Push/pop stack class program in C++? - c++

Question

I've written and modified an example code from a C++ book about pushing/popping numbers off a stack class. This is a basic question just for my understanding.. Here is the code.
//stakarray.cpp
//a stack as a class
#include <iostream>
using namespace std;
class Stack
{
private:
enum { MAX = 10 };
//int MAX=10;
int st[MAX];
int top;
public:
Stack()
{ top=0; }
void push(int var)
{ st[++top]=var; } //increments stack, input var
int pop()
{ return st[top--]; } //returns var, decrements stack
void show(int var)
{ cout << st[var]<< endl; }
};
int main()
{
//some stack operations
int i;
Stack s1;
s1.push(11);
s1.push(22);
cout<<"1: "<<s1.pop()<<endl; //22
cout<<"2: "<<s1.pop()<<endl; //11
s1.push(33);
s1.push(44);
s1.push(55);
s1.push(66);
for (i=0; i<= 10 ; i++)
{
cout<< "s1[" << i << "]= ";
s1.show(i);
}
return 0;
}
The output of this program gives
1: 22
2: 11
s1[0]= 2
s1[1]= 33
s1[2]= 44
s1[3]= 55
s1[4]= 66
s1[5]= 0
s1[6]= 0
s1[7]= 0
s1[8]= 4196896
s1[9]= 0
s1[10]= 4
Why is s1[0]=2, s1[8]=4196896, s1[10]=4? Is there also any way to access MAX from private or do i have to define it somewhere else in the class (not using as global variable or part of main())?

Answer 1

Element 0 is never used, because in push you use pre-increment (++top) instead of post-increment (top++).
Your stack has at most 4 elements in it at a time, therefore all elements following index 4 have undefined content (i.e., random garbage in s1[5]...s1[10]).

Answer 2

In your code you were incrementing top with a pre-increment ++top before setting the value. Therefore, top would go to 1 and then you would set s1[1]=33. If you switch to post-increment top++, your counter variable top will increment after setting s[0]=33.
//stakarray.cpp
//a stack as a class
#include <iostream>
using namespace std;
class Stack
{
private:
enum { MAX = 10 };
//int MAX=10;
int st[MAX];
int top;
public:
Stack()
{ top=0; }
void push(int var)
{ st[top++]=var; } //increments stack, input var
int pop()
{ return st[top--]; } //returns var, decrements stack
void show(int var)
{ cout << st[var]<< endl; }
};
int main()
{
//some stack operations
int i;
Stack s1;
s1.push(11);
s1.push(22);
cout<<"1: "<<s1.pop()<<endl; //22
cout<<"2: "<<s1.pop()<<endl; //11
s1.push(33);
s1.push(44);
s1.push(55);
s1.push(66);
for (i=0; i<= 10 ; i++)
{
cout<< "s1[" << i << "]= ";
s1.show(i);
}
return 0;
}