CParser
Using CParser for simple expression evaluator
main.cpp
#include <Core/Core.h>
using namespace Upp;
VectorMap<String, double> var;
double Exp(CParser& p);
double Term(CParser& p)
{
if(p.Id("abs")) {
p.PassChar('(');
double x = Exp(p);
p.PassChar(')');
return fabs(x);
}
if(p.IsId())
return var.Get(p.ReadId(), 0);
if(p.Char('(')) {
double x = Exp(p);
p.PassChar(')');
return x;
}
return p.ReadDouble();
}
double Mul(CParser& p)
{
double x = Term(p);
for(;;)
if(p.Char('*'))
x = x * Term(p);
else
if(p.Char('/')) {
double y = Term(p);
if(y == 0)
p.ThrowError("Divide by zero");
x = x / y;
}
else
return x;
}
double Exp(CParser& p)
{
double x = Mul(p);
for(;;)
if(p.Char('+'))
x = x + Mul(p);
else
if(p.Char('-'))
x = x - Mul(p);
else
return x;
}
CONSOLE_APP_MAIN
{
for(;;) {
Cout() << "Expresion: ";
String l = ReadStdIn();
if(l.IsEmpty())
break;
CParser p(l);
try {
String id;
if(p.IsId()) {
CParser::Pos pos = p.GetPos();
id = p.ReadId();
if(p.Char('=')) {
double x = Exp(p);
Cout() << id << " <- " << x << '\n';
var.GetAdd(id) = x;
goto done;
}
p.SetPos(pos);
}
Cout() << Exp(p) << '\n';
done:;
}
catch(CParser::Error e) {
Cout() << "ERROR: " << e << '\n';
}
}
}
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