Study this code sample... it may help you understand records based, random access filing a bit better...
Code://random access file demo
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <ctype.h>
#include <string.h>
#define FNAME "random.dat"
// test data struct
struct t_Record
{ int number;
char word[16]; }
Record;
///////////////////////////////////////////////////////
// Random Access File Handlers
//
// open or create the file
FILE *FileOpen(char* Filename)
{ FILE* pFile;
pFile = fopen(Filename,"rb+");
if (!pFile)
pFile = fopen(Filename,"wb+");
return pFile; }
// Write a record to the file
int WriteRecord(FILE *File, int RecNum)
{ if( fseek(File, RecNum * sizeof(Record), SEEK_SET) == 0 )
if ( fwrite(&Record,sizeof(Record),1,File) )
return 1;
return 0; }
// read a record from the file
int ReadRecord(FILE *File, int RecNum)
{ if( fseek(File, RecNum * sizeof(Record), SEEK_SET) == 0 )
if ( fread(&Record,sizeof(Record),1,File) )
return 1;
return 0; }
int AddRecord(FILE *File)
{ fseek(File,0,SEEK_END);
fwrite(&Record,sizeof(Record),1,File);
return (ftell(File) / sizeof(Record)) - 1; }
//////////////////////////////////////////////////////////////
// View a Record
//
int ViewRecord (FILE *File, int RecNum)
{ if (! ReadRecord(File,RecNum))
{ printf("Invalid record\n");
return -1; }
printf("-----\n");
printf("Record : %d\n",RecNum);
printf("Number Value : %d\n",Record.number);
printf("Word Value : %s\n",Record.word);
printf("-----\n");
return RecNum; }
//////////////////////////////////////////////////////////////
// Add a new record
//
int AddNewData(FILE *File)
{ memset(&Record,0,sizeof(Record));
printf("\nEnter a number : ");
scanf("%d", &Record.number);
printf("Enter a word : ");
scanf(" %s",Record.word);
return AddRecord(File); }
//////////////////////////////////////////////////////////////
// Edit a record
//
int EditRecord(FILE *File, int RecNum)
{ if (! ReadRecord(File,RecNum))
{ printf("Invalid record\n");
return -1; }
printf("\n-----\n");
printf("Record : %d\n",RecNum);
printf("Number Value : %d\n",Record.number);
printf("Word Value : %s\n",Record.word);
printf("-----\n");
do
{ while(getchar() != '\n');
printf("Change Values: Number, Word or Save (N, W or S) ? ");
switch (toupper(getchar()))
{ case 'N' :
printf("\nEnter new number : ");
scanf("%d",&Record.number);
break;
case 'W' :
printf("Enter new word : ");
scanf(" %15s", Record.word);
break;
case 'S' :
if (WriteRecord(File,RecNum))
printf("\nRecord #%d updated\n",RecNum);
return RecNum; } }
while(1);
return -1; }
////////////////////////////////////////////////////////////////
// List records
//
void ListRecords(FILE *File )
{ int i = 0;
printf("\nRecord Number\tWord\n\n");
while (ReadRecord(File,i))
{ printf("%3d%16d\t%s\n",i,Record.number,Record.word);
i++; }
printf("\n\n"); }
////////////////////////////////////////////////////////
// this is for demonstration purposes only
// you would not do this in a real program
void InitFile(FILE* File)
{ int x, y;
memset(&Record,sizeof(Record),0);
for (x = 0; x < 10; x++)
{ Record.number = rand();
for (y = 0; y < ((Record.number % 15) + 1); y++)
Record.word[y] = (rand() % 26) + 'a';
Record.word[y] = 0;
if (! WriteRecord(File,x))
printf("Oh drat!"); } }
//////////////////////////////////////////////////////////
// program mains
//
int main (void)
{ int Rec = 0; // record number
FILE *File;
srand(time(NULL));
File = FileOpen(FNAME);
if (!File)
{ printf("Curses foiled again!\n\n");
exit(-1); }
printf("Random Access File Demonstration\n\n");
do
{ printf("Menu : Dummy, Add, Edit, View, List, Quit (D, A, E, V, L or Q) : ");
switch(toupper(getchar()))
{ case 'D' :
printf("Creating dummy file of 10 entries\n");
InitFile(File);
break;
case 'A' :
Rec = AddNewData(File);
printf("Record #%d Added\n\n", Rec);
break;
case 'E' :
printf("\nRecord number (-1 Cancels): ");
scanf("%d",&Rec);
if (Rec > -1)
EditRecord(File,Rec);
break;
case 'V' :
printf("\nRecord number (-1 Cancels): ");
scanf("%d",&Rec);
if (Rec > -1)
ViewRecord(File,Rec);
break;
case 'L' :
ListRecords(File);
break;
case 'Q' :
fclose(File);
return 0; }
while(getchar() != '\n'); }
while (1);
return 0; }
