/// <summary>
/// This function return the area under curve using Gauss Legendre rule
/// </summary>
/// <param name="function">Function. function for curve</param>
/// <param name="lowerBound">double. lowerBound is lower bound of integral</param>
/// <param name="upperBound">double. upperBound is lower bound of integral</param>
/// <param name="accuracy">double. accuracy is the value of corvergence tolerance</param>
/// <returns>The area under the curve</returns>
public static double GaussLegendre(BaseFunction function,
double lowerBound, double upperBound, double accuracy)
{
double dx = upperBound - lowerBound;
double value = 1.0e+10;
double oldValue;
int numPoints = 10;
int maxPoints = 10000;
double[] x = new double[maxPoints];
double[] w = new double[maxPoints];
// Compute the integral value adding points
// until the solution convergences or maxPoints
// is reached.
do
{
// Calculate the Gauss-Legendre weights and
// abscissas
computeGaussWeights(x, w, numPoints);
// Compute the integral value by summing up the
// weights times the function value at each
// abscissa
oldValue = value;
value = 0.0;
for (int i = 0; i < numPoints; ++i)
{
value += w[i] * function.Value(lowerBound + x[i] * dx);
}
value *= dx;
// If the solution is convergences, return the
// value. Otherwise, double the points in the
// integration domain and try again.
if (Math.Abs(value - oldValue) <=
accuracy * Math.Abs(oldValue))
{
return value;
}
numPoints *= 2;
} while (numPoints < maxPoints);
// Solution did not convergence
return value; ;
}
/// <summary>
/// This function return the area under curve using Simpson's rule
/// </summary>
/// <param name="function">Function. function for curve</param>
/// <param name="lowerBound">double. lowerBound is lower bound of integral</param>
/// <param name="upperBound">double. upperBound is lower bound of integral</param>
/// <param name="accuracy">double. accuracy is the value of corvergence tolerance</param>
/// <returns>The area under the curve</returns>
public static double SimpsonsRule(BaseFunction function,
double lowerBound, double upperBound, double accuracy)
{
double dx = upperBound - lowerBound;
double value, oldValue, sum, scale;
double valueSimpson, oldValueSimpson;
int maxIter = 20;
// Set initial values for the trapezoidal
// method (value) and Simpson's rule (valueSimpson)
value = 0.5 * dx * (function.Value(lowerBound) +
function.Value(upperBound));
valueSimpson = double.MaxValue;
// Subdivide the integration range by adding more
// points. Recompute the extended trapezoidal value
// and the Simpson's rule value. If the Simpson's
// rule value convergences, return the value.
for (int i = 0; i < maxIter; ++i)
{
sum = 0.0;
scale = Math.Pow(0.5, i + 1);
for (int j = 0; j < Math.Pow(2, i); ++j)
{
sum += function.Value(
lowerBound + scale * (2 * j + 1.0) * dx);
}
oldValue = value;
value = 0.5 * value + scale * dx * sum;
oldValueSimpson = valueSimpson;
valueSimpson = (4.0 * value - oldValue) / 3.0;
if (Math.Abs(valueSimpson - oldValueSimpson) <=
accuracy * Math.Abs(oldValueSimpson))
{
return valueSimpson;
}
}
// Solution did not convergence
return value; ;
}
/// <summary>
/// This function return the area under curve using trapezoidal rule
/// </summary>
/// <param name="function">Function. function for curve</param>
/// <param name="lowerBound">double. lowerBound is lower bound of integral</param>
/// <param name="upperBound">double. upperBound is lower bound of integral</param>
/// <param name="accuracy">double. accuracy is the value of corvergence tolerance</param>
/// <returns>The area under the curve</returns>
public static double Trapezoidal(BaseFunction function,
double lowerBound, double upperBound, double accuracy)
{
double dx = upperBound - lowerBound;
double value, oldValue, sum, scale;
int maxIter = 20;
// first, compute an value of the integral using
// the two-point trapezoidal rule.
value = 0.5 * dx * (function.Value(lowerBound) +
function.Value(upperBound));
// Subdivide the integration range by adding more
// points. Recompute the integral value each time.
// If the solution convergences, return the value.
for (int i = 0; i < maxIter; ++i)
{
sum = 0.0;
scale = Math.Pow(0.5, i + 1);
for (int j = 0; j < Math.Pow(2, i); ++j)
{
sum += function.Value(lowerBound +
scale * (2 * j + 1.0) * dx);
}
oldValue = value;
value = 0.5 * value + scale * dx * sum;
if (Math.Abs(value - oldValue) <=
accuracy * Math.Abs(oldValue))
{
return value;
}
}
// Solution did not convergence
return value; ;
}
/// <summary>
/// This function return the area under curve using MidPoint rule
/// </summary>
/// <param name="function">Function. function for curve</param>
/// <param name="lowerBound">double. lowerBound is lower bound of integral</param>
/// <param name="upperBound">double. upperBound is lower bound of integral</param>
/// <param name="accuracy">double. accuracy is the value of corvergence tolerance</param>
/// <returns>The area under the curve</returns>
public static double MidPoint(BaseFunction function,
double lowerBound, double upperBound, double accuracy)
{
double dx = upperBound - lowerBound;
double value, oldValue, sum, temp;
double scale = 1.0 / 3.0;
int numPoints, numPairs;
int maxIter = 20;
// Start with a two-point midpoint
// evaluation.
value = 0.5 * dx * (function.Value(lowerBound + 0.25 * dx) +
function.Value(upperBound - 0.25 * dx));
// Refine the solution by adding points to the
// integration domain. Each iteration adds three
// times as many points as the previous iteration.
// When the solution convergences, return the result.
for (int i = 0; i < maxIter; ++i)
{
numPoints = 4 * (int)Math.Pow(3, i);
numPairs = (numPoints - 2) / 2;
temp = 1.0 / (3.0 * numPoints);
// Add the two endpoint values to the sum
sum = function.Value(lowerBound + temp * dx) +
function.Value(upperBound - temp * dx);
// Add in each pair to the sum
for (int j = 0; j < numPairs; ++j)
{
sum = sum +
function.Value(lowerBound + (6 * j + 5) * temp * dx) +
function.Value(lowerBound + (6 * j + 7) * temp * dx);
}
oldValue = value;
value = value / 3.0 + 0.5 * Math.Pow(scale, i + 1) * dx * sum;
if (Math.Abs(value - oldValue) <=
accuracy * Math.Abs(oldValue))
{
return value;
}
}
// Solution did not convergence
return value; ;
}
