsTrue(Math.Abs(x - result[0][i]) < 1e-10);
Assert.IsTrue(Math.Abs(val - result[1][i]) < 1e-10);
val -= (rk4.A * 4 * (x - 1)); // (x*x) - ((x-2)*(x-2) = 4x-4
val -= (2 * rk4.B);
}
}
public void ParabolaForward20()
{
// Parabola with A=2 and B=0
ParabolaRk4 rk4 = new ParabolaRk4()
{
A = 2d, B = 0d
};
// Setup Rk4 intgration from x=1 to x=10
double x0 = 1d; // Initial x
double y0 = (rk4.A * x0 * x0) + rk4.B; // Initial y
double h = 1d; // Step size
double x = x0 + 8.9; // Minimum final x
// Integrate Rk4 algorithm stops at (x0 + N*h),
// with N integer and N>=8.9
List <List <double> > result = rk4.Rk4_solve(x0, y0, x, h);
Assert.AreEqual(2, result.Count); // Check lists' sizes
Assert.AreEqual(10, result[0].Count);
Assert.AreEqual(10, result[1].Count);
// Check values
for (int i = 0; i < 10; ++i)
{
double xi = i + 1;
Assert.IsTrue(Math.Abs(xi - result[0][i]) < 1e-10);
double y = (rk4.A * xi + rk4.B) * xi;
Assert.IsTrue(Math.Abs(y - result[1][i]) < 1e-10);
}
}
