一个简单实用的表达式计算类

#include <stdio.h> #include <string> #include <math.h> #include <stdexcept> #include "cexpr.h" using namespace std; CExpression::CExpression() { Expression=""; } CExpression::CExpression(string exp) { Expression=exp; } void CExpression::SetExpression(string exp) { Expression=exp; } char CExpression::GetChar() { if ((unsigned int)pos < Expression.length()) ch = Expression[pos++]; else ch = '\0'; return ch; } char CExpression::GetNonWhitespace() { GetChar(); SkipWhitespace(); return ch; } void CExpression::SkipWhitespace() { if (ch == '\t' || ch == ' ') { do { GetChar(); } while (ch == '\t' || ch == ' '); } } double CExpression::CalcAddition() { double Result; bool tmp=true; double temp; Result = CalcMultiplication(); while (tmp) { switch (ch) { case '+': temp= CalcMultiplication(); Result =Result +temp; break; case '-': temp=CalcMultiplication(); Result =Result - temp; break; default: tmp=false; } } return Result; } double CExpression::CalcMultiplication() { double Result; bool tmp=true; double temp; Result = CalcPower(); while (tmp) { switch (ch) { case '*': temp= CalcPower(); Result = Result * temp; break; case '/': temp= CalcPower(); Result = Result /temp; break; default: tmp=false; break; } } return Result; } double CExpression::CalcPower() { double Result; bool tmp=true; Result = CalcSignedTerm(); while (tmp) { switch (ch) { case '^': Result = exp(log(Result) * CalcPower() ); break; default: tmp=false; } } return Result; } double CExpression::CalcSignedTerm() { double Result; switch (GetNonWhitespace()) { case '+' : Result = CalcSignedTerm(); break; case '-' : Result = -CalcSignedTerm(); break; default: Result = CalcTerm(); } return Result; } double CExpression::CalcTerm() { double Result; char s[100]; if (isdigit(ch)) Result = CalcNumber(); else if (ch=='(') Result = CalcExpressionInBrackets(); else { sprintf(s, "Syntax error in expression at position %d", pos); throw runtime_error(s); } return Result; } double CExpression::CalcNumber() { double Result ; char s[256]; *s = '\0'; while (isdigit(ch)) { sprintf(s + strlen(s), "%c", ch); GetChar(); if (ch == '.') { do { sprintf(s + strlen(s), "%c", ch); GetChar(); } while (isdigit(ch)); } } sscanf(s,"%lf",&Result); SkipWhitespace(); return Result; } double CExpression::CalcExpressionInBrackets() { double Result; char s[100]; Result = CalcAddition(); if (ch == ')') { GetNonWhitespace(); } else { sprintf(s,"Mismatched parentheses at position %d", pos ); throw runtime_error(s); } return Result; } double CExpression::Calculate() { double Result; char s[100]; pos = 0; Result = CalcAddition(); if (ch != '\0') { sprintf(s,"Unexpected end of expression at position %d", pos + 1); throw runtime_error(s); } return Result; }