#include<stdio.h>
#include <stdlib.h>
int s[100],top=0,n;

void push()
{
    int x;
    
    if(top>=n)
    {
        printf("stack overflow\n");
    }
    else
    {
        printf("enter number:");
        scanf("%d",&x);
        top++;
        s[top]=x;
    }
}
void pop()
{
    if(top<=0)
    {
        printf("stack underflow\n");
    }
    else
    {
        printf("%d is popped\n",s[top]);
        top--;
    }
}
void peep()
{
    int i;
    
    printf("enter ith position:");
    scanf("%d",&i);
    
    if(top-i+1<=0)
    {
        printf("stack underflow\n");
    }
    else
    {
        printf("%d is %dth valur from the top\n",s[top-i+1],i);
    }
}
void change()
{
    int i,x;
    
    printf("enter ith position from top to change:");
    scanf("%d",&i);
    
    if(top-i+1<=0)
    {
        printf("stack underflow\n");
    }
    else
    {
        printf("enter new value:");
        scanf("%d",&x);
        
        s[top-i+1]=x;
        printf("new value of %dth position from top is %d\n",i,x);
    }
    
}
int main()
{
    int choise;
    
    printf("enter length of stack:");
    scanf("%d",&n);
    
    printf(" 1 for push \n");
    printf(" 2 for pop \n");
    printf(" 3 for peep \n");
    printf(" 4 for change \n");
    printf(" 0 for exit \n");
    
    
    while(1)
    {
        printf("enter choise:");
        scanf("%d",&choise);
        
        switch(choise)
        {
            case 1:
                push();
                break;
            case 2:
                pop();
                break;
            case 3:
                peep();
                break;
            case 4:
                change();
                break;
            case 0:
                exit(1);
                break;
            default:
                printf("enter valid number\n");
                
        }
    }
}