Thread: delete record from random access binary file

Is it possible to completely delete a record from a random access binary file without making an entirely new file?

This is the code I've been lazily working on for a couple weeks:

Code:

#include <stdio.h>
#include <string.h>
 
typedef struct person {
    char lastName[15];
    char firstName[15];
    char age[4];
} Person;
 
void blank100();
void tenEntry();
void printTen();
void updateRecord();
void deleteRecord();
 
int main(void) {
 
    blank100();
    tenEntry();
    printTen();
    updateRecord();
    printTen();
    deleteRecord();
    printTen();
}
 
// Creates 100 blank entries
void blank100() {
    
    FILE* wfPtr;
    if ((wfPtr = fopen("nameage.dat", "wb")) == NULL) {
        puts("Error opening nameage.dat for writing");
        return;
    }
 
    Person blankPerson = { "", "", "0" };
 
    for (int i0 = 0; i0 < 100; ++i0) {
        fwrite(&blankPerson, sizeof(Person), 1, wfPtr);
    }
    fclose(wfPtr);
}
 
// Fills ten entries
void tenEntry() {
 
    FILE* wfPtr;
    if ((wfPtr = fopen("nameage.dat", "wb")) == NULL) {
        puts("Error opening nameage.dat for writing");
        return;
    }
 
    Person p0 = { "Anderly", "Charley", "54" };
    Person p1 = { "Schultz", "Gilberto", "67" };
    Person p2 = { "Cuevas", "Abbigail", "34" };
    Person p3 = { "Rhodes", "Gregory", "83" };
    Person p4 = { "Taylor", "Giovanna", "78" };
    Person p5 = { "Glenn", "Kole", "54" };
    Person p6 = { "Reilly", "Dennis", "41" };
    Person p7 = { "Morrix", "Shannon", "27" };
    Person p8 = { "Herman", "Mekhi", "84" };
    Person p9 = { "Beltran", "Damari", "30" };
 
    fwrite(&p0, sizeof(Person), 1, wfPtr);
    fwrite(&p1, sizeof(Person), 1, wfPtr);
    fwrite(&p2, sizeof(Person), 1, wfPtr);
    fwrite(&p3, sizeof(Person), 1, wfPtr);
    fwrite(&p4, sizeof(Person), 1, wfPtr);
    fwrite(&p5, sizeof(Person), 1, wfPtr);
    fwrite(&p6, sizeof(Person), 1, wfPtr);
    fwrite(&p7, sizeof(Person), 1, wfPtr);
    fwrite(&p8, sizeof(Person), 1, wfPtr);
    fwrite(&p9, sizeof(Person), 1, wfPtr);
 
    fclose(wfPtr);
}
 
// Prints the first ten entries
void printTen() {
 
    FILE* rfPtr;
    if ((rfPtr = fopen("nameage.dat", "rb")) == NULL) {
        puts("Error opening nameage.dat for reading");
        return;
    }
 
    Person toScreen = { "", "", "0" };
 
    for (int i0 = 0; fread(&toScreen, sizeof(Person), 1, rfPtr) && i0 < 10; ++i0) {
        printf("%s, %s, %s\n", toScreen.lastName, toScreen.firstName, toScreen.age);
    }
    puts("");
    fclose(rfPtr);
}
 
// updates one record with a new age value
void updateRecord() {
    
    FILE* rpfPtr;
    if ((rpfPtr = fopen("nameage.dat", "rb+")) == NULL) {
        puts("Error opening nameage.dat r+");
    }
 
    Person newData = { "", "", "0" };
    printf("Enter existing lastName, firstName, newAge: ");
    char line[50];
    fgets(line, 50, stdin);
    char* token = strtok(line, ", \n");
    strcpy(newData.lastName, token);
    token = strtok(NULL, ", \n");
    strcpy(newData.firstName, token);
    token = strtok(NULL, ", \n");
    strcpy(newData.age, token);
    
    Person search = { "", "", "0" };
    while (fread(&search, sizeof(Person), 1, rpfPtr) && strcmp(search.lastName, newData.lastName)) {
        
    }
    if (strcmp(search.lastName, newData.lastName) == 0) {
        fseek(rpfPtr, -1 * (int)sizeof(Person), SEEK_CUR);
        fwrite(&newData, sizeof(Person), 1, rpfPtr);
    }
    else {
        puts("Record not found.\n");
    }
 
    fclose(rpfPtr);
}
 
// deletes one record
void deleteRecord() {
    FILE* fPtr;
    if ((fPtr = fopen("nameage.dat", "rb+")) == NULL) {
        puts("Error opening nameage.dat r+");
    }
 
    Person userEntry = { "", "", "0" };
    printf("Delete existing lastName, firstName, age: ");
    char line[50];
    fgets(line, 50, stdin);
    char* token = strtok(line, ", \n");
    strcpy(userEntry.lastName, token);
    token = strtok(NULL, ", \n");
    strcpy(userEntry.firstName, token);
    token = strtok(NULL, ", \n");
    strcpy(userEntry.age, token);
 
    Person search = { "", "", "0" };
    while (fread(&search, sizeof(Person), 1, fPtr) && strcmp(search.lastName, userEntry.lastName)) {
 
    }
    if (strcmp(search.lastName, userEntry.lastName) == 0) {
        fseek(fPtr, -1 * (int)sizeof(Person), SEEK_CUR);
        Person blank = { "", "", "" };
        fwrite(&blank, sizeof(Person), 1, fPtr);
    }
    else {
        puts("Record not found.\n");
    }
 
    fclose(fPtr);
}

Output:

Code:

Anderly, Charley, 54
Schultz, Gilberto, 67
Cuevas, Abbigail, 34
Rhodes, Gregory, 83
Taylor, Giovanna, 78
Glenn, Kole, 54
Reilly, Dennis, 41
Morrix, Shannon, 27
Herman, Mekhi, 84
Beltran, Damari, 30

Enter existing lastName, firstName, newAge: Rhodes, Jeff, 44
Anderly, Charley, 54
Schultz, Gilberto, 67
Cuevas, Abbigail, 34
Rhodes, Jeff, 44
Taylor, Giovanna, 78
Glenn, Kole, 54
Reilly, Dennis, 41
Morrix, Shannon, 27
Herman, Mekhi, 84
Beltran, Damari, 30

Delete existing lastName, firstName, age: Rhodes, Abe, 3
Anderly, Charley, 54
Schultz, Gilberto, 67
Cuevas, Abbigail, 34
, ,
Taylor, Giovanna, 78
Glenn, Kole, 54
Reilly, Dennis, 41
Morrix, Shannon, 27
Herman, Mekhi, 84
Beltran, Damari, 30

It is the , , that bothers me a little. It comes from saving the blank record { "", "", "" } into the record I want to delete. I suppose I could modify the printing of records.

> I suppose I could modify the printing of records.
That's pretty much what you have to do.
You make a specific instance of your record (say all the strings begin with \0) which means 'deleted record' and then code around it.

The only way to physically remove the record from the file is to copy all the records you want to keep to another file.