给一个包含小数的中缀表达式 求出它的值
首先转换为后缀表达式然后利用stack求出值
转换规则：
如果字符为'(' push
else if 字符为 ')'
出栈运算符直到遇到‘('
else if 字符为‘+’，’-‘，’*‘，’/‘
{
if 栈为空或者上一个运算符的优先级小于当前运算符
push
else
{
运算符优先级小于等于栈顶运算符的优先级，出栈
然后！将当前运算符入栈！
}
}
代码
#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<sstream>
#include<algorithm>
#include<queue>
#include<deque>
#include<iomanip>
#include<vector>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<fstream>
#include<memory>
#include<list>
#include<string>
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
#define MAXN 1100
#define L 31
#define INF 1000000009
#define eps 0.00000001
/*
1.000+2/4=
((1+2)*5+1)/4=
首先把中缀表达式转换为后缀表达式！（注意点运算符求值）
转换后的结果用一个string vector来表示
然后从前到后求值，pop两个数字 计算结果然后插入到stack中
*/
string str;
vector<string> trans;
stack<char> S;
stack<float> cal;
map<char, int> pri;
void Read()
{
string tmp;
trans.clear();
while (!S.empty())
S.pop();
while (!cal.empty())
cal.pop();
for (int i = ; i < str.size() - ; i++)// 特殊考虑( ) .
{
if (str[i] == '(')
{
if (!tmp.empty())
{
trans.push_back(tmp);
tmp.clear();
}
S.push(str[i]);
}
else if (str[i] == ')')
{
if (!tmp.empty())
{
trans.push_back(tmp);
tmp.clear();
}
while (!S.empty() && S.top() != '(')
{
string ttt = "";
ttt.push_back(S.top());
trans.push_back(ttt);
S.pop();
}
if (!S.empty() && S.top() == '(')
S.pop();
}
else if (str[i] == '+' || str[i] == '-' || str[i] == '*' || str[i] == '/')
{
if (!tmp.empty())
{
trans.push_back(tmp);
tmp.clear();
}
if (S.empty() || pri[S.top()]<pri[str[i]])
{
S.push(str[i]);
continue;
}
else
{
while (!S.empty() && pri[S.top()] >= pri[str[i]])
{
string ttt = "";
ttt.push_back(S.top());
trans.push_back(ttt);
S.pop();
}
S.push(str[i]);
}
}
else
{
tmp.push_back(str[i]);
}
}
if (!tmp.empty())
{
trans.push_back(tmp);
tmp.clear();
}
while (!S.empty())
{
string ttt = "";
ttt.push_back(S.top());
trans.push_back(ttt);
S.pop();
}
}
float solve()//计算转化出的后缀表达式的值
{
while (!cal.empty())
cal.pop();
for (int i = ; i < trans.size(); i++)
{
if (trans[i] == "+" || trans[i] == "-" || trans[i] == "*" || trans[i] == "/")
{
float a, b;
a = cal.top();
cal.pop();
b = cal.top();
cal.pop();
if (trans[i] == "+")
cal.push(a + b);
else if (trans[i] == "-")
cal.push(b - a);
else if (trans[i] == "*")
cal.push(a * b);
else
cal.push(b / a);
}
else
{
cal.push(atof(trans[i].c_str()));
}
}
return cal.top();
}
int main()
{
int n;
cin >> n;
pri['+'] = pri['-'] = , pri['*'] = pri['/'] = , pri['('] = pri[')'] = -;
while (n--)
{
cin >> str;
Read();
printf("%.2f\n",solve());
}
return ;
}