上節實現了計算器的界面,這節實現核心算法,下節實現界面與業務邏輯的交互
目錄
1、計算器核心算法
分離算法:將中綴表達式進行數字和運算符的分離
中綴轉後綴算法:將中綴表達式轉換爲後綴表達式
後綴表達式計算 :通過後綴表達式計算最終結果
1、分離算法思想
思想:以符號作爲標誌對錶達式中的字符逐個訪問
- 初始累計變量num爲空,遍歷字符串每個字符
- 若遇到數字或小數點(即當前符號是數字的一部分),累加到num變量,並用pre變量記錄當前符號(用來判斷正負號)
- 若遇到其它符號(+, -, *, / , ( , ) ,+, -)
✔ 先看num變量,若有值,num的值放入隊列,清空num
✔ 若爲正負號,累計到num變量
✔ 否則當前字符(即運算符或括號)入隊列
✔ pre記錄當前字符
- 遍歷結束後若num不爲空,入隊列(遍歷的最後一個字符是數字還沒機會入隊列)
區分正負號和加減號準則:+ 和 - 在表達式的第一個位置,括號後的 + 和 - ,運算符後的 + 和 -
bool QCalculatorDec::isDigitOrDot(QChar c)
{
return (('0' <= c) && (c <= '9')) || (c == '.');
}
bool QCalculatorDec::isSymbol(QChar c)
{
return isOperator(c) || (c == '(') || (c == ')');
}
bool QCalculatorDec::isSign(QChar c)
{
return (c == '+') || (c == '-');
}
bool QCalculatorDec::isNumber(QString s)
{
bool ret = false;
s.toDouble(&ret);
return ret;
}
bool QCalculatorDec::isOperator(QString s)
{
return (s == "+") || (s == "-") || (s == "*") || (s == "/");
}
bool QCalculatorDec::isLeft(QString s)
{
return (s == "(");
}
bool QCalculatorDec::isRight(QString s)
{
return (s == ")");
}
int QCalculatorDec::priority(QString s)
{
int ret = 0;
if( (s == "+") || (s == "-") )
{
ret = 1;
}
if( (s == "*") || (s == "/") )
{
ret = 2;
}
return ret;
}
QQueue<QString> QCalculatorDec::split(const QString& exp)
{
QQueue<QString> ret;
QString num = "";
QString pre = "";
for(int i=0; i<exp.length(); i++)
{
if( isDigitOrDot(exp[i]) )
{
num += exp[i];
pre = exp[i];
}
else if( isSymbol(exp[i]) )
{
if( !num.isEmpty() )
{
ret.enqueue(num);
num.clear();
}
if( isSign(exp[i]) && ((pre == "") || (pre == "(") || isOperator(pre)) )
{
num += exp[i];
}
else
{
ret.enqueue(exp[i]);
}
pre = exp[i];
}
}
if( !num.isEmpty() )
{
ret.enqueue(num);
}
return ret;
}
2、括號匹配算法
合法的四則運算表達式中
- 括號匹配成對出現
- 左括號必然先於右括號出現
bool QCalculatorDec::match(QQueue<QString>& exp)
{
bool ret = true;
int len = exp.length();
QStack<QString> stack;
for(int i=0; i<len; i++)
{
if( isLeft(exp[i]) )
{
stack.push(exp[i]);
}
else if( isRight(exp[i]) )
{
if( !stack.isEmpty() && isLeft(stack.top()) )
{
stack.pop();
}
else
{
ret = false;
break;
}
}
}
return ret && stack.isEmpty();
}
3、中綴轉後綴算法
轉換過程:
- 當前元素e爲數字,輸出
- 當前元素e爲運算符: 與棧頂運算符進行優先級比較
✔ 小於等於:將棧頂元素輸出,轉1
✔ 大於:將當前元素e入棧
- 當前元素e爲左括號:入棧
- 當前元素e爲右括號:
1. 彈出棧頂元素並輸出,直至棧頂元素爲左括號
2. 將棧頂的左括號從棧中彈出
bool QCalculatorDec::transform(QQueue<QString>& exp, QQueue<QString>& output)
{
bool ret = match(exp);
QStack<QString> stack;
output.clear();
while( ret && !exp.isEmpty() )
{
QString e = exp.dequeue();
if( isNumber(e) )
{
output.enqueue(e);
}
else if( isOperator(e) )
{
while( !stack.isEmpty() && (priority(e) <= priority(stack.top())) )
{
output.enqueue(stack.pop());
}
stack.push(e);
}
else if( isLeft(e) )
{
stack.push(e);
}
else if( isRight(e) )
{
while( !stack.isEmpty() && !isLeft(stack.top()) )
{
output.enqueue(stack.pop());
}
if( !stack.isEmpty() )
{
stack.pop();
}
}
else
{
ret = false;
}
}
while( !stack.isEmpty() )
{
output.enqueue(stack.pop());
}
if( !ret )
{
output.clear();
}
return ret;
}
4、後綴表達式計算
遍歷後綴表達式中的數字和運算符
- 當前元素爲數字,進棧
- 當前元素爲運算符:
