Computation Using A Stack [Update]

[DoABarrelRoll94]

I changed up a bit of my code from the last post I made of this assignment. This time, the code actually runs now but when I put in an expression e.g., "3+4-6", I keep getting either 51 or 52.

#include <iostream>
#include <cctype>       //isdigit(),
#include <string>       //size(), string, find_first_of
using namespace std;

class Node                                  //CLASS Node
{
private:
    int val;                                //variable for values
    char op;                                //variable for operators
    Node* next;                             //pointer which points to next node
    friend class LinkedStack;               //let class LinkedStack use variables of class Node
};

class LinkedStack                           //CLASS linkedstack
{
public:
    LinkedStack();                          //constructor
    void pushVal(const int&);               //push value
    void pushOp(const char&);               //push operator
    int popVal();                           //pop value
    char popOp();                           //pop operator
    const char& topOp();                    //returns reference of top of opStack
    const int& topVal();                    //returns reference of top of valStack
    bool isValue(const char&);              //checks if there is a value in string
    bool isOp(const char&);                 //checks for operator in string and precedence
    bool empty() const;                     //is stack empty?
private:
    Node* top;                              //points to top of stack
    Node* curr;                             //current pointer points to current node
};

//LINKEDSTACK FUNCTIONS//

LinkedStack::LinkedStack()                  //constructor
: top(NULL){};                              //init top to NULL

void LinkedStack::pushVal(const int& e)     //push value onto valStack
{
    Node* temp = new Node;                  //create new node
    temp->val = e;                          //assign value
    temp->next = top;                       //new node's next pointer points to where top ptr points to
    
    top = temp;                             //top ptr points to new node
}

void LinkedStack::pushOp(const char& e)     //push operator onto opStack
{
    Node* temp = new Node;                  //create new node
    temp->op = e;                           //assign operator
    temp->next = top;                       //new node's next ptr points to where top ptr points to
    
    top = temp;                             //top ptr points to new node
}

int LinkedStack::popVal()                   //pop value from valStack
{
    Node* old = top;                        //old ptr points to the same node as top ptr(the top)
    top = old->next;                        //top points to the same node as old's next ptr
    int e = old->val;                       //assign e with old's value
    delete old;                             //delete old ptr
    return e;                               //return value e
}

char LinkedStack::popOp()                   //pop value from opStack
{
    Node* old = top;                        //old ptr points to the same node as top ptr(the top)
    top = old->next;                        //top points to the same node as old's next ptr
    char e = old->op;                       //assign e with old's operator
    delete old;                             //delete old ptr
    return e;                               //return operator e
}

const char& LinkedStack::topOp()                 //return reference of top operator of opStack
{
    return top->op;                              //return top operator of stack
}

const int& LinkedStack::topVal()                 //return reference of top value of valStack
{
    return top->val;                             //return top value of stack
}

bool LinkedStack::isValue(const char& e)         //checks if there is a value in the string
{
    if(isdigit(e))
    {
        return true;
    }
    else
    {
        return false;
    }
}

bool LinkedStack::isOp(const char& e)     //checks for operator precedence
{
    if((e == '*' || e == '/') && (topOp() == '*' || topOp() == '/'))        //(from string) == (from top of stack)
    {
        return true;
    }
    else if((e == '+' || e == '-') && (topOp() == '+' || topOp() == '-'))   //(from string) == (from top of stack)
    {
        return true;
    }
    else if((e == '*' || e == '/') && (topOp() == '+' || topOp() == '-'))   //(from string) > (from top of stack)
    {
        return false;
    }
    else if((e == '+' || e == '-') && (topOp() == '*' || topOp() == '/'))   //(from string) < (from top of stack)
    {
        return true;
    }
    else
    {
        return false;
    }
}

bool LinkedStack::empty() const         //return true if stack is empty, else false
{
    return top == NULL;
}


//MAIN//

int main()
{
    LinkedStack valStack;
    LinkedStack opStack;
    
    string exp;
    cout << "Enter a mathematical expression involving numbers and operators(+,-,*,/):\n";
    cin >> exp;
    
    opStack.pushOp(exp.find_first_of("+-*/"));
    
    for(int i = 0; i < exp.size(); i++)
    {
        if(valStack.isValue(exp[i]) == true)        //checks if there is a value
        {
            valStack.pushVal(exp[i]);
        }
        else if(opStack.isOp(exp[i]) == true)       //checks if there is an operator and checks precedence
        {
            int x = valStack.popVal();
            int y = valStack.popVal();
            char op = opStack.popOp();
            switch(op)
            {
                case '*': valStack.pushVal(y * x);
                    break;
                case '/': valStack.pushVal(y / x);
                    break;
                case '+': valStack.pushVal(y + x);
                    break;
                case '-': valStack.pushVal(y - x);
                    break;
                default: cout << "No operators found.\n";
            }
        }
            else
            {
                opStack.pushOp(exp[i]);
            }
     }
    
    
    cout << valStack.topVal();
    
    return 0;
}

[ne555]

> when I put in an expression e.g., "3+4-6", I keep getting either 51 or 52.
let me see if I understand, you write 3+4-6 as input, and sometimes you've got 51 as output, but other times you've got 52, when it is obvious that you should get 54.

opStack.pushOp(exp.find_first_of("+-*/")); ¿what are you trying to do here?

valStack.pushVal(exp[i]);, you are not pushing 3, but '3', whose ASCII code is 51.

            int x = valStack.popVal();
            int y = valStack.popVal();
            char op = opStack.popOp();

that looks like the algorithm to evaluate a postfix expresion.


> I changed up a bit of my code from the last post I made of this assignment
please don't create new threads, just reply to the old one. That way we can have the history, like knowing what the assignment is about.

[DoABarrelRoll94]

My apologies. I'll make sure to reply to an old thread next time.

For opStack.pushOp(exp.find_first_of("+-*/"));, I'm looking for the 1st occurrence of an arithmetic operator to push onto the operator stack. Then I compare the rest of the operators in the string with that operator which is on the top of the stack.

For

int x = valStack.popVal();
            int y = valStack.popVal();
            char op = opStack.popOp();

, yes, I am converting from infix to postfix, computing the two popped values with the popper operator.

valStack.pushVal(exp[i]);, You're right. I totally forgot to convert from character to integer. Is it the int data type parameter in void pushVal that converts '3' to 51? How would I convert the numbers in the string to integer data type then push onto stack?