Dynamically allocating array of struct from file in C

Question

I have a program that I am trying to write that will open a file named list.txt, this file will contain a num, a ID, and a string name on each line. This program will read list.txt file and sort ID numbers and print sorted ID with number and name to index.txt file. I wrote a program code and it's working...

Here is my list.txt

(num, ID, name)

0 3 AB
1 2 BC
2 28 DC
3 1 EF
4 13 BB
10 30 CC
11 23 FF
14 16 GG

After compiled this program sorted ID with number and name, print to index.txt and it should be:

(ID, num, name)

1 3 EF
2 1 BC
3 0 AB
13 4 BB
16 14 GG
23 11 FF
28 2 DC
30 10 CC

Here is my program code:

#include <stdio.h>
#include <conio.h>
#include <process.h>
#include <stdlib.h>
#include <string.h>

#define NUM_NUMBERS 9

typedef struct student
{   
    int num;
    int id;
    char name[100];
}end;

void update();
void Sort(student array[], int n); 
void load_menu();
void add(end *e);
void search(end e);
void view(end e);

FILE *fp;
FILE *f1;

int main(int argc, char** argv)
{

    load_menu();

    return 0;
}
void update()
{
    end st[15];
    int sayi[NUM_NUMBERS], number, i=0, j=0;
    fp=fopen("list.txt", "r");
    if( fp == NULL ) 
    {
        printf("File is not found at add();\n");
        exit(0);
    }
    while(!feof(fp))
    {
        fscanf(fp,"%d%d%s",&st[i].num,&st[i].id,st[i].name);
        i++;
    }
    Sort(st, NUM_NUMBERS);
    f1=fopen("index.txt", "w");
    for(int i=0; i<NUM_NUMBERS;i++)
    {
        fprintf(f1, "%d %d %s\n", st[i].id, st[i].num, st[i].name);
    }
}
void Sort(end array[], int n)
{
    int Min;
    for(int i=0; i<n-1;i++)
    {
        Min=i;
        for(int j=i+1;j<n;j++)
        {
            if(array[j].id<array[Min].id)
            {
                Min=j;
            }
        }
        end temp=array[i];
        array[i]=array[Min];
        array[Min]=temp;
    }
}
void load_menu(void)
{
    end e; 
    int choice;
    do
    {
        printf("1. Find a record given its ID value \n");
        printf("2. Add a new record to the file \n");
        printf("3. View Records\n");
        printf("4. Exit\n\n");
        printf("Please choose one: ");
        scanf("%d",&choice);
        switch(choice)
        {
            case 1:
                search(e);
                break;
            case 2: add(&e);
                    update();
                break;
            case 3: view(e);
                break;
            case 4: printf("Done.");
                return;
                break;
            default: printf("Invalid choice\n");
        }
    }while (choice != 5);
    system("cls");
} 
void add(end *e)
{
    int i=0;
    system("cls");
    fp = fopen ( "list.txt", "a" );
    if( fp == NULL ) 
    {
        printf("File is not found at add();\n");
        exit(0);
    }
    printf("\n-----Add a new record-----\n");
    printf("Enter number: ");
    scanf("%d", &e->num);
    printf("\nEnter ID : ");
    scanf("%d",&e->id);
    printf("\nEnter name: ");
    scanf("%s",e->name);
    fscanf(fp,"%d %d %s\n\n",&e->num, &e->id, e->name);
    fprintf(fp,"%d %d %s\n\n",e->num ,e->id, e->name);
    fclose(fp);
    return;
}
void search(end e)
{
    int i=0;
    int sid;
    system("cls");
    fp = fopen ("list.txt", "r");
    if(fp==NULL)
    {
        printf("File is not found at search();");
    }
    printf("\n-----Search ID-----\n");
    printf("\nEnter ID : ");
    scanf("%d",&sid);
    printf("\nNumber   ID       Name");
    while(!feof(fp))
    {
        fscanf(fp,"%d %d %s", &e.num, &e.id, &e.name);
        if(sid==e.id)
        {   
            printf("\n%d        %d        %s",e.num ,e.id, e.name);
        }
    }
    printf("\n\n");
    fclose(fp);
}
void view(end e)
{
    int i=0;
    system("cls");
    printf("\n-----list.txt-----\n");
    fp = fopen("list.txt", "r");
    if(fp == NULL)
    {
        printf("File is not found at view();\n");
        exit(0);
    }
    printf("\nNumber   ID       Name");
    printf("\n");
    while(fscanf (fp, "%d %d %s ",&e.num, &e.id, &e.name) != EOF )
    printf("\n%d        %d        %s",e.num ,e.id, e.name);
    printf("\n\n");

    printf("-----index.txt-----\n");
    f1 = fopen("index.txt", "r");
    if(fp == NULL)
    {
        printf("File is not found.\n");
        exit(0);
    }
    printf("\nNumber   ID       Name");
    printf("\n");
    while(fscanf (f1, "%d %d %s ",&e.id, &e.num, &e.name) != EOF )
    printf("\n%d        %d        %s",e.id ,e.num, e.name);
    printf("\n\n");

    fclose(fp);
    fclose(f1);
    return;
}

But I used only array so I need to dynamically allocate at array of struct to store the info. Still don't know how to use dynamically allocate (malloc). Could you please show me how to use dynamically with code? Thanks for helps. (Sorry for bad english.)

Answer 1

sample code:

#include <stdio.h>
#include <stdlib.h>

typedef struct student{   
    int num;
    int id;
    char name[100];
}end;

int cmp(void const *a, void const *b){
    const end *x = a;
    const end *y = b;
    return x->id < y->id ? -1 : x->id > y->id;
}

int main(void){
    end *st = NULL, tmp;
    FILE *fp;
    size_t i = 0, n = 0;

    fp=fopen("list.txt", "r");
    if( fp == NULL ) {
        perror("fopen at XXX");
        exit(EXIT_FAILURE);
    }
    while(3 == fscanf(fp,"%d %d %s", &tmp.num, &tmp.id, tmp.name)){
        end *temp = realloc(st, ++n * sizeof(*st));//Secure multiple records when the number of records is large
        if(temp == NULL){
            perror("realloc at XXX");
            free(st);
            exit(EXIT_FAILURE);
        }
        st = temp;
        st[i++] = tmp;
    }
    fclose(fp);

    qsort(st, n, sizeof(*st), cmp);

    fp=fopen("index.txt", "w");
    for(i = 0; i < n; ++i){
        fprintf(fp, "%d %d %s\n", st[i].id, st[i].num, st[i].name);
    }
    fclose(fp);
    free(st);
}

Answer 2

malloc() returns a pointer to the memory you allocated. The argument is the size of the memory you want to allocate, so in your case its the size of of "end".

You need to declare a pointer to "end" first and then call malloc().

end * ptr = malloc(sizeof(end));

But thats just one Element. You should definitely check out a tutorial for lists in c.