Not able to push when implementing linked list in C

Question

here's part of my code for the linked list:

struct node {
    float data;
    int key;
    struct node* next;
};

typedef struct{
    struct node *head;
    struct node *current;
    int length;
} linked_list;

linked_list *init_list(){
    linked_list *out = malloc(sizeof(linked_list));
    struct node *head = NULL;
    struct node *current = NULL;
    out->head = head;
    out->current = current;
    out->length = 0;
    return out;
}

void push_core(struct node *head, int key, float data){
    struct node *link = malloc(sizeof(struct node));
    link->data = data;
    link->key = key;
    link->next = head;
    // readjust to point at the new first node
    head = link;
    printf("%f; ", head->data);
}
void push(linked_list *list, int key, float data){
    push_core(list->head, key, data);
    list->length ++;
}

void print_list_core(struct node *head){
    struct node* ptr = head;
    printf("\n[");
    while(ptr != NULL){
        printf("(%d,%f)", ptr->key, ptr->data);
        ptr = ptr->next;
    }
}

void print_list(linked_list *list){
    print_list_core(list->head);
}

But in the main, after I initialized the linked list structure, I wasn't able to use push() to link new pointers, why is that?

linked_list *S = init_list();
for (int i = 0; i < n; i++)
{
    push(S,i,0);
    print_list(S);
    printf("%d;", S->length);
}

To clarify, the length of the list does update correctly. But when I try to print the list it doesn't work. Also, it's interesting that in another file when I initially just worked with the node struct and defined global variables for head and current, the code works fine. But when I try to wrap them up inside this linked_list struct, things aren't quite working as expected.

The C language passes function arguments by value. So updating head in push_core doesn't do anything to the head pointer in the list structure. — user3386109
– user3386109, Commented Feb 18, 2021 at 1:02
@user3386109 I thought I passed a pointer so it should write on that? So how should I edit the code to make it work? Should I change that to a double pointer? Thanks! — Macrophage
– Macrophage, Commented Feb 18, 2021 at 1:04
If I was writing the code, I'd get rid of the push_core and print_list_core functions, and just implement push and print_list. — user3386109
– user3386109, Commented Feb 18, 2021 at 1:07
I don't mean to be rude, but there are quite a few issues that you should look into with the debugger — Jack Lilhammers
– Jack Lilhammers, Commented Feb 18, 2021 at 1:07

biqarboy · Accepted Answer · 2021-02-18 01:12:17Z

1

The problem occurred because you passed the pointer value of list->head to your push_code function as a parameter. This is a function call-by-value. So, when you change the head pointer inside the push_core function, it actually do not change the list->head pointer that you are expecting to. One quick fix would be returning the newly created link pointer from the push_core function and save it as list->head. The following code should fix your problem.

struct node * push_core(struct node *head, int key, float data){
    struct node *link = malloc(sizeof(struct node));
    link->data = data;
    link->key = key;
    link->next = head;
    
    return link;
}

void push(linked_list *list, int key, float data){
    list->head = push_core(list->head, key, data);
    list->length ++;
}

answered Feb 18, 2021 at 1:12

biqarboy

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