Infinite loop when calling the stack print function

Question

This program should takes a postfix arithmetic expression then compiles the values of that expression.. Each time an integer is read, its gonna get pushed into the stack.. Otherwise, Two integers would be popped if +,-,* is read.

class Stack {
    Node *head;

public:
    Stack() {
        head = NULL;
    };

    void push(int data);
    int pop();
    bool isEmpty();
    void print();
};

void Stack::push(int data)
{
    Node * temp = new Node(data);
    temp->next = head;
    head = temp;
    delete temp;
}

int Stack::pop()
{
    int x = head->data;
    head = head->next;
    return x;
}

bool Stack::isEmpty(){
    return head == NULL;
}

void Stack::print(){
    Node * temp = head;
    while (temp != NULL){
        cout << temp->data << " ";
        temp = temp->next;
    }
    delete temp;
}


int main() {

    Stack st;
    char exp [] = "23+", c;
    int i, a;

    for (i = 0; exp[i] != '\0'; i++){
        c = exp[i];

        if (c == '+'&&!st.isEmpty()){
            a = st.pop() + st.pop();
            st.push(a);
        }
        else if (c == '-'&&!st.isEmpty()){
            a = st.pop() - st.pop();
            st.push(a);
        }
        else if (c == '/'&&!st.isEmpty()){
            a = st.pop() / st.pop();
            st.push(a);
        }
        else if (c == '*'&&!st.isEmpty()){
            a = st.pop() * st.pop();
            st.push(a);
        }
        else if (c == '0')
            st.push(0);
        else if (c == '1')
            st.push(1);
        else if (c == '2')
            st.push(2);
        else if (c == '3')
            st.push(3);
        else if (c == '4')
            st.push(4);
        else if (c == '5')
            st.push(5);
        else if (c == '6')
            st.push(6);
        else if (c == '7')
            st.push(7);
        else if (c == '8')
            st.push(8);
        else if (c == '9')
            st.push(9);

        cout << c << endl;
        st.print();
    }
    cin >> a;
    return 0;    
}

When I call the print function in main, I get an infinite loop as an output.. I tried looking for the thing that's causing an infinite loop but I couldn't find it.

Answer 1

Problems I see:

1. Using delete in push():

   void Stack::push(int data)
   {
       Node * temp = new Node(data);
       temp->next = head;
       head = temp;
       delete temp;  // This needs to go.
   }
   

2. Not using delete in pop():

   int Stack::pop()
   {
       // Problem 1.
       // What if head is NULL?
   
       int x = head->data;
   
       // Problem 2
       // The old value of head is gone. It's a memory leak.
       head = head->next; 
       return x;
   }
   

   You need:

   int Stack::pop()
   {
      if ( head != NULL )
      {
         int x = head->data;
         Node * temp = head;
         head = head->next;
         delete temp; 
         return x;
      }
      else
      {
         // Figure out what you want to do if head is NULL
      }
   }
   

3. Using delete in print().

   void Stack::print(){
       Node * temp = head;
       while (temp != NULL){
           cout << temp->data << " ";
           temp = temp->next;
       }
       delete temp; // This needs to go.
   }
   

4. Missing user defined destructor. You need to delete the objects in the object. Otherwise, you are leaking memory. Something along the lines of the code below should work.

   Stack::~Stack()
   {
      while (head)
      {
         Node * temp = head;
         head = head->next;
         delete temp;
      }
   }
   

Answer 2

Here is suggestion for push and pop.

try to understand the logic.

void Stack::push(int data) 
{
    Node * temp = new Node(data);
    temp->next=head;
    head=temp;
    //Do not delete temp; deleting temp will delete the new added Node
}

int Stack::pop() 
{ 
    Node* temp = Head;
    int x=head->data;
    head=head->next;
    delete temp; //here you free the released memory.
    return x;
}

Also, instead all the if/else, for each digit in your code, you can do as follows:

else if(c>=0 && c<=9){
     st.push(c-'0');
}