1. 從棧中彈出右操作數
2. 從棧中彈出左操作數
3. 根據符號進行運算
4 將運算結果壓入棧中
遍歷結束
- 棧中的唯一數字爲運算結果
QString QCalculatorDec::calculate(QString l, QString op, QString r)
{
QString ret = "Error";
if( isNumber(l) && isNumber(r) )
{
double lp = l.toDouble();
double rp = r.toDouble();
if( op == "+" )
{
ret.sprintf("%f", lp + rp);
}
else if( op == "-" )
{
ret.sprintf("%f", lp - rp);
}
else if( op == "*" )
{
ret.sprintf("%f", lp * rp);
}
else if( op == "/" )
{
const double P = 0.000000000000001;
if( (-P < rp) && (rp < P) )
{
ret = "Error";
}
else
{
ret.sprintf("%f", lp / rp);
}
}
else
{
ret = "Error";
}
}
return ret;
}
QString QCalculatorDec::calculate(QQueue<QString>& exp)
{
QString ret = "Error";
QStack<QString> stack;
while( !exp.isEmpty() )
{
QString e = exp.dequeue();
if( isNumber(e) )
{
stack.push(e);
}
else if( isOperator(e) )
{
QString rp = !stack.isEmpty() ? stack.pop() : "";
QString lp = !stack.isEmpty() ? stack.pop() : "";
QString result = calculate(lp, e, rp);
if( result != "Error" )
{
stack.push(result);
}
else
{
break;
}
}
else
{
break;
}
}
if( exp.isEmpty() && (stack.size() == 1) && isNumber(stack.top()) )
{
ret = stack.pop();
}
return ret;
}
2、完整代碼
QCalculatorDec.h
#ifndef _CALCULATORCORE_H_
#define _CALCULATORCORE_H_
#include <QString>
#include <QStack>
#include <QQueue>
class QCalculatorDec
{
protected:
QString m_exp;
QString m_result;
bool isDigitOrDot(QChar c);
bool isSymbol(QChar c);
bool isSign(QChar c);
bool isNumber(QString s);
bool isOperator(QString s);
bool isLeft(QString s);
bool isRight(QString s);
int priority(QString s);
bool match(QQueue<QString>& exp);
QString calculate(QQueue<QString>& exp);
QString calculate(QString l, QString op, QString r);
bool transform(QQueue<QString>& exp, QQueue<QString>& output);
QQueue<QString> split(const QString& exp);
public:
QCalculatorDec();
~QCalculatorDec();
bool expression(const QString& exp);
QString expression();
QString result();
};
#endif
QCalculatorDec.cpp
#include "QCalculatorDec.h"
QCalculatorDec::QCalculatorDec()
{
m_exp = "";
m_result = "";
}
QCalculatorDec::~QCalculatorDec()
{
}
bool QCalculatorDec::isDigitOrDot(QChar c)
{
return (('0' <= c) && (c <= '9')) || (c == '.');
}
bool QCalculatorDec::isSymbol(QChar c)
{
return isOperator(c) || (c == '(') || (c == ')');
}
bool QCalculatorDec::isSign(QChar c)
{
return (c == '+') || (c == '-');
}
bool QCalculatorDec::isNumber(QString s)
{
bool ret = false;
s.toDouble(&ret);
return ret;
}
bool QCalculatorDec::isOperator(QString s)
{
return (s == "+") || (s == "-") || (s == "*") || (s == "/");
}
bool QCalculatorDec::isLeft(QString s)
{
return (s == "(");
}
bool QCalculatorDec::isRight(QString s)
{
return (s == ")");
}
int QCalculatorDec::priority(QString s)
{
int ret = 0;
if( (s == "+") || (s == "-") )
{
ret = 1;
}
if( (s == "*") || (s == "/") )
{
ret = 2;
}
return ret;
}
bool QCalculatorDec::expression(const QString& exp)
{
bool ret = false;
QQueue<QString> spExp = split(exp);
QQueue<QString> postExp;
m_exp = exp;
if( transform(spExp, postExp) )
