The document discusses three ways to implement a stack in C using arrays, structures, and dynamic arrays. It provides code samples to demonstrate pushing and popping elements from the stack as well as displaying the stack contents. Functions like push(), pop(), and display() are used to add/remove elements and view the stack. The arrays and structure implementations use fixed size buffers while the dynamic array version allocates and reallocates memory as needed when the stack size changes.

1. Write a C program to implement a stack using arrays
#include<stdio.h>
#define SIZE 5
int s[SIZE],top=-1;
void push();
void pop();
void display();
int main()
{
int ch;
for(;;)
{
printf("n1.Pushn2.Popn3.Shown4.Exit");
printf("n Enter your choice n");
scanf("%d",&ch);
switch(ch){
case 1: push(); break;
case 2: pop(); break;
case 3: display(); break;
default: printf(" Invalidn”); exit(0);}
}
return 0;
}

2. void push()
{
int item;
if(top==SIZE-1)
{
printf(“STACK IS FULL”);
return;
}
printf(“enter an element to be inserted”);
scanf(“%d”,&item);
s[++top]=item;
}
void pop()
{
if(top==-1)
{
printf(“STACK IS EMPTY”);
return;
}
printf(“Element deleted is %d”,s[top--]);
}
void display()
{
int i;
if(top==-1)
{
printf(“STACK IS EMPTY”);
return;
}
printf(“Elements of the stack are:n”);
for(i=top;i>=0;i--)
printf(“%dn”,s[i]);
}

3. Write a C program to implement a stack using structures
#define MAX 10
struct stack
{ int a[SIZE]; int top; };
struct stack s;
s.top=-1;
void main()
{// same as previous code}
void pop()
{
if(s.top==-1)
{
printf(“STACK IS EMPTY”);
return;
}
printf(“Element deleted is %d”,s.a[s.top--]);
}
void push()
{
int item;
if(s.top==SIZE-1)
{ // same as previous push fush() }
printf(“Enter an item:”);
scanf(“%d”,&item);
s.top++;
s.a[s.top]=item;
}
void display()
{
int i;
if(s.top==-1)
{
printf(“STACK IS EMPTY”);
return;
}
printf(“Elements of the stack are:n”);
for(i=s.top;i>=0;i--)
printf(“%dn”,s.a[i]);
}

4. Write a C program to implement a stack using dynamic arrays
#include<stdio.h>
#include<stdlib.h>
int *s;
int stackSize=1;
int top=-1;
s=(int*)malloc(stackSize * sizeof(int));
void push()
{
if(top == stackSize-1)
{printf(“stack full allocate extra memory”);
stackSize++;
s=(int*)realloc(s,stackSize * sizeof(int));
s[++top]=item;
}
void pop()
{
if(top==-1)
{
printf(“STACK IS EMPTY”);
return;
}
printf(“Element deleted is %d”,s[top--]);
printf(“stack size decreased”);
stackSize--;
s=(int*)realloc(s,stackSize * sizeof(int));
}
MAIN FUNCTION
REMAINS SAME