77. Math Parser

Learn how to parse mathematical expressions represented as strings and get calculation results.

77.1 Parsing Mathematical Expressions

• There are functions to parse mathematical expressions and get calculation results:

Code	Description
double Eval(expression)	Parses mathematical expression and returns result as double type. Returns NaN if expression is invalid
Optional<double> EvalOpt(expression)	Parses mathematical expression and returns result as double type. Returns none if expression is invalid

• You can use the following operators and functions in expressions:
  • +, -, *, /, %, ^
  • log2, log10, log, ln, exp, sqrt, sign, abs, min, max, sin, cos, tan, asin, acos, atan, sinh, cosh, tanh, asinh, acosh, atanh

# include <Siv3D.hpp>

void Main()
{
	{
		const String expression = U"100 + 10 * 2 + sqrt(9) + 10 * 0.2 ^2";
		Print << Eval(expression);
		Print << EvalOpt(expression);
	}

	{
		// Invalid expression
		const String expression = U"100 +";
		Print << Eval(expression);
		Print << EvalOpt(expression);
	}

	while (System::Update())
	{

	}
}

Output

123.4
(Optional)123.4
nan
none

77.2 Adding Constants

• If you want to make new elements like custom constants available in expressions, use the MathParser class

# include <Siv3D.hpp>

void Main()
{
	MathParser parser;

    // Add constants
	parser.setConstant(U"pi", Math::Pi);
	parser.setConstant(U"x", 100);

    // Set expression
	parser.setExpression(U"x * pi");

	Print << parser.eval();
	Print << parser.evalOpt();

	while (System::Update())
	{

	}
}

Output

314.15927
(Optional)314.15927

77.3 Adding Variables

• By registering the address of double type variables with MathParser, you can use variables in expressions

# include <Siv3D.hpp>

void Main()
{
	double x = 0.0;

	MathParser parser;
	parser.setVariable(U"x", &x);
	parser.setExpression(U"100 + x");

	for (int32 i = 0; i < 10; ++i)
	{
		x = (i * 0.1);
		Print << parser.eval();
	}

	while (System::Update())
	{

	}
}

Output

100
100.1
100.2
100.3
100.4
100.5
100.6
100.7
100.8
100.9

77.4 Adding Functions

• You can register custom functions with MathParser

# include <Siv3D.hpp>

double CircleArea(double r)
{
	return (Math::Pi * r * r);
}

double TriangleArea(double a, double b, double c)
{
	double s = ((a + b + c) / 2.0);
	return std::sqrt(s * (s - a) * (s - b) * (s - c));
}

void Main()
{
	MathParser parser;
	parser.setFunction(U"circleArea", CircleArea);
	parser.setFunction(U"triangleArea", TriangleArea);

	parser.setExpression(U"circleArea(10)");
	Print << parser.eval();

	parser.setExpression(U"triangleArea(3, 4, 5)");
	Print << parser.eval();

	while (System::Update())
	{

	}
}

Output

314.15927
6