Question

so I’m currently trying to migrate my Java experience to C++ by implementing various Data Structures for the sake of having them implemented at least once.

Would you mind giving me some advise? The problem I am having is mainly concentrated around the pointers in push(int value) and especially pop(). As push seems to be working correctly I found myself struggling to get the correct value back when pop’ing things. What’s the matter?

PS: I also think, that since I allocate my array space manually I’d need to delete it aswell. How do I do that?

#ifndef STACK_H
#define STACK_H

class Stack
{
private:
    int *stackArray;
    int elementsInArray;
    int allocatedArraySize;
    int alpha;
    int beta;

public:
    Stack();
    void push(int aValue);
    int pop();
    bool isEmpty();
    int size() const;
};

#endif

and the implementation:

#include <iostream>
#include "Stack.h"

Stack::Stack() 
{
    alpha = 4;
    beta = 2;
    elementsInArray = 0;
    allocatedArraySize = 1;
    stackArray = new int[1];
}

void Stack::push(int aValue)
{
    if (elementsInArray == allocatedArraySize) 
    {
        int temporaryArray[allocatedArraySize*beta];

        for (int i = 0; i < elementsInArray; i++)
            temporaryArray[i] = stackArray[i];

        stackArray = temporaryArray;
        allocatedArraySize *= beta;
    }

    elementsInArray++;
    stackArray[elementsInArray] = aValue;
}

int Stack::pop()
{
    int result = -INT_MAX;

    if (elementsInArray == 0)
        return result;

    if (elementsInArray > 0) 
    {
        result = stackArray[elementsInArray-1];
        elementsInArray--;

        if (elementsInArray <= allocatedArraySize/alpha) 
        {
            int temporaryArray[allocatedArraySize/alpha];

            for (int i = 0; i < elementsInArray; i++) 
                temporaryArray[i] = stackArray[i];

            stackArray = temporaryArray;
            allocatedArraySize /= beta;
        }
    }

    return result;
}

bool Stack::isEmpty()
{
    if (elementsInArray == 0) 
        return true;

    return false;
}

int Stack::size() const
{
    return allocatedArraySize;
}

Answer 1

For starters, you should be post incrementing the index on the array, so change:

elementsInArray++;
stackArray[elementsInArray] = aValue;

to:

stackArray[elementsInArray++] = aValue;

or:

stackArray[elementsInArray] = aValue;
elementsInArray++;

Second, when you create the new temp array you are doing it inside the if statement… therefore it is a local variable and placed on the system stack and lost after you exit the if statement. So change

int temporaryArray[allocatedArraySize*beta];

to:

int *temporaryArray = new int[allocatedArraySize*beta];

Third, add in the delete you were talking about by saving the original pointer from stackArray before copying the location of tempArray and then perform the delete after you’ve made the pointer copy.

Finally, you’ll have to make similar changes to your pop function…