Values in dynamic array cannot be assigned - c++

Question

I am using a stack to reverse a given string. Each character of the string is pushed onto the stack, then they are popped back into a new string which is returned to main. But for some reason, the value of stack[top] in the stack class code never seems to change.
#include <iostream>
#include <string>
using namespace std;
const int size = 4;
class Stack{
private:
//char stack[size];
char* stack;
int top;
public:
Stack()
{
top = -1;
}
void Init_Size(int size)
{
stack = new char[size];
}
bool isFull()
{
if (top == (size - 1))
return true;
return false;
}
bool isEmpty()
{
if (top == -1)
return true;
return false;
}
void push(char c)
{
if (isFull())
{
cout << "STACK IS FULL" << endl;
return;
}
top++;
stack[top] == c;
cout << "PUSHED" << stack[top] << " ONTO STACK" << endl;
}
char pop()
{
if (isEmpty())
{
cout << "STACK IS EMPTY" << endl;
return '/'; //error
}
char temp = stack[top];
cout << stack[top] << endl;
top--;
cout << "POPPED" << temp << " OFF STACK" << endl;
return temp;
}
};
string ReverseString(string str)
{
Stack stack;
string result;
stack.Init_Size(str.length());
for (int i=0; i < str.length(); i++)
{
stack.push(str[i]);
}
for (int i=0; i < str.length(); i++)
{
result[i] = stack.pop();
}
return result;
}
int main()
{
string str;
cin >> str;
cout << ReverseString(str);
}
I tried using a normal array for the stack instead of a dynamic one, but no change, however, I did notice that the value of the stack array can be changed but only in the void Init_Size(int size) function and I'm not sure why.

Answer 1

I see a number of errors in your program:
size: You have a global constant size set to 4 which is used in your isFull function, independent of the local size parameter used in Init_Size. Make this a class variable instead.
Your push function has the line stack[top] == c with two = instead of one - thus comparing the value against stack instead of setting stack. Turn on warnings and you should have found this yourself
ReverseString assigns directly to result[i] without ever setting the size of result. You could have used the str variable instead, since that is just a copy.
Those three are the major errors that need fixing for your program to work, but there are several minor issues as well, like using namespace std; thereby making size ambigious, making unnecessary copies of strings, using new and c-style arrays etc.

Answer 2

You have a problem with creation and deletion of your arrays here in your program as well as a few small misshaps on the way.
Fix:
#include <iostream>
#include <string>
using namespace std;
class Stack{
private:
//char stack[size];
int size;
char* stack;
public:
Stack() : size(0), stack(NULL) {}
~Stack() {if (stack != NULL) delete stack;} //THIS IS NEEDED HERE
void Init_Size(int newSize)
{
if (stack != NULL) delete stack;
size = newSize;
stack = new char[newSize]; //CREATES DYNAMIC ARRAY WHICH YOU NEED TO REMEMBER TO REMOVE OTHERWISE MEMORY LEAK
}
void push(char c) //adds
{
//adding here should work in a way:
//a) create new dynamic array with different size
//b) copy all elements into new array
//c) delete array
if (stack == NULL) Init_Size(1);
char* newArray = new char[size+1];
for(int i = 0; i < size; i++)
{
newArray[i] = stack[i];
}
newArray[size] = c;
delete stack;
stack = newArray;
size++;
cout << "PUSHED " << stack[size-1] << " ONTO STACK" << endl;
}
char pop()
{
//removing here should work in a way:
//a) create new dynamic array with different size
//b) copy all elements into new array
//c) delete array
if (stack == NULL)
{
cout << "Stack empty" << endl;
return '\0';
}
char* newArray = new char[size-1];
for(int i = 0; i < size-1; i++)
{
newArray[i] = stack[i];
}
char temp = stack[size-1];
delete stack;
stack = newArray;
size--;
cout << "POPPED " << temp << " OFF STACK" << endl;
return temp;
}
};
string ReverseString(string str)
{
Stack stack;
string result;
stack.Init_Size(str.length());
for (int i=0; i < str.length(); i++)
{
stack.push(str[i]);
}
for (int i=0; i < str.length(); i++)
{
result[i] = stack.pop();
}
return result;
}
int main()
{
string str;
cin >> str;
cout << ReverseString(str);
}