Loading information from file into structures

Question

I am wondering how to edit my code to swap loading iformation b typing it, to load it from file. I was looking at it for 2 hours but I did not bring up any result so I am trying to find some help here. In file it should look like this for students:

name study_year score

my code:

#include <stdio.h>
#include <string.h>

struct Teacher
{
    char name[50];
    int age;
};

struct Student
{
    char name[50];
    int study_year;
    float score; // 0.0 -- 100.0
};


void getT(struct Teacher *t)
{
    printf("Enter teacher's name: ");
    fgets(t->name, sizeof t->name, stdin);

    size_t len = strlen(t->name);
    if ('\n' == t->name[len - 1]) t->name[len - 1] = '\0';

    printf("Enter techaer's age: ");
    scanf("%d", &t->age);
    getchar();

    putchar('\n');
}

void getS(struct Student *s)
{
    printf("Enter student's name: ");
    fgets(s->name, sizeof s->name, stdin);

    size_t len = strlen(s->name);
    if ('\n' == s->name[len - 1]) s->name[len - 1] = '\0';

    printf("Enter student's study year: ");
    scanf("%d", &s->study_year);
    getchar();

    printf("Enter student's score: ");
    scanf("%f", &s->score); // tu bolo chybne "%d"
    getchar();

    putchar('\n');
}


void printT(struct Teacher t)
{
    printf(
        "Ucitel: %s\n"
        "   Vek: %d\n\n",
        t.name, t.age);
}

void printS(struct Student s)
{
    printf(
        "Student: %s\n"
        "  Rok studia:       %d\n"
        "  Studijny priemer: %.2f\n\n",
        s.name, s.study_year, s.score);
}


void main()
{
    struct Teacher t1;

    int count = 2; //pocet studentov

    struct Student ss[count];

    getT(&t1);

    for (int i = 0; i < count; ++i)
        getS(&ss[i]);

    printT(t1);
    // printS(s1);

    for (int i = 0; i < count; ++i)
        printS(ss[i]);

    printf("Teacher's size: %d\n", sizeof(struct Teacher));
    printf("Student's size: %d\n", sizeof(struct Student));

    return 0;
}

Thank you for all suggestions and answers.

Suggest editing post and changing out `"name study_year score"` for actual values you might find in the file, eg: `"William 2020 98.0"`, just to eliminate any questions on interpretation. Aside, an [easier way to remove a new line](https://stackoverflow.com/a/28462221/645128): `s->name[strcspn(s->name, "\n")] = 0;` — ryyker, Feb 17 '21 at 18:36

score 0 · Answer 1 · answered Feb 17 '21 at 18:44

I would use the fread function to read the structures, and the fwrite function to write them. Say you have an array of SIZE students you want to write to a file:

struct Student students[SIZE];

// Populate your structs...

FILE *fp = fopen("myfile.bin", "wb");
fwrite(students, sizeof(struct Student), SIZE, fp);
fclose(fp);

Then, to read them back, first verify that the size of the file is a multiple of the size of your struct, in bytes, then read them from the file:

struct Student *students = NULL; // Dynamic since we don't know the size until we look at the file
FILE *fp = fopen("myfile.bin", "rb");
struct stat stats; // include <sys\stat.h>

stat("myfile.bin", &stats);
if( (stats.st_size % sizeof(struct Student)) != 0) 
{
    // This file is not composed of a series of 'struct Students' in order; print 'invalid file' message and exit
}

students = malloc(stats.st_size);
if(NULL == students)
{
     // Malloc failed; handle error
}
fread(students, sizeof(struct Student), (stats.st_size / sizeof(struct Student)), fp);
fclose(fp);

// 'students' is now populated with the file contents


free(students);
students = NULL;

Also note that the 'size check' to verify that you're working with a correct file is a very simplistic approach; proprietary file formats usually use magic numbers to identify what type of file they are.

score 0 · Answer 2 · answered Feb 17 '21 at 18:58

Look into the fscanf family of functions, which are the opposite of the fprintf family. You for example your teacher structure could be encoded using:

void write_teacher(FILE *f, struct Teacher *teacher) {
    // This prints the name, followed by a space, and then the age
    fprintf(f, "%s %d\n", &teacher->name[0], teacher->age);
}

On the other hand, reading it almost the exact opposite:

int read_teacher(FILE *f, struct Teacher *teacher) {
    int count;
    // zero out the structure, you may also want to replace that 50
    // with a sizeof expression
    memset(&teacher->name[0], 0, 50);

    // here the 49 means that a maximum of 49 characters should be read
    // for the name. This reading happens until whitespace is encountered
    // so if there are spaces in the name, you will have to break it up.
    count = fscanf(f, "%49s %d\n", &teacher->name[0], &teacher->age);

    // fscanf returns the number of arguments converted, we want 2 conversions
    // so it should be 2 if it was successful.
    return count != 2; // returns 1 on error, 0 on success
}

scanf is quite well suited for applications like this, but it does have its shortcomings, see this article for some common issues and how to resolve them.

Loading information from file into structures

2 Answers2