Dynamic struct array in function in C

Question

I'm trying to create adjacency list to represent a graph from a list of edges in file "input.txt". I know the basics of how pointers work but I have problem with dynamic struct array which consists of singly linked lists.

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

struct list {
    int vertex;
    struct list *next;
};

void create_list(int v, struct list* **array);

int main()
{
    int i, v = 5;
    struct list *ptr, **array = (struct list **)malloc(sizeof(struct list *) * v);
    for (i = 0; i < v; i++)
        array[i] = NULL;
    array[i] = NULL;

    create_list(v, &array);

    for(i = 0; i < v; i++) {
        ptr = array[i];
        printf("%d: ", i);
        while(ptr != NULL) {
            printf(" ->%d", ptr->vertex);
            ptr = ptr->next;
        }
        printf("\n");
    }
    return 0;
}

void create_list(int v, struct list* **array)
{
    int m, n;
    struct list *ptr, *tmp;
    FILE *luki;
    luki = fopen("input.txt", "r");
    while(fscanf(luki, "%d %d\n", &m, &n) == 2) {
        tmp = (struct lista*)malloc(sizeof(struct list));
        tmp->vertex = n;
        tmp->next = NULL;
        if (*array[m] == NULL)      //Here my program crashes when m changes from 0 to 1
            *array[m] = tmp;
        else {
            ptr = array[m];
            while(ptr->next != NULL)
                ptr = ptr->next;
            ptr->next = tmp;
        }
    }
    fclose(luki);
}

Could you please help me figure out how it should look like?

Also, at first I made the function without using pointer to array:

void create_list(int v, struct list **array)

create_list(v, array);

And it worked really good when debugging and (I'm using CodeBlocks):

0: 4 ->3 ->1
1: 2
2: 3
3:
4:

but while running the program normally I got this:

0:
1:
2:
3:
4:

Why the output while debugging was right if passing array to function create_list was wrong?

Welcome to Stack Overflow! [Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, May 12 '16 at 18:40
looks good `void create_list(int v, struct list **array); ... create_list(v, array);` — BLUEPIXY, May 12 '16 at 22:57
`create_list(v, &array);` isn't necessary. Because `create_list` doesn't change `array` itself. — BLUEPIXY, May 12 '16 at 23:12

score 0 · Accepted Answer · answered May 12 '16 at 23:02

(1) void create_list(int v, struct list **array); ... create_list(v, array);

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

struct list {
    int vertex;
    struct list *next;
};

void create_list(int v, struct list **array);

int main(void) {
    int i, v = 5;
    struct list *ptr, **array = (struct list **)malloc(sizeof(struct list *) * v);
    for (i = 0; i < v; i++)
        array[i] = NULL;

    create_list(v, array);

    for(i = 0; i < v; i++) {
        ptr = array[i];
        printf("%d: ", i);
        while(ptr != NULL) {
            printf("%d", ptr->vertex);
            ptr = ptr->next;
            if(ptr)
                printf(" -> ");
        }
        printf("\n");
    }
    return 0;
}

void create_list(int v, struct list **array){
    int m, n;
    struct list *ptr, *tmp;
    FILE *luki;
    luki = fopen("input.txt", "r");
    while(fscanf(luki, "%d %d\n", &m, &n) == 2) {
        tmp = (struct list*)malloc(sizeof(struct list));
        tmp->vertex = n;
        tmp->next = NULL;
        if (array[m] == NULL)
            array[m] = tmp;
        else {
            ptr = array[m];
            while(ptr->next != NULL)
                ptr = ptr->next;
            ptr->next = tmp;
        }
    }
    fclose(luki);
}

(2) void create_list(int v, struct list ***array); ... create_list(v, &array);

void create_list(int v, struct list ***array){
    int m, n;
    struct list *ptr, *tmp;
    FILE *luki;
    luki = fopen("input.txt", "r");
    while(fscanf(luki, "%d %d\n", &m, &n) == 2) {
        tmp = (struct list*)malloc(sizeof(struct list));
        tmp->vertex = n;
        tmp->next = NULL;
        if ((*array)[m] == NULL)
            (*array)[m] = tmp;
        else {
            ptr = (*array)[m];
            while(ptr->next != NULL)
                ptr = ptr->next;
            ptr->next = tmp;
        }
    }
    fclose(luki);
}

Dynamic struct array in function in C

1 Answers1