gistfile1.cpp

#include<iostream>
#include<fstream>
#include <cstdlib>
using namespace std;

#define NUM 100
class Stackf
{
	private:
			int top;
			float array[NUM];
	public:
			Stackf(){
				top=0;	
			}

			void push(float item)
			{
				array[top]=item;
				top++;
			}
			int pop()
			{
				if(top==0)
				cout<<"Stack is empty"<<endl;
				else
				{
					float item;
					top--;
					item=array[top];
					return item;
				}
			}
			bool isEmpty(){
				return top==0? true:false;
			}
};

class Stackc
{
	private:
			int top;
			char array[NUM];
	public:
			Stackc(){
				top=0;	
			}
			void push(char item)
			{
				array[top]=item;
				top++;
			}
			void pop()
			{
				if(top!=0)
					top--;
			}
			char getTop(){
				if(top!=0)
					return array[top-1];
				return NULL;
			}
			bool isEmpty(){
				return top==0? true:false;
			}
};


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

bool isDigit(char input){
	if(input=='0' || input=='1' || input=='2' || input=='3' || input=='4' || input=='5' || input=='6' || input=='7' || input=='8' || input=='9')
		return true;
	return false;
}

int getPriority(char input){
	if(input=='*' || input=='/') return 4;
	else if (input=='+' || input=='-') return 3;
	else if (input=='(') return 2;
	else if (input==')') return 1;
}


int main()
{

	float sumf;
	float tmp;
	int point=0,divide=1;

	//Read the file into some buffer.
	ifstream fin("input1.txt");
	string input;
	fin >> input;
	//Do infix to prefix.
	//Create two stack, one for float number, one for operator.
	Stackf stackf ;
	Stackc stackc ;
	Stackc output;
	for(int i=input.size()-1;i>=0;i--)
	{
		//if it is a digit, we calculate
		if(isDigit(input[i]))
		{
			tmp = input[i]-'0';
			tmp*=divide;
			sumf+=tmp;
			divide*=10;
			output.push(input[i]);
        }
	   //if it is a '.', we rest the divide.
       else if(input[i]=='.')
       {
			sumf/=divide;
			divide=1;
			output.push(input[i]);
	   }
	   // if it is a operator, match the operator's priority
	   else if(isChar(input[i]))
	   {
			sumf=0;
			divide=1;

			//push to stack
			if(stackc.isEmpty()) //如果是空白的，就放進去
				stackc.push(input[i]);
			else //不是空白的話，只要遇到比我強的，就丟掉
			{	
				while(getPriority(stackc.getTop()) > getPriority(input[i])){
					output.push(stackc.getTop());
					stackc.pop();
				}
				stackc.push(input[i]);
	   		}
	   }
	   else if(input[i]=='('){ //一路丟掉，直到遇到)
			while(stackc.getTop()!=')'){
				output.push(stackc.getTop());
				stackc.pop();
			}
			stackc.pop();
	   }
	   else if(input[i]==')'){ //(直接放進去
	   	stackc.push(input[i]);
	   }
	}
	while(!stackc.isEmpty()){
		output.push(stackc.getTop());
		stackc.pop();
	}
	while(!output.isEmpty()){
		cout<<output.getTop();
		output.pop();
	}
	//Do infix to postfix.


}