Help with infix/postfix

[nysp10]

I'm writing a program that reads in a mathematical expression from a user ands stores the expression in a string, then it parses the expression into tokens and stores each token in a queue, then it converts the infix expression to postfix. Everything works except when I have a variable. If i try using a variable it gives me an "Expression: deque iterator not dereferencable" error. Also if I try using a power, for example, 2^2, I get "Expression: deque empty before pop" error.

Any help I can get would be appreciated. Thank you.

#include <iostream>
#include <string>
#include <cctype>
#include <queue>
#include <stack>
#include <cmath>
using namespace std;


bool isoperator(char symbol)
{
	if (symbol == '+' ||
		symbol == '-' ||
		symbol == '*' ||
		symbol == '/' ||
		symbol == '^')
		return true;
	
	else
		return false;
}

int priority(char ch)
{
	switch (ch)
	{
		case '(': return 0; 
			break;
		case '+':
		case '-': return 1; 
			break;
		case '*':
		case '/': return 2; 
			break;
		case '^': return 3; 
			break;
		default: return 0; 
			break;
	}
}

void processToInfix(string expression, queue<string>& infix)
{
	int lcount = 0, rcount = 0, i = 0, len = expression.length( ), letterIndex;
	string value;
	char ch;
	string token = "";
	string variable[26];

	for (int j = 0; j < 26; j++)
		variable[j] = "?";

	if(expression == "")
		infix.push("0");

	while(i < len)
	{
		ch = expression[i];
		token = "";

		if(isdigit(ch) || ch == '.')
		{
			while (i < len && (isdigit(expression[i]) || expression[i] == '.'))
			{
				token += ch;
				i++;
				if (i != len)
					ch = expression[i];
			}
				
			infix.push(token);
		}

		else if ((i < len) && (isalpha(ch)))
		{
			ch = toupper(ch);
			letterIndex = ch - 65;

			if (variable[letterIndex] == "?")
			{
				cout << "Enter the value of " << ch << ": ";
				cin >> value;
				infix.push(variable[letterIndex]);
			}
			else
				infix.push(variable[letterIndex]);
			
			i++;
		}

		else if ((i < len) && (isoperator(ch) || ch == '(' || ch == ')'))
		{
			token = ch;
			infix.push(token);
			i++;

			if (ch == '(')
				lcount++;
			else if (ch == ')')
				rcount++;
		}

		else if ((i < len) &&  (ch == ' '))
			i++;

		else
		{
			cout << "Error! Invalid symbol!" << endl;
			system("pause");
			exit(-1);
		}
	}

	if (isoperator(infix.back( )[0]) || isoperator(infix.front( )[0]))
	{
		cout << endl << "Error! Invalid expression!" << endl;
		system ("pause");
		exit(-1);
	}

	if (rcount != lcount)
	{
		cout << endl << "Error! Mismatch parenthesis!" << endl;
		system ("pause");
		exit (-1);
	}
}	


void convertToPostfix(queue<string> infix, queue<string>& postfix)
{
	string token;
	stack<string> operators;


	if (infix.empty( ))
		return;

	do
	{
		token = infix.front( );
		infix.pop( );

		if (token == "(")
			operators.push(token);
	
		else if (isdigit(token[0]))
			postfix.push(token);

		else if (token == ")")
		{
			do
			{
				postfix.push(operators.top( ));
				operators.pop( );
			} 
			
			while(operators.top( ) != "(");
				operators.pop( );
		}

		else if (isoperator(token[0]))
		{
			if (operators.empty( ))
				operators.push(token);

			else if (priority(operators.top( )[0]) < priority(token[0]))
				operators.push(token);
			
			else
			{
				do
				{
					postfix.push(operators.top( ));
					operators.pop( );
				} 
				
				while (!operators.empty( ) && (priority(operators.top( )[0]) > priority(token[0]) || priority(operators.top( )[0]) == priority(token[0])));
					operators.push(token);
			}
		}
	} 
	
	while(!infix.empty( ));

	
	while (!operators.empty( ))
	{
		if (operators.top( ) == "(")
		{
			cout << endl << "Error! Parenthesis mismatch!" << endl;
			exit (-1);
		}
		
		postfix.push(operators.top( ));
		operators.pop( );
	}
}


double evaluate(queue<string>& postfix)
{
	double val1 = 0.0, val2 = 0.0, answer;
	string token;
	stack <double> results;


	if (postfix.empty( ))
		return 0;

	do
	{
		token = postfix.front( );
		postfix.pop( );

		if (isdigit(token[0]))
		{
			results.push(atof(token.c_str( )));
		}

		else if (isoperator(token[0]))
		{
			switch(token[0])
			{
				case('+'):
				val2 = results.top( );
				results.pop( );
				val1 = results.top( );
				results.pop( );
				results.push(val1 + val2);
				break;

				case('-'):
				val2 = results.top( );
				results.pop( );
				val1 = results.top( );
				results.pop( );
				results.push(val1 - val2);
				break;

				case('*'):
				val2 = results.top( );
				results.pop( );
				val1 = results.top( );
				results.pop( );
				results.push(val1 * val2);
				break;

				case('/'):
				val2 = results.top( );
				results.pop( );
				val1 = results.top( );
				results.pop( );
				if (val2 == 0)
				{
					cout << endl << "Error! Division by zero!" << endl;
					exit(-1);
				}
				else
					results.push(val1 / val2);
				break;

				case('^'):
				val2 = results.top( );
				postfix.pop( );
				val1 = results.top( );
				postfix.pop( );
				results.push(pow(val1, val2));
				break;
			}
		}
	} 
	
	while (!postfix.empty( ));
		answer = results.top( );

	results.pop( );

	return answer;
}

int main( )
{
	string expression;
	queue <string> infix;
	queue <string> postfix;

	cout << "Please enter an expression: ";
	getline(cin, expression);

	processToInfix(expression, infix);

	convertToPostfix(infix, postfix);

	cout << endl << "The result is: " << evaluate(postfix) << endl << endl;

	return 0;
}

[norm b]

I get "Expression: deque empty before pop" error.

Lines 265 & 267 - aren't you removing from the wrong container?

[nysp10]

Wow I can't believe I did that. Thank you so much. Now the only problem left is with variables.

[norm b]

Line 83 - shouldn't you be pushing "value" onto infix here? I don't see where you use "value" anywhere.


edit: looks like you just need to assign value to variable[letterIndex] before the push onto infix.