關于字符串四則運算表達式的求解，網上有很多利用變字符串為前綴表達式或后綴表達式實驗的方法，本人覺得模擬筆算的方法簡單易懂，這里給出了QT的具體實現，未經大量測試，僅供參考。

mainwindow.h

#ifndef MAINWINDOW_H#define MAINWINDOW_H#include <QMainWindow>#include <QQueue>#include <QStack>namespace Ui {class MainWindow;}class MainWindow : public QMainWindow{    Q_OBJECTpublic:    explicit MainWindow(QWidget *parent = 0);    ~MainWindow();    void parseStringToStack(QStack<double> &q = stact_num, const QString &s = input);    void parseSymbolToStack(QStack<QChar> &q = stact_smb, QString &s = input);    int findTheLastNonNum(const QString &s, int pos);    double calc(QStack<double> &stact_n, QStack<QChar> &stact_s);    double calccc(QStack<double> sd, QStack<QChar> ss);PRivate slots:    void on_pushButton_clicked();    void on_lineEdit_num_returnPressed();    void on_lineEdit_num_textChanged(const QString &arg1);    void on_pushButton_reset_clicked();    void on_textEdit_input_textChanged();private:    Ui::MainWindow *ui;    QString static input;    QStack<double> static stact_num;    QStack<QChar> static stact_smb;};#endif // MAINWINDOW_Hmainwindow.cpp
#include "mainwindow.h"#include "ui_mainwindow.h"#include <QMessageBox>QString MainWindow::input;QStack<double> MainWindow::stact_num;QStack<QChar> MainWindow::stact_smb;MainWindow::MainWindow(QWidget *parent) :    QMainWindow(parent),    ui(new Ui::MainWindow){    ui->setupUi(this);    input = "";    ui->lineEdit_num->setStyleSheet("font-size : 26px");    ui->lineEdit_result->setStyleSheet("font-size : 24px");    ui->textEdit_input->setStyleSheet("font-size : 24px");    QObject::connect(ui->lineEdit_num, SIGNAL(returnPressed()), this, SLOT(on_pushButton_clicked()));}MainWindow::~MainWindow(){    delete ui;}double MainWindow::calc(QStack<double> &stact_n, QStack<QChar> &stact_s){    double result = 0;    while(!stact_s.isEmpty())    {        QStack<double> stack_num1;        QStack<QChar> stack_smb1;        double cc;        QChar ch = stact_s.pop();        switch (ch.toLatin1()) {        case '+':            result += stact_n.pop();            break;        case '-':            result -= stact_n.pop();            break;        case '*':        case '/':            stack_smb1.push(ch);            stack_num1.push(stact_n.pop());            while(!stact_s.isEmpty())            {                ch = stact_s.pop();                if(!(ch == '*' || ch == '/'))                {                    stact_s.push(ch);                    break;                }                stack_smb1.push(ch);                stack_num1.push(stact_n.pop());            }            stack_num1.push(stact_n.pop());            cc = calccc(stack_num1, stack_smb1);            //QMessageBox::warning(this, "cc", QString::number(cc));            stact_n.push(cc);            break;        default:            break;        }    }    if(!stact_n.isEmpty())        result += stact_n.pop();    return result;}void MainWindow::on_pushButton_clicked(){    QString num = ui->lineEdit_num->text();    if("" != num)    {        QString str = num.mid(0, num.length());        bool ok = false;        str.toDouble(&ok);        if(!ok)        {            QMessageBox::warning(this, "輸入警告", "輸入值錯誤！");            return;        }        str = ui->textEdit_input->toPlainText();        str += num;        ui->textEdit_input->setText(str);        ui->lineEdit_num->clear();    }    input = ui->textEdit_input->toPlainText();    if("" == input)        return;    int pos1 = input.lastIndexOf('(');    int pos2 = input.indexOf(')', pos1);    while(pos1 != -1)    {        QString str = input.mid(pos1+1, pos2-pos1-1);        input.remove(pos1, pos2 - pos1 + 1);        parseStringToStack(stact_num, str);        double val;        if(stact_num.isEmpty())        {            QMessageBox::warning(this, "輸入警告", "輸入的()內為空！");            exit(0);        }        if(stact_num.length()==1)        {            val = stact_num.pop();            input.insert(pos1, QString::number(val));            continue;        }        if(stact_num.length() != stact_smb.length() + 1)        {            