The functions && Linked List - c

Question

I did my best with this program but I could not know where is the error?? I'll explain the program. In this program I should implement a stack of integers as linked list, using a global variable to point to the top of the stack by using these methods:
int push(int i);
push i on the stack, return 1 if successful else return 0.
int pop();
pop number from stack. if stack empty return 0;
I did create new method call int stackEmpty(); and the two method above.
Every time I run my program it's push the numbers into the stack but the pop doesn't work. Here my code:::
#include <stdio.h>
#include <stdlib.h>
typedef struct stack Stack;
struct stack
{
int number;
Stack *next;
};
Stack *top = NULL;
int push(int i);
int count();
int stackEmpty();
int pop();
int main()
{
char op;
int i, x;
printf("Welcome to my stack\n");
printf("p to pop, s to push, c to count, q to quit\n");
while (op != 'q')
{
scanf("%c", &op);
if (op == 'p')
{
x = pop();
if (x == 0)
{
printf("Stack is empty\n");
}
else
{
printf("%d popped\n", pop());
}
}
else if (op == 'c')
{
i = count();
printf("%d numbers on stack\n", i);
}
else if (op == 's')
{
printf("Enter number: ");
scanf("%d", &i);
x = push(i);
if (x == 1 || x == 2)
{
printf("%d puched :: state%d\n", i, x);
}
else
{
printf("faill %d\n", x);
}
}
else if (op == 'q')
{
return 0;
}
}
return 0;
}
int stackEmpty()
{
if (top == NULL)
{
return 1;
}
else
{
return 0;
}
}
int count()
{
int counter = 0;
if (top == NULL)
{
return counter;
}
else
{
while (top != NULL)
{
top = top->next;
counter++;
}
return counter;
}
}
int push(int i)
{
Stack *head;
Stack *next;
Stack *new;
int state;
int m;
head = top;
new = (Stack *) malloc(sizeof(Stack));
if (new == NULL)
{
state = 0;
} new->number = i;
m = stackEmpty();
if (m == 1)
{
head = new;
top = head;
head->next = NULL;
state = 1;
}
else
{
while (head != NULL)
{
if ((next = head->next) == NULL)
next = new;
next->next = NULL;
state = 2;
break;
head = top->next;
next = head->next;
}
top = head;
}
return state;
}
int pop()
{
Stack *head;
int state;
int m;
head = top;
if (head == NULL)
{
state = 0;
}
m = stackEmpty();
if (m == 1)
{
state = 0;
}
else
{
state = head->number;
top = head->next;
free(head);
}
return state;
}

Answer 1

Several problems:
top is your supposed head of the stack I assume. In count you advance top until it is NULL - thus once you called count you have "lost" your stack.
A stack is a LIFO queue (last in first out). Your push would implement a FIFO (first in first out) by appending new elements at the end.
Your push is not actually adding anything to the list. You are just assiging new to next but you are not pointing to next from anywhere in your list.
When using pop you are calling it twice (once for removing the element and once for printing). Therefore you remove two elements whenever you go down that code path. A better implementation would be to write a peek function which returns the top element without removing it and the pop function simply removes it (indicating success with 1 and fail with 0)
A push for a stack goes like this:
Create a new element
Point to your current head as the next element
Make your new element the new head of the stack
No loop needed. It's an O(1) operation.

Answer 2

You are not pushing correctly. You are changing next which is a local variable. you are not changing the "next" value in you list tail.

Answer 3

One problem is that you pop(), then check result, then pop() again while printing. You're popping twice for each time you try to print.

Answer 4

Another error:
while (head != NULL)
{
if ((next = head->next) == NULL)
next = new;
next->next = NULL;
state = 2;
break;
head = top->next;
next = head->next;
}
Should be:
while (head != NULL)
{
if ((next = head->next) == NULL)
{
next = new;
next->next = NULL;
state = 2;
break;
}
head = top->next;
next = head->next;
}
At least, that's what your original indentation seems to indicate.