//简易计算器
#include <iostream>
#include <cmath>
#include <string>
int main()
{
	using namespace std;
	double num1, num2;
	char op;
	double result;
	string caption1("Error!Divided by 0!\n");
	string caption2("Invalid opereator!\n");
	while (1)
	{
		cout << "Please input the expression\n";
		cin >> num1 >> op >> num2;
		switch (op) //op不要加单引号！！！
		{
		case '+':
			result = num1 + num2;
			cout << num1 << op << num2 << "=" << result << endl; break;
		case '-':
			result = num1 - num2;
			cout << num1 << op << num2 << "=" << result << endl; break;
		case '*':
			result = num1 * num2;
			cout << num1 << op << num2 << "=" << result << endl; break;
		case '/':
			if (fabs(num2)<1.0e-8)
			{
				cout << caption1 << endl;
			}
			else
			{
				result = num1 / num2;
				cout << num1 << op << num2 << "=" << result << endl; break;
			}
		default: cout << caption2 << endl;
		}
	}
	return 0;
}

这个语句主要利用了开关语句来判断对应的关系，注意switch (op) op不要加单引号！！！