如何从相对二进制文件中删除记录？

Question

我正在考虑手动删除等级为5 [5]的记录。我还没有尝试过任何东西，因为我不知道从哪里开始。我不想要一个快速完成工作的功能，我想要实现自己的功能并理解它，但我不知道如何删除相关文件中的记录。我的代码应该这样工作： - 二进制文件应该从文本文件接收数据并显示搜索到的学生，如果学生不存在，我应该添加它。在完成所有这些之后，我应该能够删除严格低于5的等级[5]的学生。这只是我的代码中缺少的删除功能。

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <conio.h>

typedef struct {
    int nrm;             //enrolment number - works as relative key
    char CNP[14];        //string ASCIIZ
    char nume[30];       //string
    int an;              //study year
    int grupa;           //group
    unsigned char note[20];  //grades
    char is;//this is status indicator
} STUDENT;

int filesize(FILE* f, int rec_size)
{//from the crt.pos substract one and i ll know the record of...
    long crt_pos;
    int size;
    //below compute the size of the file in records, withouth altering the crt.pozition
    crt_pos = ftell(f);//save the current pozition in a var, before going to the end of the file
    fseek(f, 0, SEEK_END);//go to the end of the file
    size = ftell(f) / rec_size;//ftell gives the current position and the dim of the record
    fseek(f, crt_pos, SEEK_SET);//ne readuce la positia curenta, initiala pe care am salvat o n var
    return size;
}

void main()
{
    char numefr[30] = "..\\Studenti_r_f.dat";//why \\? changes the meaning of the foll char; afiseaza un singur
    FILE* f;
    STUDENT x;//record variable
    int i, key, dim;//key=key for the record;dim=size of file in records
    //dim changes when computing the sizeof(file), at every iteration
    fopen_s(&f, numefr, "wb+");
    //optionally preform the file

    freopen("..\\input.txt", "rt", stdin);  //remove if reading data from keyboard and uncomment the printf lines

    //printf("Enrollment number: ");
    scanf_s("%d", &key);//read the first field, always the 1 field is the key
    while (!feof(stdin))//process the key read another and so on
    {
        //check key range
        dim = filesize(f, sizeof(STUDENT));//dim=nr de studenti din fisier;nr de recorduri;
        //check the poz indicated by the key, it may have changed since the last iteration
        if (key >= dim)//if the key=10, position 10 doesn t exist
        { //extend the file
            x.is = 0;
            fseek(f, 0, SEEK_END);//go to the end of file and write this record
            for (i = 0; i < key - dim; i++)
                fwrite(&x, sizeof(STUDENT), 1, f);
        }
        fseek(f, key * sizeof(STUDENT), SEEK_SET);//change the pos in the file, go to the pos indicated by the key
        fread(&x, sizeof(STUDENT), 1, f);//read rec from file
        if (x.is == 1)//if is=1, the position is taken, it must always be 0
            printf("\nError: duplicate key %d. Skipping.", key);//read rest of field for student, write record from file then
        else
        {
            x.nrm = key;
            //printf_s("Name: ");
            gets_s(x.nume);
            gets_s(x.nume);
            //printf_s("CNP: ");
            gets_s(x.CNP);
            //printf_s("Year: "); 
            scanf_s("%d", &x.an);
            //printf_s("Group: ");
            scanf_s("%d", &x.grupa);
            for (i = 0; i < 20; i++)
                //x.note[i]=0;
                scanf_s("%d", &x.note[i]);
            x.is = 1;
            //the cur poz is after this record before we write the fseek, which brings us to the beginning of this record
            fseek(f, key * sizeof(STUDENT), SEEK_SET);//ne aduce la inceputul recordului nou adaugat;
            fwrite(&x, sizeof(STUDENT), 1, f);//write a record
        }//everything repets with a new key
        //printf_s("Enrollment number (or CTRL+Z): ");
        scanf_s("%d", &key);
    }

    fclose(f);
    fclose(stdin);

    printf("\nDone. File <%s> was created. Press a key.", numefr);
    _getch();
}

Answer 1

有两种方法可以从文件中删除记录：

1）将要删除的记录之前的所有记录复制到新文件中，跳过要删除的记录，然后复制所有其余记录，然后删除原始文件，然后将新文件重命名为原始文件名。

2）逐步浏览文件，直到找到要删除的记录。然后，在循环中将下一条记录复制到当前记录，然后将文件截断1个记录长度

所有这些（上面）可能非常混乱和耗时。建议：只需将记录中的字段'is'设置为'已删除'为0即可