C++ Mathematical Expression Parser Issue

Question

#include <iostream>
#include <string>
#include <queue>
#include <stack>
#include "NodeType.h"
using namespace std;

// Test if token is an operator
bool isOperator(char token);

int getPrecedence(char token);

bool comparePrecedence(char tokenA, char tokenB);


int main()
{

    stack<char> tokenStack;
    queue<char> tokenQueue;
    string expression= "", postfix= "";
    char x;
    cout<<"Please enter a mathematical expression: "<<endl;
    getline(cin, expression);
    cout<<expression.length()<<endl;
    for(int i = 0; i <= expression.length(); i++)
    {
        x = expression[i];
        if(isdigit(x))
        {
            tokenQueue.push(x);
        }
        if(isOperator(x))
        {
            while((!tokenStack.empty()) && (comparePrecedence(x, tokenStack.top() == true)))
            {
               char z = tokenStack.top();
               tokenQueue.push(z);
               tokenStack.pop();
            }
            tokenStack.push(x);
        }
        if(x == '(')
        {
            tokenStack.push(x);
        }
        if(x == ')')
        {
            while((!tokenStack.empty()) && (tokenStack.top() != '('))
            {
                char z = tokenStack.top();
                tokenQueue.push(z);
                tokenStack.pop();
            }
            tokenStack.pop();
        }
        while(!tokenStack.empty())
        {
            char z = tokenStack.top();
            tokenQueue.push(z);
            tokenStack.pop();
        }
    }

    return 0;
}

int getPrecedence(char token)
{
    if((token == '+') || (token == '-'))
    {
        return 1;
    }
    else if((token == '*') || (token == '/'))
    {
        return 2;
    }
    else if ((token == '(') || (token == ')'))
        return 0;
    else
        return 99;
}

// Test if token is an operator
bool isOperator(char token)
{
    return token == '+' || token == '-' ||
    token == '*' || token == '/';
}

bool comparePrecedence(char tokenA, char tokenB)
{
    if(getPrecedence(tokenA) < getPrecedence(tokenB))
        return true;
    else
        return false;
}

For some reason I cannot get my code to work correctly. It always throws a Thread 1: EXC_BAD_ACCESS (code=EXC_1386_GPFLT) error. It is also not correctly placing the '+' sign when I test using a simple string such as: (3+4). The Queue should look like: 34+ but it hold 3+4. It seems to me that the '+' operator never gets pushed onto the stack. Can anyone please help me find what I should be focussing my attention on?

Answer 1

Debugging code is a valuable skill to learn, it's my opinion that it should form a much more important part of curricula in schools.

For example, if you modify your code to output all the stack and queue operations thus:

int main()
{

    stack<char> tokenStack;
    queue<char> tokenQueue;
    string expression= "", postfix= "";
    char x;
    cout<<"Please enter a mathematical expression: "<<endl;
    getline(cin, expression);
    cout<<expression.length()<<endl;
    for(int i = 0; i <= expression.length(); i++)
    {
        x = expression[i];
        if(isdigit(x))
        {
            tokenQueue.push(x);
            cout << "qpush A " << x << '\n';
        }
        if(isOperator(x))
        {
            while((!tokenStack.empty()) && (comparePrecedence(x, tokenStack.top() == true)))
            {
               char z = tokenStack.top();
               tokenQueue.push(z);
               cout << "spop  G " << z << '\n';
               cout << "qpush B " << z << '\n';
               tokenStack.pop();
            }
            tokenStack.push(x);
            cout << "spush E " << x << '\n';
        }
        if(x == '(')
        {
            tokenStack.push(x);
            cout << "spush F " << x << '\n';
        }
        if(x == ')')
        {
            while((!tokenStack.empty()) && (tokenStack.top() != '('))
            {
                char z = tokenStack.top();
                tokenQueue.push(z);
                cout << "spop  H " << z << '\n';
                cout << "qpush C " << z << '\n';
                tokenStack.pop();
            }
            cout << "spop  I " << tokenStack.top() << '\n';
            tokenStack.pop();
        }
        while(!tokenStack.empty())
        {
            char z = tokenStack.top();
            tokenQueue.push(z);
            cout << "spop  J " << z << '\n';
            cout << "qpush D " << z << '\n';
            tokenStack.pop();
        }
    }

    return 0;
}

and run it with a simple 3+4 , you'll see the following output:

qpush A 3
spush E +
spop  J +
qpush D +
qpush A 4

So you are placing the operation on the stack. However, you later take it off the stack and put it on the queue before you place the next digit on the queue.

That's definitely the wrong order but, if you examine the code, it's not just a small snippet that has two lines in the wrong order (that would be too easy).

The code that's doing that transfer from stack to queue is the final while loop in main() which, after every single character, transfers all the items from the stack to the queue, effectively rendering your stack superfluous.

That's where you should be looking but I'll give you a clue. You don't want to be transferring stack to queue after every character, only for those that involve numbers.

There may well be other problems after you solve that one but that method (debugging output every time you do something important) should be able to give you enough information to fix whatever comes along.