75 lines
2.4 KiB
C++
75 lines
2.4 KiB
C++
#include <algorithm> // remove()
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#include <cmath> // pow()
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#include <iostream>
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#include <stack> // stack<T>
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#include <stdexcept> // runtime_error
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#include <string> // string
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using std::string;
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// 返回运算符的优先级，值越大，优先级越高
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inline size_t precedence(const char op) {
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if (op == '+' || op == '-')
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return 1;
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if (op == '*' || op == '/')
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return 2;
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if (op == '^')
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return 3;
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throw std::runtime_error{string{"表达中包含无效的运算符"} + op};
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}
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// 计算
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double execute(std::stack<char> &ops, std::stack<double> &operands) {
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double result{};
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double rhs{operands.top()}; // 得到右操作数
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operands.pop();
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double lhs{operands.top()}; // 得到做操作数
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operands.pop();
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switch (ops.top()) // 根据两个操作数之间的运算符，执行相应计算
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{
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case '+':
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result = lhs + rhs;
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break;
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case '-':
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result = lhs - rhs;
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break;
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case '*':
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result = lhs * rhs;
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break;
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case '/':
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result = lhs / rhs;
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break;
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case '^':
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result = std::pow(lhs, rhs);
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break;
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default:
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throw std::runtime_error{string{"invalid operator: "} + ops.top()};
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}
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ops.pop(); //计算完成后，该运算符要弹栈
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operands.push(result); //将新计算出来的结果入栈
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return result;
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}
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double cal(string exp) {
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static std::stack<double> operands; //存储表达式中的运算符
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static std::stack<char> operators; //存储表达式中的数值
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//移除用户输入表达式中包含的无用的空格
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size_t index{};
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//每个表达式必须以数字开头,index表示该数字的位数
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operands.push(std::stod(exp, &index)); // 将表达式中第一个数字进栈
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while (true) {
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operators.push(exp[index++]); // 将运算符进栈
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size_t i{};
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operands.push(
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std::stod(exp.substr(index),
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&i)); //将运算符后的数字也进栈,并将数字的位数赋值给 i。
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index += i; //更新 index
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if (index == exp.length()) {
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while (!operators.empty()) //如果 operators不为空，表示还没有计算完
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execute(operators, operands);
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break;
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}
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//如果表达式还未遍历完，但子表达式中的运算符优先级比其后面的运算符优先级大，就先计算当前的子表达式的值
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while (!operators.empty() &&
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precedence(exp[index]) <= precedence(operators.top()))
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execute(operators, operands);
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}
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return operands.top();
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}
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