用栈实现中缀表达式转后缀表达式并构建计算机求值

#include

#include

#include

#include

#include

using namespace std;

class calculator

{

public:

calculator() {

}

calculator(string expression) {

getexpression(expression);

}

//获得算术表达式

void getexpression(string expression) {

e = expression;

}

~calculator() {

}

//返回中缀表达式

string ShowMiddleExpression() {

return e;

}

//计算函数, 输入为两个数和一个操作符, 返回值为计算结果(字符串形式) string calculate(string num2, string num1, string op) {

double n2, n1;

string result;

stringstream x;

x << num2;

x >> n2;

x.clear();

x << num1;

x >> n1;

x.clear();

if (op == "+")

x << (n2 + n1);

if (op == "-")

x << (n2 - n1);

if (op == "*")

x << (n2 * n1);

if (op == "/")

x << (n2 / n1);

x >> result;

x.clear();

return result;

}

//判别符号的优先级

int Priority(char a) {

if (a == '(')

return 0;

else if (a == '+' || a == '-')

return 1;

else if (a == '*' || a == '/')

return 2;

return 0;

}

//用于去除输入中多余的空格输入为要除去空格的算术表达式返回去掉空格的算术表达式

string RidSpace(string origin) {

string transfer;

for (int i = 0; i < origin.length(); i++) {

if (origin[i] != ' ')

transfer += origin[i];

}

return transfer;

}

//中缀表达式转后缀表达式, 因为要兼容多位数, 括号, 负数和小数等功能,

//由于多位数在转为后缀表达式时会分不清, 故在每个数和运算符后面加上一个空格作为区别

//expression为输出的后缀表达式

string MidToLast(string str) {

str = RidSpace(str);

string expression = "";

string number = "";

char x;

stack op;

for (int i = 0; i < str.length(); ) {

//第一位做特殊处理判断是否为负号

if (i == 0 && str[i] == '-') {

number = "-";

i++;

}

x = str[i];

//录入数字

if ((x >= '0' && x <= '9') || number == "-" || x == '.') {

while ((str[i] >= '0' && str[i] <= '9') || str[i] == '.') {

number += str[i];

i++;

}

expression += number;

//加空格以区别

expression += " ";

number = "";

continue;

} else {

//判断是括号还是运算符

if (x == '(') {

op.push(x);

//判断是否为负号

if (str[i+1] == '-') {

number = "-";

i++;

}

} else {

//遇到右括号直接弹出运算符栈里的运算符到表达式中运算符后加空格以区别

if (x == ')') {

while (op.top() != '(') {

expression += op.top();

expression += " ";

op.pop();

}

//弹出右括号

op.pop();

} else {

//弹出栈中优先级较高的运算符运算符后加空格以区别

while (!op.empty() && (Priority(op.top()) >= Priority(x))) {

expression += op.top();

expression += " ";

op.pop();

}

//判断是否为负号

if (str[i+1] == '-') {

number = "-";

i++;

}

//将运算符压入栈

op.push(x);

}

}

}

i++;

}

while (!op.empty()) {

expression += op.top();

expression += " ";

op.pop();

}

return expression;

}

//获得后缀表达式

string GetLastExpression() {

return MidToLast(RidSpace(e));

}

string exe() {

//获取后缀表达式

string le = GetLastExpression();

string tem = "";

stack num;

for (int i = 0; i < le.length(); ) {

while (le[i] != ' ') {

tem += le[i];

i++;

}

//若为数字, 压栈

if (tem.length() > 1 || (tem.length() == 1 && tem[0] >= '0' && tem[0] <= '9')) { num.push(tem);

} else {

//若为运算符, 执行运算后压栈

//处理如-()的情况

if (num.empty()) {

tem = "-";

i++;

continue;

}

string tem1 = num.top();

num.pop();

if (!num.empty()) {

string tem2 = num.top();

num.pop();

num.push(calculate(tem2, tem1, tem));

} else {

tem1 = "-" + tem1;

num.push(tem1);

}

}

i++;

tem = "";

}

return num.top();

}

private:

string e;

};