题解 | 简单计算器

#include<bits/stdc++.h>
using namespace std;

//判断字符是否为数字字符
bool isnum(char a)
{
    return a>='0'&&a<='9';
}
//判断是否为运算符
bool isyun(char a)
{
    if(a == '+'||a=='-'||a=='*'||a=='/')
    {
        return true;
    }
    return false;
}
//比较运算符的优先级
int  isbigger(char a,char b)
{
    if(a == '*'||a == '/')
    {
        if(b == '+'||b == '-')
        {
            return 1;
        }
        else {
            return 0;
        }
    }
    else {
        if(b == '*'||b == '/')
        {
            return -1;
        }
        else {
        return 0;
        }
    }
}

int main()
{
    string str;
    while(getline(cin,str))
    {
        if(str == "0")break;
        int len = str.length();
        stack<char> s;
        string output = "";
        int i = 0;
        while(i<len)
        {
            int sum = 0;
            if(isnum(str[i]))
            {
                while(str[i] != ' '&&i<len)
                {
                    sum = sum*10 + str[i]-'0';
                    i++;
                }
                output += to_string(sum) + " ";
            }
            else if(isyun(str[i]))
            {
                while(!s.empty()&&isbigger(str[i],s.top())<=0)
                {
                    output += s.top();
                    output += " ";
                    s.pop();
                }
                s.push(str[i]);
                i++;
            }
            else {
            i++;
            }
        }
        while(!s.empty())
        {
            output += s.top();
            output += " ";
            s.pop();
        }
    //对后缀表达式进行计算求值
    stack<float> st;
    int l = output.length();
    int j = 0;
    while(j<l)
    {
        float sum = 0.00;
        if(isnum(output[j]))
        {
            while(isnum(output[j])&&j<l)
            {
                sum = sum*10 + output[j]-'0';
                j++;
            }
            st.push(sum);
        }
        else if(isyun(output[j]))
        {
            float b = st.top();st.pop();
            float a = st.top();st.pop();
            if(output[j] == '+')st.push(a+b);
            if(output[j] == '-')st.push(a-b);
            if(output[j] == '*')st.push(a*b);
            if(output[j] == '/')
            {
                if(b == 0)
                {
                    cout<<"除零错误！"<<endl;
                    return -1;
                }
                st.push(a/b);
            }
            j++;
        }
        else {
        j++;
        }
    }
    printf("%.2f",st.top());
    st.pop();
    cout<<endl;
    }

    return 0;
}