static void Main(string[] args)
{
// Create some constants.
Expression A = 2;
Constant B = Constant.New(3);
// Create some variables.
Expression x = "x";
Variable y = Variable.New("y");
// Create a basic expressions.
Expression f = A*x + B*y + 4;
// This expression uses the implicit conversion from string to
// Expression, which parses the string.
Expression g = "5*x + C*y + 8";
// Create a system of equations from the above expressions.
var system = new List<Equal>()
{
Equal.New(f, 0),
Equal.New(g, 0),
};
// We can now solve the system of equations for x and y. Since the
// equations have a variable 'C', the solutions will not be
// constants.
List<Arrow> solutions = system.Solve(x, y);
Console.WriteLine("The solutions are:");
foreach (Arrow i in solutions)
Console.WriteLine(i.ToString());
// A fundamental building block of ComputerAlgebra is the 'Arrow'
// expression. Arrow expressions define the value of one expression
// to be the value given by another expression. For example, 'x->2'
// defines the expression 'x' to have the value '2'.
// The 'Solve' function used above returns a list of Arrow
// expressions defining the solutions of the system.
// Arrow expressions are used by the 'Evaluate' function to
// substitute values for expressions into other expressions. To
// demonstrate the usage of Evaluate, let's validate the solution
// by using Evaluate to substitute the solutions into the original
// system of equations, and then substitute a value for C.
Expression f_xy = f.Evaluate(solutions).Evaluate(Arrow.New("C", 2));
Expression g_xy = g.Evaluate(solutions).Evaluate(Arrow.New("C", 2));
if ((f_xy == 0) && (g_xy == 0))
Console.WriteLine("Success!");
else
Console.WriteLine("Failure! f = {0}, g = {1}", f_xy, g_xy);
// Suppose we need to evaluate the solutions efficiently many times.
// We can compile the solutions to delegates where 'C' is a
// parameter to the delegate, allowing it to be specified later.
var x_C = x.Evaluate(solutions).Compile<Func<double, double>>("C");
var y_C = y.Evaluate(solutions).Compile<Func<double, double>>("C");
for (int i = 0; i < 20; ++i)
{
double C = i / 2.0;
Console.WriteLine("C = {0}: (x, y) = ({1}, {2})", C, x_C(C), y_C(C));
}
}
// Expand N(x)/D(x) using partial fractions.
private static Expression ExpandPartialFractions(Expression N, Expression D, Expression x)
{
List<Expression> terms = new List<Expression>();
List<Variable> unknowns = new List<Variable>();
List<Expression> basis = new List<Expression>();
foreach (Expression i in Product.TermsOf(D))
{
// Get the multiplicity of this basis term.
Expression e = i;
int n = Power.IntegralExponentOf(e);
if (n != 1)
e = ((Power)i).Left;
// Convert to a polynomial.
Polynomial Pi = Polynomial.New(e, x);
// Add new terms for each multiplicity n.
for (int j = 1; j <= n; ++j)
{
// Expression for the unknown numerator of this term.
Expression unknown = 0;
for (int k = 0; k < Pi.Degree; ++k)
{
Variable Ai = Variable.New("_A" + unknowns.Count.ToString());
unknown += Ai * (x ^ k);
unknowns.Add(Ai);
}
terms.Add(Product.New(unknown, Power.New(e, -j)));
}
basis.Add(i);
}
// Equate the original expression with the decomposed expressions.
D = Sum.New(terms.Select(j => (Expression)(D * j))).Expand();
Polynomial l = Polynomial.New(N, x);
Polynomial r = Polynomial.New(D, x);
// Equate terms of equal degree and solve for the unknowns.
int degree = Math.Max(l.Degree, r.Degree);
List<Equal> eqs = new List<Equal>(degree + 1);
for (int i = 0; i <= degree; ++i)
eqs.Add(Equal.New(l[i], r[i]));
List<Arrow> A = eqs.Solve(unknowns);
// Substitute the now knowns.
return Sum.New(terms.Select(i => i.Evaluate(A)));
}
