Infix to Postfix Using Stacks C++: Error Code 6

Question

My assignment is to implement stacks using singly linked lists to convert a string which is in infix form to postfix form. For simplicity, this string does not contain any spaces.

My algorithm in a nutshell is:

1. read character from infix string

2. create a temp node with the character and its associated precedence in the order of operations

3. push it onto the stack if it is an operation and not a number/if it is a number, automatically append it to the postfix string

4. every time a character is pushed onto the stack, if the top node of the stack has a higher precedence than the temp node of the next character, pop it from the stack and append it to the postfix string.

These steps work when doing an infix to postfix by hand. Whenever I try to run my code, I keep getting an error 6 SIGABRT. My code should be easy to understand. Can anyone tell me what this error means, why I am getting it, and how to fix it so that my code outputs the postfix string properly?

#include <iostream>
#include <string>

using namespace std;
string postfix; //infix and postfix strings

//function to return true if a character is an operation
bool isoperator(char a)
{
    if(a == '(' ||  ')' || '*' || '/' || '+' || '-') //might need to 
change to "" instead of ''
    {
        return(true);
    }
    else
    {
        return(false);
    }
}

//node class
class node
{
public:
    char character;
    //double number;
    int level; //to check for precedence of operations
    node *ptr;
    void assignlevel()
    {
        switch(character)
        {
            case ')':
                level = 3;
                break;
            case '(':
                level = 0;
                break;
            case '+':
                level = 1;
                break;
            case '-':
                level = 1;
                break;
            case '*':
                level = 2;
                break;
            case '/':
                level = 2;
                break;
            default:
                level = 0;
        }
}
friend class stack;
};

//stack class
class stack
{
public:
    node *top, *temp;

//Constructor Function
stack()
{
    top = new node;
    top->character = '&';
    top->ptr = NULL;
    top->level = 0;
    temp = new node;
    temp->ptr = NULL;
}

//Empty
bool empty()
{
    return(top->character == '&');
}

//Read character from string
void readchar(char a)
{
    temp->character = a;
    temp->assignlevel();
}

//Check Precedence of top and temp
bool precedence()
{
    return(top->level >= temp->level);
}

//Push function for infix to postfix
void push1(char a)
{
    readchar(a);
    if(isoperator(temp->character)) //Push onto stack if character is an operation
    {
        if(empty())
        {
            top->character = temp->character;
            top->assignlevel();
        }
        else
        {
            node *v = new node;
            v->character = temp->character;
            v->level = temp->level;
            v->ptr = top;
            top = v;
            delete v;
        }
    }
    else //append to string if character is number
    {
        postfix += temp->character;
    }

    if(precedence()) //we check if we have to pop every time we push 
onto the stack
        {
            pop1();
        }
    }

    void pop1() //Pop onto postfix string
    {
        postfix += top->character;
        node *w = top->ptr;
        delete &top;
        top = w;
        delete w;
    }
};

int main()
{
    string infix = "2+3-5*(7+1)";
    stack op;
    for(int i = 0; i < infix.size(); ++i)
    {
        op.push1(infix[i]);
    }


    for(int j = 0; j < infix.size(); j++)
    {
        cout << postfix[j];
    }
    return 0;
}

Answer 1

Why do you do "delete v" in push? This deletes the node you have just created.