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);}
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