{
m_result = calculate(postExp);
ret = (m_result != "Error");
}
else
{
m_result = "Error";
}
return ret;
}
QString QCalculatorDec::result()
{
return m_result;
}
QQueue<QString> QCalculatorDec::split(const QString& exp)
{
QQueue<QString> ret;
QString num = "";
QString pre = "";
for(int i=0; i<exp.length(); i++)
{
if( isDigitOrDot(exp[i]) )
{
num += exp[i];
pre = exp[i];
}
else if( isSymbol(exp[i]) )
{
if( !num.isEmpty() )
{
ret.enqueue(num);
num.clear();
}
if( isSign(exp[i]) && ((pre == "") || (pre == "(") || isOperator(pre)) )
{
num += exp[i];
}
else
{
ret.enqueue(exp[i]);
}
pre = exp[i];
}
}
if( !num.isEmpty() )
{
ret.enqueue(num);
}
return ret;
}
bool QCalculatorDec::match(QQueue<QString>& exp)
{
bool ret = true;
int len = exp.length();
QStack<QString> stack;
for(int i=0; i<len; i++)
{
if( isLeft(exp[i]) )
{
stack.push(exp[i]);
}
else if( isRight(exp[i]) )
{
if( !stack.isEmpty() && isLeft(stack.top()) )
{
stack.pop();
}
else
{
ret = false;
break;
}
}
}
return ret && stack.isEmpty();
}
bool QCalculatorDec::transform(QQueue<QString>& exp, QQueue<QString>& output)
{
bool ret = match(exp);
QStack<QString> stack;
output.clear();
while( ret && !exp.isEmpty() )
{
QString e = exp.dequeue();
if( isNumber(e) )
{
output.enqueue(e);
}
else if( isOperator(e) )
{
while( !stack.isEmpty() && (priority(e) <= priority(stack.top())) )
{
output.enqueue(stack.pop());
}
stack.push(e);
}
else if( isLeft(e) )
{
stack.push(e);
}
else if( isRight(e) )
{
while( !stack.isEmpty() && !isLeft(stack.top()) )
{
output.enqueue(stack.pop());
}
if( !stack.isEmpty() )
{
stack.pop();
}
}
else
{
ret = false;
}
}
while( !stack.isEmpty() )
{
output.enqueue(stack.pop());
}
if( !ret )
{
output.clear();
}
return ret;
}
QString QCalculatorDec::calculate(QString l, QString op, QString r)
{
QString ret = "Error";
if( isNumber(l) && isNumber(r) )
{
double lp = l.toDouble();
double rp = r.toDouble();
if( op == "+" )
{
ret.sprintf("%f", lp + rp);
}
else if( op == "-" )
{
ret.sprintf("%f", lp - rp);
}
else if( op == "*" )
{
ret.sprintf("%f", lp * rp);
}
else if( op == "/" )
{
const double P = 0.000000000000001;
if( (-P < rp) && (rp < P) )
{
ret = "Error";
}
else
{
ret.sprintf("%f", lp / rp);
}
}
else
{
ret = "Error";
}
}
return ret;
}
QString QCalculatorDec::calculate(QQueue<QString>& exp)
{
QString ret = "Error";
QStack<QString> stack;
while( !exp.isEmpty() )
{
QString e = exp.dequeue();
if( isNumber(e) )
{
stack.push(e);
}
else if( isOperator(e) )
{
QString rp = !stack.isEmpty() ? stack.pop() : "";
QString lp = !stack.isEmpty() ? stack.pop() : "";
QString result = calculate(lp, e, rp);
if( result != "Error" )
{
stack.push(result);
}
else
{
break;
}
}
else
{
break;
}
}
if( exp.isEmpty() && (stack.size() == 1) && isNumber(stack.top()) )
{
ret = stack.pop();
}
return ret;
}
main.cpp
#include <QtGui/QApplication>
#include "QCalculatorDec.h"
#include <QDebug>
int main(int argc, char *argv[])
{
QCalculatorDec c;
c.expression(" (5 - 8) * (5 - 6) ");
qDebug() << c.result();
return 0;
}