Problem with my stack implementation in C [duplicate] - c

Question

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Why does call-by-value example not modify input parameter?
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How to modify a struct in a function and return to main?
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Closed 10 months ago.
I'm learning Data Structures and was implementing a stack on C. The code compiles correctly but the stack remains unedited i.e. nothing is being pushed into the stack even after a push operation and it stays empty. I'm not sure where the problem with this code is. Please help me with this. Thanks.
Here is my code:
#include <stdio.h>
#include <stdlib.h>
#define MAX_SIZE 101
//Define a struct
struct stack{
int A[MAX_SIZE]; //Array of size 101
int top; //Variable that stores the index position of the recently inserted element in the stack.
};
//Function to create a stack, set top to -1 and return it.
struct stack CreateStack(){
struct stack p;
p.top = -1;
return p;
}
//Function to insert a number at the end of the stack.
void Push(int x, struct stack p){
//If the array is full, return an error message.
if (p.top == MAX_SIZE - 1){
printf("Error: Stack Overflow!");
return;
}
//Increment top and set insert x at the last of A.
p.top++;
p.A[p.top] = x;
};
//Function to delete an element from the last in a stack.
void Pop(struct stack p){
//If stack is already empty, print a message.
if (p.top == -1){
printf("Empty Stack!");
return;
}
//Decrement top.
p.top--;
};
//Function to return the top element in the stack.
int Top(struct stack p){
return p.A[p.top];
};
//Function to check if the stack is empty.
int IsEmpty(struct stack p){
return p.top == -1;
};
//Function to display all the elements in the stack.
void Print(struct stack p)
{
printf("Stack: ");
for(int i = 0; i <= p.top; i++){
printf("%d", p.A[i]);
}
printf("\n");
};
int main(){
struct stack mystack = CreateStack(); //Creates a stack called mystack.
Push(22, mystack); //Pushes 22 on the stack.
Print(mystack);// Should display 22.
}

Answer 1

You pass struct stack p by value (copy); instead pass it in via pointer so the modifying functions can change state:
void Push(int x, struct stack *p,) {
if (p->top == MAX_SIZE - 1){
printf("Error: Stack Overflow!");
return;
}
p->A[p->top++] = x;
};
It's a convention to pass the abstract data type (here struct stack *p) as the first argument. Also consider returning a status so caller can tell if the function failed. Finally, it's a good idea to separate i/o (printing error messages) from logic, as it may depend on the context of the caller (console/browser/gui, but also it may not be an error for caller if the stack is full). For example:
struct stack *Push(struct stack *p, int x) {
if (p->top == MAX_SIZE - 1) return NULL;
p->A[p->top++] = x;
return p;
};