QMessageBox::warning(this, "輸入警告", "輸入的()有誤！");        }        val = calc(stact_num, stact_smb);        input.insert(pos1, QString::number(val));        //3+4*(3-(2*3))-2*(1+2)        QMessageBox::warning(this, "輸入警告", input);        /*QString sisi = "+13.14";        double xixi = sisi.toDouble();        QMessageBox::warning(this, "ceshi", QString::number(xixi));*/        pos1 = input.lastIndexOf('(');        pos2 = input.indexOf(')', pos1);    }    parseStringToStack();    if(stact_num.isEmpty())    {        ui->lineEdit_result->clear();        return;    }    if(stact_num.length()==1)    {        ui->lineEdit_result->setText(QString::number(stact_num.pop()));        return;    }    double result = calc(stact_num, stact_smb);    ui->lineEdit_result->setText(QString::number(result));}double MainWindow::calccc(QStack<double> sd, QStack<QChar> ss){    double z = sd.pop();    while(!ss.isEmpty())    {        QChar ch = ss.pop();        switch (ch.toLatin1())        {            case '*':                z *= sd.pop();                break;            case '/':                z /= sd.pop();                break;            default:                break;        }    }    return z;}void MainWindow::on_lineEdit_num_returnPressed(){}void MainWindow::on_lineEdit_num_textChanged(const QString &arg1){    QString num = ui->lineEdit_num->text();    if(num=="")        return;    int len = num.length();    QChar ch = num[len-1];    if(ch == 'r' || ch == 'R')    {        ui->lineEdit_num->clear();        ui->lineEdit_result->clear();        return;    }    if(ch == 'c' || ch == 'C')    {        input.clear();        ui->lineEdit_num->clear();        ui->lineEdit_result->clear();        ui->textEdit_input->clear();        return;    }    if((ch>='0' && ch<='9') || (ch=='.'))    {        return;    }    else if(ch=='+' || ch=='-' || ch=='*' || ch=='/')    {        int len = num.length();        if(len > 1)        {            QString str = num.mid(0, len-1);            bool ok = false;            str.toDouble(&ok);            if(!ok)            {                QMessageBox::warning(this, "輸入警告", "輸入值錯誤！");                return;            }        }        input += num;        ui->textEdit_input->setText(input);        ui->lineEdit_num->clear();    }    else    {        QMessageBox::warning(this, "輸入警告", "輸入值錯誤！");    }}void MainWindow::on_pushButton_reset_clicked(){    input.clear();    ui->lineEdit_num->clear();    ui->lineEdit_result->clear();    ui->textEdit_input->clear();}int MainWindow::findTheLastNonNum(const QString &s, int pos){    if(s.length() - 1 < pos)        return -1;    for(int i = pos; i > 0; i--)    {        if(s[i]=='+' || s[i]=='-' || s[i]=='*' || s[i]=='/')        {            if(i>0 && (s[i-1]=='*' || s[i-1]=='/'))            {                i--;            }            return i;        }    }    return -1;}void MainWindow::parseStringToStack(QStack<double> & q, const QString & s){    stact_num.clear();    stact_smb.clear();    if(s == "")    {        q.clear();        return;    }    s.trimmed();    int len = s.length();    int pos = len - 1;    if(s[len-1] == '.' || s[len-1]=='+' || s[len-1]=='-' || s[len-1]=='*' || s[len-1]=='/')    {        if(len == 1)        {            q.clear();            this->ui->lineEdit_num->clear();            this->ui->lineEdit_result->clear();            this->ui->textEdit_input->clear();            return;        }        pos--;    }    QString tmp;    int pos1;    while(1)    {        pos1 = this->findTheLastNonNum(s, pos);        if(pos1 == -1)        {            tmp = s.mid(0, pos+1);            stact_num.push_front(tmp.toDouble());            return;        }        else {            tmp = s.mid(pos1+1, pos-pos1);            stact_num.push_front(tmp.toDouble());            stact_smb.push_front(s[pos1]);            pos = pos1 - 1;        }    }}void MainWindow::on_textEdit_input_textChanged(){    ui->lineEdit_result->clear();    QTextCursor cursor = ui->textEdit_input->textCursor();    cursor.movePosition(QTextCursor::End);    ui->textEdit_input->setTextCursor(cursor);}