[Failed Practice]Suffix Expression

#include <iostream>
#include <stack>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;

bool isDigit(char c)
{
	return (c <= '9' && c >= '0');
}

const char allOperator[] = "+-*/^^()";

bool isOperator(char c)
{
	for (int j = 0; j < strlen(allOperator); ++j)
	{
		if (c == allOperator[j])
			return 1;
	}
	return 0;
}

//expression verification
bool isLegal(const char* str)
{
	int l = strlen(str);
	for (int i = 0; i < l; ++i)
	{
		if ( !( isDigit(str[i]) || isOperator(str[i]) || str[i] == '.' ) )
			return 0;
	}
	return 1;
}

int ofPosition(char c)
{
	for (int i = 0; i < strlen(allOperator); ++i)
	{
		if (c == allOperator[i])
			return i;
	}
	return -1;
}

//extract into double
double extractDigit(const char* s , int& len)
{
	double value = 0;
	int i = 0;
	int point = 0;
	while(isDigit(s[i]) || s[i] == '.')
	{
		++len;
		if ( s[i] != '.' )
		{
			value = s[i] - '0' + value * 10;
		}
		else 
		{
			++point;
		}
	}
	for (int k = 0; k < point; ++k)
	{
		value *= 0.1;
	}
	return value;
}

//return 1 if a > b, need pop
bool priority(char a, char b)
{
	if (a_pos == b_pos)
		return 0;
	if (b == '(')	return 0;
	int a_pos = ofPosition(a);
	int b_pos = ofPosition(b);
	if (a_pos > b_pos && abs(b_pos - a_pos) != 1 )
		return 1;
	return 0;
}

double compute(double a, double b, char v)
{
	switch(v)
	{
	case '+':
		return a + b;
	case '-':
		return a - b;
	case '*':
		return a * b;
	case '/':
		return a / b;
	case '^':
		return pow(a, b);
	}
}

int main(int argc, char const *argv[])
{
	input:
	string expression;
	cin >> expression;
	// verify legal or not
	if ( !( isLegal(expression.c_str() ) ) )
	{
		cout << "Format error. Please enter again.\n";
		goto input;
	}

	stack<double> Operand;
	stack<char> Operator;
	for (int i = 0; i < expression.length(); ++i)
	{
		//Digit process
		if (isDigit(expression[i]) )
		{
			int nextPos = 0;
			Operand.push( extractDigit(&expression[i], nextPos) );
			i += nextPos - 1;
		}
		//Operator verification
		else if (isOperator(expression[i]) )
		{
			if (Operator.empty())
				Operator.push(expression[i]);
			else 
			{
				while ( priority(Operator.top(), expression[i]) )
				{
					double num1 = Operand.top();
					Operand.pop();
					double num2 = Operand.top();
					Operand.pop();
					char sign = Operator.top();
					Operator.pop();
					Operand.push( compute(num1, num2, sign) );
				}
				Operator.push(expression[i]);
			}
		}
	}
	while (!Operator.empty())
	{
		double num1 = Operand.top();
		Operand.pop();
		double num2 = Operand.top();
		Operand.pop();
		char sign = Operator.top();
		Operator.pop();
		Operand.push( compute(num1, num2, sign) );
	}
	cout << Operand.top()<< endl;
	return 0;
}