Thread: Linked list scanning from a file?

Make a program that reads entries from a file one at a time until it reaches the end. Once you can do that, make a program read entries one at a time, and then it copies the one it has just read into a structure. Once you can do that, make those structures be added to a linked list one at a time after you read them.


Quzah.

Well... this is what I have now, and it's still not working. What am I missing? D:

It says the friend_t head; variable isn't working.

This is completely incomplete so ignore everything except the linked list part.

Code:

#include <stdio.h>
#include <strings.h>
#include <stdlib.h>
#define SIZE 20
#define SIZE2 100
#define LEN 20


typedef struct friend friend_t;


typedef struct person
{
    char *name;
    int age;
    int id_number;
    friend_t* friends;
}    person_t;


struct friend
{
    person_t *user;
    struct friend* next;
};


void print_users(person_t *s)
{
    int i;


    for(i=0;i<SIZE;i++)
        printf("\n%s: Age: %d, ID Number: %d\n",s[i].name,s[i].age,s[i].id_number);
}


void find_user(char *tempc, person_t *t, person_t *arr)
{
    int i;


    for(i=0;i<SIZE;i++)
    {
        if(strcmp(tempc, arr[i].name) == 0)
            t = arr;
    }
}


friend_t *find_friends(person_t *arr, char *separator, FILE* data)
{


    friend_t *t1, *h, *p;
    person_t *t2;
    char *tempc = 0; // used to check for separator


    fscanf(data, "%s", tempc);
    while(tempc != separator)
     {
        t1 = (friend_t*)malloc(sizeof(friend_t)); // memory allocation
        t2 = (person_t*)malloc(sizeof(person_t)); // "        "
        find_user(tempc, t2, arr); // ?!
        t1->next = NULL;
        t1->user = t2;
        t1->user->name = tempc;


        if(p == NULL)
            h = t1;
        else
         {
            p->next = t1;
            p = t1;
         }
     }
    return h;
}


int main(void)
{
    person_t s[SIZE];
    /* Say you array of people is person_t people[20] in your main function.  Then people[0]. friend_list is a head pointer to a struct of friend_t.*/
    friend_t *head = 0;
    char *separator = "~";
    int i;
    int temp = 1;
    int get;


    FILE* data = fopen("final_data.dat", "r");


    for(i=0;i<SIZE;i++)
        fscanf(data, "%s %d %d" ,s[i].name,&s[i].age,&s[i].id_number);


    head = find_friends(s, separator, data);


    while(temp == 1) {
    printf("List of commands: \n\t(1) View users \n\t(2) Add Friend \n\t(3) Delete Friend \n\t(4) Modify Attribute \n\t(5) Sort by attribute \n\t(6) Find degree of separation \n\t(7) Find user with most friends \n\t(8) Find user with least friends \n\t(9) Check to see if a friendship exists \n\t(10) Quit");
    printf("\n\nEnter a command: ");
    scanf("%d", &get);


    switch(get)
     {
        case 1:
            print_users(s);
            break;
        case 2:
            // add_friend();
            break;
        case 3:
            // delete_friend();
            break;
        case 4:
            // change_atr();
            break;
        case 5:
            // sort();
            break;
        case 6:
            // separation();
            break;
        case 7:
            // most_fr();
            break;
        case 8:
            // least_fr();
            break;
        case 9:
            // check_fr();
            break;
        case 10:
            temp = 0;
            break;
     }
    }


    printf("\n\n");
    return 0;
}

> fscanf(data, "%s %d %d" ,s[i].name,&s[i].age,&s[i].id_number);
You're missing out the fact that each .name is a POINTER which is presently uninitialised.

> char *tempc = 0; // used to check for separator
> fscanf(data, "%s", tempc);
Well it's initialised at least, but it is a NULL pointer, which is a guaranteed crash on most systems.

Each time you write a * in your code to create a pointer, you need to stop and think for a while exactly how it's going to be used and where the memory it points to is coming from.