#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAX 50


typedef struct
        {
        char  name[10];
        int key;
      } file_record;

/* this function adds the relatiuve addres to the index for a key */
void create_index(long index[], int key, long rel_add )
    {
     index[key] = rel_add;
   }

/* this function writes a record to the file */
void write_rec(FILE *fp, file_record rec)
    {
   fwrite(&rec,sizeof(rec),1,fp);
    }

void main()
    {
       long rel_add;
       int key;
       file_record frec;
       long index[MAX];/* an index list*/
int n,i;

       FILE *recfile=NULL,*ifile=NULL;
      /* this initializes the index list to all –1 */
for(i=0; i< MAX; i++)
                index[i]= (-1);

       recfile=fopen("mfile","w");
       if(recfile == NULL)
       {
        printf("Error in openeing file mfile\n");
        exit(0);
       }
       rel_add = 0 ;
       do
       {
          printf(" Enter the data vlue and the key of the record to be added to file mfile\n");
          scanf("%s %d",frec.name,&frec.key);
            while(index[frec.key] != (-1))
              {
              printf(" A record with this key value already exist in a file enter record key
   value\n");
               scanf("%s %d",frec.name,&frec.key);
              }
             create_index(index,frec.key,rel_add);
             write_rec(recfile,frec);
             rel_add =  ftell(recfile); /* this sets the relative address for the next record to be 
      the value of current file position pointer in bytes from 
      the beginning of the file */

             printf("Enter 1 to continue adding records to the file\n");
             scanf("%d",&n);
           }while(n == 1);
         ifile=fopen("index_file","w");
         if(ifile == NULL)
           {
                printf("Error in openeing file index_file\n");
             exit(0);
           }
          fwrite(index,sizeof(index),1,ifile);/*writes the complete index into the index_file */

          fclose(recfile);
          fclose(ifile);
          printf("Enter 1 if you want to retrieve a record\n");
          scanf("%d",&n);
          if( n == 1)
              {
              ifile=fopen("index_file","r");
              if(ifile == NULL)
                   {
                        printf("Error in openeing file index_file\n");
                     exit(0);
                }
              fread(index,sizeof(index),1,ifile);/* reads the complete index into the index list 
from the index_file*/

              fclose(ifile);
              recfile=fopen("mfile","r");
              if(recfile == NULL)
                   {
                        printf("Error in openeing file mfile\n");
                     exit(0);
                }
              }
              printf("THE CONTENTS OF FILE IS \n");
              while( (fread(&frec,sizeof(frec),1,recfile)) != 0)
                    printf("%s %d\n",frec.name,frec.key);
              do
              {
                printf("Enter the key of the record to be retrieved\n");
                scanf("%d",&key);
                rel_add = index[key]; /*gets the relative address of the record from index list */
if( (fseek(recfile,rel_add,SEEK_SET))!= 0)
                   {
                     printf("Error\n");
                     exit(0);
                   }
                fread(&frec,sizeof(frec),1,recfile);
                printf("The data value of the retrieved record is %s\n",frec.name);
                printf("Enter 1 if you want to retrieve a record\n");
                scanf("%d",&n);
              } while(n == 1);
            fclose(recfile);
          }