Evaluation Of postfix Expression using stack【1月21日学习笔记】

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//Evaluation Of postfix Expression using stack
#include<iostream>
#include<stack>//stack from standard template library（STL）
#include<string>
using namespace std;

int EvaluatePostfix(string exp);

bool IsOperator(char c);

int PerformOperation(char operation, int  op1, int op2);

bool IsNumericDigit(char c);

int main() {
	string expression;
	getline(cin, expression);//与cin>>不同，可以读取空格，直到整行换行符结束
	int result = EvaluatePostfix(expression);
	cout << "Output = " << result << "\n";
}


int EvaluatePostfix(string exp) {
	stack<int> S;//存放操作数及运算结果

	for (int i = 0; i < exp.length(); i++) {
		if (exp[i] == ' '||exp[i]==',')  continue;//设置操作数之间分隔符
		else if (IsOperator(exp[i])) {//拿出最后两个操作数进行运算
			int op2 = S.top();//类型转换
			S.pop();
			int op1 = S.top();
			S.pop();
			int result = PerformOperation(exp[i], op1, op2);
			S.push(result);//运算结果放回
		}
		else if (IsNumericDigit(exp[i])) {//一位一位的读取操作数
			int operand = 0;//初始化为0
			while (i < exp.length() && IsNumericDigit(exp[i])) {
				operand = operand * 10 + (exp[i] - '0');//右移进位
				                                        //使用exp[i]-'0'将ASCII值转化成目标整数
				i++;
			}
			i--;//退回到已读最后一位
			S.push(operand);//放入完整操作数
		}
	}
	return S.top();//最终结果 
}

bool IsOperator(char c) {
	if (c == '+' || c == '-' || c == '*' || c == '/')
		return true;
	return false;
}

int PerformOperation(char operation, int  op1, int op2) {
	if (operation == '+')  return op1 + op2;
	else if (operation == '-')  return op1 - op2;
	else if (operation == '*')  return op1 * op2;
	else if (operation == '/')  return op1 / op2;

	else cout << "Unexpected Error \n";
	return -1;
}

bool IsNumericDigit(char c) {
	if (c >= '0' && c <= '9')  return true;
	return false;
}