creating temp stack to print it in c - c

Question

current uni student and i have been tasked with creating a stack using the following structure
typedef char stackitem;
struct stack {
stackitem d;
struct stack *next;
};
typedef struct stack ELEMENT;
typedef ELEMENT *POINTER;
I am having issues with printing my stack. What i am trying to do is create a temp stack where I print the element and then pop it off. however it seems to be removing it from the original stack which I dont want.
here is the code:
void print_stack(POINTER Top)
/*Print the contents of the stack. Do not modify the stack in any way. */
{
POINTER temp = Top;
printf("Start printing the stack ...\n");
while (temp != NULL){
printf("%c\n",temp->d);
pop(&temp);
}
}
void pop(POINTER *Top)
/* Remove the top item */
{
POINTER Top1 = *Top;
if (Top != NULL)
{
*Top = Top1->next;
// printf("Remove element %c\n", Top1->d);
free(Top1);
}
else
printf("Empty stack.\n");
}
In my mind I am printing and popping from the temp pointer which shouldn't affect the original pointer top but when i insert into the stack and print it the first time it prints correctly and then the second time it prints nothing.
this is my main
POINTER top;
top= (POINTER) NULL;
stackitem A='A';
stackitem B='B';
push(&top,A);
push(&top,B);
print_stack(top); // prints B A
print_stack(top); // prints nothing
Any help with what is going wrong?

Answer 1

Your print_stack() method currently looks like this:
void print_stack(POINTER Top) {
POINTER temp = Top;
printf("Start printing the stack ...\n");
while (temp != NULL){
printf("%c\n",temp->d);
pop(&temp); // Calling `pop()` will modify your Stack
}
}
You're calling pop() in your loop which modifies the stack; which is not good since print_stack is a read operation and should not be modifying the list. You can instead just increment the pointer so that it points to next value:
void print_stack(POINTER Top) {
POINTER temp = Top;
printf("Start printing the stack ...\n");
while (temp != NULL){
printf("%c\n",temp->d);
temp = temp->next; // Go to next element of stack
}
}
I tested this code(I just needed to add additional push and createNewStackNode methods that weren't present:
// ...
POINTER createNewStackNode(stackitem i) {
POINTER newNode = (POINTER) malloc(sizeof(ELEMENT));
newNode->d = i;
newNode->next = NULL;
return newNode;
}
void push(POINTER *top, char item) {
POINTER curr = createNewStackNode(item);
curr->next = *top;
*top = curr;
}
// ...
int main() {
POINTER top;
top= (POINTER) NULL;
stackitem A='A';
stackitem B='B';
push(&top,A);
push(&top,B);
print_stack(top); // prints B A
push(&top, 'C');
push(&top, 'D');
push(&top, 'E');
push(&top, 'F');
print_stack(top);
}
This prints the following output:
Start printing the stack ...
B
A
Start printing the stack ...
F
E
D
C
B
A

Answer 2

As another user suggested, you don't create a second stack rather you just create a second pointer that shows you the same memory position as the original stack.
If I was you I would try to add the stack into an array (you can search for guidance here) and then modify that array. That way your original stack will be unharmed.

Answer 3

The print_stack function claims it doesn't modify the stack, but it does. For example, it calls pop which calls free, destroying the object on the top of the stack.