任意输入含加减乘除乘方开方的文本数学表达式文本，自动计算结果

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#include<iostream>
#include<string>
using namespace std;
int main()
{
	string str;
	int num1, num2;
	char op;
	while (cin >> str)
	{
		for (int i = 0; i < str.length(); i++)
		if ((str[i] == '+') || (str[i] == '-') || (str[i] == '*') || (str[i] == '/'))
		{
			op = str[i];
			str[i] = ' ';
			break;
		}
		num1 = stoi(str);
		cin >> num2;
		switch (op)
		{
		case '+':cout << num1 + num2; break;
		case '-':cout << num1 - num2; break;
		case '*':cout << num1 * num2; break;
		case '/':cout << num1 /