计算前缀表达式的值

/*
*  参考(https://blog.csdn.net/qq_42325947/java/article/details/104219251)
*/
#include <iostream>
#include <stack>
#include <string>
using namespace std;
stack <double> st;
int main()
{
    
    
	string s;
	getline(cin, s);
	for (int i = s.size() - 1; i >= 0; i--)
	{
    
    
		if (isdigit(s[i]))
		{
    
    
			double mul = 10, num = s[i] - '0';
			for (i--; i >= 0; i--)
			{
    
    
				if (isdigit(s[i]))
				{
    
    
					num += (s[i] - '0') * mul;
					mul *= 10;
				}
				else if (s[i] == '.')
				{
    
    
					num /= mul;
					mul = 1;
				}
				else if (s[i] == '-')
					num = -num;
				else
					break;
			}
			st.push(num);
		}
		else if (s[i] != ' ')   //else
		{
    
    
			double a, b, sum;
			a = st.top();
			st.pop();
			b = st.top();
			st.pop();
			switch (s[i])
			{
    
    
			case '+':
				sum = a + b;
				break;
			case '-':
				sum = a - b;
				break;
			case '*':
				sum = a * b;
				break;
			case '/':
			{
    
    
				if (b == 0)
				{
    
    
					cout << "ERROR";
					return 0;
				}
				sum = a / b;
			}
			}
			st.push(sum);
		}
	}
	printf("%.1lf", st.top());
}

其中用到了getline 函数以及isdigit 函数。 整个思路是从后向前遍历整个字符串，遇到符号直接运算即可，不用cin 是因为cin遇到空格会结束。
参考
https://blog.csdn.net/qq_42325947/java/article/details/104219251