Quadratic probing

#include<stdio.h>
#include<stdlib.h>
#define MAX   10

int a[MAX];
void qp(int ,int []);
void qpsr(int key,int a[MAX]);
void display(int a[MAX]);


 int main()
 {
   int i,key,ch;
   for(i=0;i<MAX;i++)
   a[i]='\0';
   do
   {
   printf("\n \n Program for intersection/searching keys with the quadratic  probing");
   printf("\n \n ************************************************************\n\n");
   printf("\n 1.Insert Keys");
   printf("\n 2.Search keys");
   printf("\n 3.Display keys");
   printf("\n 4.Exit");
   printf("\nSelect the operation");
   scanf("%d",&ch);
   switch(ch)
   {
       case 1:do{
             printf("\nEnter the key value [type-1]for termination");
             scanf("%d",&key);
             if(key!=-1)
             {
               qp(key,a);

             }
             }while(key!=-1);
             display(a);
             break;

         case 2:printf("\nEnter the search key value");
                scanf("%d",&key);
                qpsr(key,a);
                break;

        case 3:display(a);
                break;

        default:printf("\nInvalid input given");
     }
   }while(ch!=4);
 }

 void qp(int key,int a[MAX])
 {
     int loc;
     int i=1;
     loc=key%MAX;
     while(a[loc]!='\0')
     {
        loc=(key %MAX+i*i)%MAX;
        i++;
     }
     a[loc]=key;
 }

 void qpsr(int key,int a[MAX])
 {
     int loc;
     loc=key% MAX;
     while((a[loc]!=key)&& (a[loc]!=-1))
        loc=++loc%MAX;
     if(a[loc]!='\0')
        printf("\nSearch successful at index %d",loc);
     else
        printf("\nSearch successful");
 }

 void display(int a[MAX])
 {
     int i;
     printf("\nList of keys('0' indicate that the location is empty):\n");
     for(i=0;i<MAX;i++)
     {
         printf("%d\t",a[i]);
     }
 }