stack.c

/*
 * Author: Sam Jones
 * Description: Stack revision
 * Created: 18/04/2013
 */

#include <stdio.h>
#include <stdlib.h>

typedef struct node {
    int item;
    struct node *next;
} Node;

typedef struct {
    Node *top;
    int size;
} Stack;

void initialise(Stack *s) {
// this function is to initialise the stack
    s->size = 0;  // set the size of the stack to 0
    s->top = NULL; // top of the stack now points to NULL
}

void push(Stack *s, int value) {
// Push function to add data to the top of the stack
    Node *np;  // make a node pointer

    np = (Node *)malloc(sizeof(Node));  // we now have some space for a node
    if (np == NULL) {
        // The program will crash if the memory was not allocated
        exit(1);
    }

    np->item = value; // put the value we want to allocate in the node
    np->next = s->top; // copy the last address that was at the top of the stack
    s->top = np; // now the top is pointing to the new top of the stack

    // increment the size of the stack count
    s->size++;
}

int pop(Stack *s) {
// Pop function to remove data to the top of the stack
    int temp;
    Node *np;

    if (s->top == NULL) {
        // The program will crash if the stack is empty
        exit(1);
    }

    np = s->top; // node pointer is now pointing to the top of the stack
    temp = s->top->item; // store the valuse currently at the top of the stack
    s->top = np->next; // make the top of the stack point to the node below
    s->size--; // decrement the size of the stack count
    free(np); // free the memory at the address the np is pointing to

    return temp; // return the value that was popped off the stack
}

int main() {
    int pv; // make a pop value variable
    Stack stk; // this is a stack object

    initialise(&stk); // initialise the data structure to 0
    
    push(&stk, 7); // push the value 7 on to the stack
    push(&stk, 9); // push the value 9 on to the stack
	// The stack now contains 9, 7 and NULL
	
    pv = pop(&stk);
    // the value 9 has been popped off the stack so now only 7 and NULL remains
    pv = pop(&stk);
    // the value 7 has been popped off the stack so now only NULL remains
	
    return 0;
}