Unable to print linked list

Question

I cannot print the entire linked list.. what's my mistake? I'm new to linked list..

This is the base structure

struct data {
    int id;
    int age;
    struct data *next;
};

main function

int main() {
    int mode;
    struct data *initial;
    struct data *insert;
    struct data *temp;

    insert = initial;
    printf("1. Insert\n5. Print List\n");

    while (1) {
        printf("Mode: ");
        scanf("%d", &mode);
        if (mode == 1) {
            insert = malloc(sizeof(struct data));

            printf("Id: ");
            scanf("%d", &(insert->id));

            printf("Age: ");
            scanf("%d", &(insert->age));

            insert=insert->next;
        } else if (mode == 5) {
            temp = initial;
            while (temp != insert) {
                printf("Id: %d\tAge: %d\n", temp->id, temp->age);
                temp = temp->next;
            }
        }
    }
}

thank you for your help.

Answer 1

You never initialize initial to anything, not even NULL or 0, so there's no way you can chase a list from it sensibly.

You also need to set insert->next = NULL; after the two scanf() calls and before the assignment insert = insert->next;. You need to hook insert into the original list somehow; maybe with struct data *initial = 0; when it is declared and if (initial == 0) initial = insert; before the insert = insert->next;.

You also need to do more error checking: scanf() and malloc() can both fail, and you should do something sensible when they do.

Answer 2

In addition to the points made by @JonathanLeffler, I see problems in this block of code:

if(mode==1)
{
    insert=malloc(sizeof(struct data));
    printf("Id: ");scanf("%d",&(insert->id));
    printf("Age: ");scanf("%d",&(insert->age));
    insert=insert->next;
}

First time you execute this block, you do the following:

1. Create memory for a struct data and let insert point to it.
2. Fill up the data in the struct data.
3. Make insert become insert->next. But insert->next was not initialized to anything before the statement. At the end of the statement insert points to something completely unpredictable.
4. You have lost track of the memory you allocated since nothing points to it.

Second time you execute this block, you do the following:

1. Create memory for a struct data and let insert point to it.
2. Fill up the data in the struct data.
3. Make insert become insert->next. But insert->next was not initialized to anything before the statement. At the end of the statement insert points to something completely unpredictable.
4. You have lost track of the memory you allocated since nothing points to it.

What happened here?

1. You repeated the same process.
2. You have lost track of the memory allocated by two calls to malloc.
3. insert still points to something unpredictable.

This process repeats every time you execute that loop.

Here's what you can to to fix it.

if(mode==1)
{
    // Allocate memory and let temp point
    // to the allocated memory.
    temp=malloc(sizeof(struct data));

    // Fill up the data of the newly allocated memory.
    printf("Id: ");scanf("%d",&(temp->id));
    printf("Age: ");scanf("%d",&(temp->age));

    // Make temp clean by making its next point to NULL.
    temp->next = NULL;

    // Now figure out where to link this newly allocated
    // memory to the linked list.

    // Assuming you initialize insert correctly...
    if ( insert == NULL )
    {
       initial = insert = temp;
    }
    else
    {
       insert->next = temp;
       insert = temp;
    }
}

Hope that makes sense.