How to delete the record from file - c

Question

# include <stdio.h>
# include <conio.h>
# include <string.h>
# include <stdlib.h>
struct dnode{
char dlname[20],
dfname[20],
dtel[15];
struct dnode *dnext;
};
int dmenu(); //main menu
void dadd(); //add to list
void dfind(); //search from the list
void dedit(); //edit the record
void ddel(); //delete from the list
void ddisp(); //display all
dnode* sort(dnode* head_node);
typedef struct dnode node;
node *dstart, *dtemp;
int dmenu()
{
int dch;
int i,j,n;
printf(" TELEPHONE DIRECTORY ");
printf(" =================== ");
printf(" \n1. Add ");
printf(" \n2. Find ");
printf(" \n3. Sort ");
printf(" \n4. Delete ");
printf(" \n5. Display All ");
printf(" \n6. EXIT ");
printf(" \nEnter your choice(1-6):");
scanf("%d", &dch);
return dch;
}
void dadd()
{
FILE *fp;
fp=fopen("record.txt","a");
node *dptr,*dprev;
dtemp=(node *)malloc(sizeof(node));
printf("First name: ");
scanf("%s", dtemp->dfname);
printf("Last name:");
scanf("%s", dtemp->dlname);
printf("Telephone No.: ");
scanf("%s", dtemp->dtel);
dtemp->dnext=NULL;
if(dstart==NULL) dstart=dtemp;
else {
dprev=dptr=dstart;
while(strcmp(dtemp->dfname,dptr->dfname)>0){
dprev=dptr;
dptr= dptr->dnext;
if (dptr == NULL) break;
}
if(dptr==dprev) {
dtemp->dnext=dstart;
dstart=dtemp;
}
else if(dptr==NULL)
dprev->dnext=dtemp;
else {
dtemp->dnext=dptr;
dprev->dnext=dtemp;
}
}
fprintf(fp,"%20s %20s %s\n",dtemp->dfname,dtemp->dlname,dtemp->dtel);
fclose(fp);
}
void dfind()
{
FILE *fp;
node *dptr;
char dstr[20];
fp=fopen("record.txt","r");
if(dstart==NULL){
printf("\n\t\t\tTelephone Directory is Empty....\n");
getch();
return;
}
printf("First Name to Find : ");
scanf("%s",dstr);
dptr=dstart;
while(strcmp(dptr->dfname,dstr)!=0)
{
dptr= dptr->dnext;
if (dptr == NULL) break;
}
if(dptr!=NULL) {
printf("First Name : %s\n",dptr->dfname);
printf("Last Name : %s\n",dptr->dlname);
printf("Phone Number : %s\n",dptr->dtel);
}
else {
printf("No Matching Records Found .......\n");
}
fclose(fp);
getch();
}
This is my sorting function, how can i sort the record in file?
dnode* sort(dnode* head_node)
{
dnode *prev, *curr = head_node;
int i, list_size = 0;
/* Determine the size first */
while (curr != NULL)
{
list_size++;
curr = curr->dnext;
}
/* sorting */
for (i = 1; i < list_size; i++)
{
curr = head_node;
while (curr->dnext != NULL)
{
if (strcmp(curr->dfname, curr->dnext->dfname) > 0)
{
dnode* next_node = curr->dnext;
curr->dnext = next_node->dnext;
next_node->dnext = curr;
if (curr == head_node)
head_node = next_node;
else
prev->dnext = next_node;
curr = next_node;
}
prev= curr;
curr = curr->dnext;
}
}
return head_node;
}
Here is my delete function, anyone can help me to solve the delete record in a file?
void ddel()
{
node *dptr,*dprev,*dtemp;
char dstr[20],dyn='n';
struct dnode record;
if(dstart==NULL) {
printf("\n\t\t\tTelephone Directory is Empty....\n");
getch();
return;
}
printf("First Name to Delete : ");
scanf("%s",&dstr);
dprev=dptr=dstart;
while (strcmp(dptr->dfname,dstr)!=0)
{
dprev=dptr;
dptr= dptr->dnext;
if (dptr == NULL)
break;
}
if(dptr!=NULL){
printf("\nDeleting Record.....Confirm [y/n]: ");
dyn=getch();
printf("\n\n---------------------------------------------------------");
printf("\nFirst Name : %s\n",dptr->dfname);
printf("Last Name : %s\n",dptr->dlname);
printf("Phone Number : %s\n",dptr->dtel);
printf("---------------------------------------------------------");
if(dyn=='y') {
if (dptr==dstart) {
dtemp=dstart->dnext;
free(dstart);
dstart=dtemp;
}
else {
dtemp=dptr->dnext;
free(dptr);
dprev->dnext=dtemp;
}
printf("\n\n1 Record Deleted....");
}
else
printf("\n\nRecord not Deleted....");
}
else {
printf("\nNo Matching Records Found .......");
}
getch();
}
void ddisp()
{
FILE *fp;
struct dnode rec;
fp=fopen("record.txt","r");
node *dptr;
if(dstart==NULL) {
printf("\n\t\t\tTelephone Directory is Empty....\n");
getch();
return;
}
printf("\t\t------------------------------\n");
for(dptr=dstart; dptr!=NULL; dptr=dptr->dnext) {
printf("\t\tFirst name: %s", dptr->dfname);
printf("\n\t\tLast name:%s", dptr->dlname);
printf("\n\t\tTelephone No.: %s", dptr->dtel);
printf("\n\t\t------------------------------\n");
}
fclose(fp);
getch();
}
int main()
{
int dch;
int i,j,n;
dstart=(node *)malloc(sizeof(node));
dstart=NULL;
do{
dch=dmenu();
switch(dch) {
case 1: dadd();
break;
case 2: dfind();
break;
case 3: dstart = sort(dstart);;
break;
case 4: ddel();
break;
case 5: ddisp();
break;
}
}while(dch!=6);
}
I have done this code but i can not delete and sorting the record in record.txt file.

Answer 1

To sort a file: read all the records, sort them in memory, then write all of them in the refreshed file (open it in w mode to reset it to an empty file). If there is too many records to handle them in memory you may need to find some alternative (an index would be more appropriate).
To delete: you can't delete something in a file, you need to rewrite most of the file. If there is not so much record, read all of them, remove the one from the memory structure and then rewrite a fresh file. You may find some alternative in the case of too many records.

Answer 2

Ask yourself some questions first on how is your program supposed to deal with adding/storing/searching and deleting records. Only after this start to modify it.
E.g. do you wan to process your data on disk, or in memory? If you prefer to process the phone list in memory in the form of a list – when are you going to synchronize what you have in memory with what you have on disk? As for now it is not clear from your code when you have to open of close the data file, or if our advice should concentrate on deleting/sorting records in memory or on disk.
Without even a sketchy design you have too many ways to change your program.