#include <iostream>
#include <conio.h>
#include <string>
void main()
{
?? ?char str[0xFF];
?? ?do
?? ?{
?? ??? ?system("cls");
?? ??? ?float a{};
?? ??? ?float b{};
?? ??? ?int count = 1;
?? ??? ?int isOperating = -1;
?? ??? ?bool isFirst = true;
?? ??? ?std::string text{ "按任意鍵重置，繼續計算\n" };
?? ??? ?std::cout << "------計算機------\n";
?? ??? ?std::cout << "請輸入加減乘除：";
?? ??? ?std::cin >> str;
?? ??? ?for (int i = strlen(str) - 1; i >= 0; i--)
?? ??? ?{
?? ??? ??? ?float* num = isFirst ? &b : &a;
?? ??? ??? ?switch (str[i])
?? ??? ??? ?{
?? ??? ??? ?case '*':
?? ??? ??? ??? ?isOperating = 0;
?? ??? ??? ??? ?break;
?? ??? ??? ?case '/':
?? ??? ??? ??? ?isOperating = 1;
?? ??? ??? ??? ?break;
?? ??? ??? ?case '+':
?? ??? ??? ??? ?isOperating = 2;
?? ??? ??? ??? ?break;
?? ??? ??? ?case '-':
?? ??? ??? ??? ?isOperating = 3;
?? ??? ??? ??? ?break;
?? ??? ??? ?}
?? ??? ??? ?if (!isFirst&&!isdigit(str[i])&& str[i]<=127&& str[i]>=0)
?? ??? ??? ?{
?? ??? ??? ??? ?isOperating = 4;
?? ??? ??? ??? ?text = "暫只支持正整數加減乘除，不能輸入中文,其他符號或運算符\n按任意鍵重新輸入\n";
?? ??? ??? ??? ?break;
?? ??? ??? ?}
?? ??? ??? ?if (isOperating != -1 && count != 1 && isFirst)
?? ??? ??? ?{
?? ??? ??? ??? ?isFirst = false;
?? ??? ??? ??? ?count = 1;
?? ??? ??? ??? ?if (i == 0 || i == strlen(str) - 1)
?? ??? ??? ??? ?{
?? ??? ??? ??? ??? ?isOperating = 4;
?? ??? ??? ??? ??? ?break;
?? ??? ??? ??? ?}
?? ??? ??? ??? ?continue;
?? ??? ??? ?}
?? ??? ??? ?*num += ((int)str[i] - 48) * count;
?? ??? ??? ?count *= 10;
?? ??? ?}
?? ??? ?switch (isOperating)
?? ??? ?{
?? ??? ?case 0:
?? ??? ??? ?std::cout << "結果為:" << a * b << std::endl;
?? ??? ??? ?break;
?? ??? ?case 1:
?? ??? ??? ?if (a == 0)
?? ??? ??? ??? ?text = "除數不能為0\n按任意鍵重置，繼續計算\n";
?? ??? ??? ?else
?? ??? ??? ??? ?std::cout << "結果為:" << a / b << std::endl;
?? ??? ??? ?break;
?? ??? ?case 2:
?? ??? ??? ?std::cout << "結果為:" << a + b << std::endl;
?? ??? ??? ?break;
?? ??? ?case 3:
?? ??? ??? ?std::cout << "結果為:" << a - b << std::endl;
?? ??? ??? ?break;
?? ??? ?default:
?? ??? ??? ?text = text == "按任意鍵重置，繼續計算\n" ? "錯誤格式,按任意鍵重新輸入\n" : text;
?? ??? ?}
?? ??? ?std::cout << text;
?? ??? ?_getch();
?? ?} while (true);
}

#include<iostream>
using namespace std;

int main()
{
? ?double a,b;
? ?char op;
? ?cout << "enter a" << endl;
? ?cin >> a;
? ?cout << "enter operator" << endl;
? ?cin >> op;
? ?cout << "enter b" << endl;
? ?cin >> b;
? ?double result;
? ?if (op=='+')
? ?{
? ? ? result = a + b;
? ?}
? ?else if (op=='-')
? ?{
? ? ? result = a-b;
? ?}
? ?else if (op=='*')
? ?{
? ? ? result = a*b;
? ?}
? ?else if (op=='/'){
? ? ? if (b==0){
? ? ? ? ?cout << "cannot devided by 0\n";
? ? ? ? ?return 0;

? ? ? }
? ? ? else{
? ? ? ? ?result =a/b;
? ? ? }
? ?}
? ?else
? ?{
? ? ? cout << "invalid operator\n";
? ? ? return 0；
? ?}

? ?cout << "result is " << result << endl;
? ?return 0;
}