c - Movement & starting point of s1->top in the following functions, push, display & pop -

can me understand whats happening in following functions? use of s1->top? , movement of s1->top in functions push &pop & display? because if in function push, s1->top moves right whenever number pushed? why in display function, says s1->top first in traversing, while in push s1->top n right, while in printing values, need firstly in left , traverse..why?

typedef struct node* nodeptr; typedef char dataitem;  typedef struct node{ dataitem data; nodeptr next; }node;  typedef struct{ int count; nodeptr top; }stack_head;  typedef stack_head* stack;  stack createstack() {   stack s1;   s1 = (stack) malloc(sizeof(stack_head));   s1 - > count = 0;   s1 - > top = null;   return s1; }  nodeptr createnode(dataitem item) {   nodeptr temp;   temp = (nodeptr) malloc(sizeof(node));   temp - > data = item;   temp - > next = null;   return temp; }  void push(stack s1, dataitem item) {   nodeptr temp = createnode(item);   temp - > next = s1 - > top;   s1 - > top = temp;   s1 - > count++; }  void display(stack s1) {   nodeptr ptr = s1 - > top;   while (ptr != null) {     printf("%d", ptr - > data);     ptr = ptr - > next;   }   printf("\n"); }  void pop(stack s1) {     nodeptr temp;     if (isempty(s1))       printf("list empty");     else {       temp = s1 - > top;       s1 - > top = temp - > next;       temp - > next = null;       free(temp);       s1 - > count;     }      int isempty(stack s1) {       return s1 - > top == null;     }  

first off, let's fix bug - in function display(), assume line :

while (ptr1 = null) { 

should :

while (ptr != null) { 

in question, refer "left" , "right", variable "top" implies, it's easier visualising vertical stack.

for example, imagine stack of dinner plates. new plates "push"ed onto top, , on demand "popped" top.

so can see, sort of stack called last-in-first-out (or lifo) stack, , code implementing.

the s1->top variable pointer top of stack - ie. last node added, first removed. each node has pointer next node "under" it. null used indicate "no more nodes", both s1->top, , node->next pointers.

so in push(), 2 things must happen consistency :

1. the new arrival's ->next pointer must set current "top" of stack (so existing top "under" new arrival), ,
2. the new arrival must set new "top".

note order of operations here important - if (2) performed before (1), end new arrival's "->next" pointer pointing !

pop() performs operations in reverse, , display() runs down elements in stack (notice display() not alter value of s1->top ).