Using pointers and structures

**SolderGuy** · 11-28-2021

Hi everyone!

I hav been using C for a while and want to learn more and how to use pointers and structures but I need some help.

I have a structure like this:

Code:

struct Items
{
int* integerValue
struct Items* Next
};

in main I have this:

Code:

struct Items* MyNewList;

MyNewList = AddNewValue(9,NULL);
MyNewList = AddNewValue(93,MyNewList);

How do I add the new values in the function AddNewValue?
I'm out of ideas.

All help highly appreciated.

**aghast** · 11-28-2021

What you are trying to build is a Singly Linked list. If you're asking this question, I strongly encourage you to read the Wikipedia article.

Singly linked lists are not "the most important data structure everrrrh" but they are a very effective data structure for teaching students the basics of pointers and dynamic memory in C. For this reason, it is essential that you figure this stuff out - it's the most basic form of everything else you'll do with C and data structures.

In order to add new nodes to a list, you will need to do two things:

create a new node (probably using malloc)
insert the node in the list.

Creating the node is the simplest and most direct thing. You probably know how to do this already. I encourage you to write a function that creates a node, initializes it with values from the parameters, and returns the result.

**john.c** · 11-28-2021

The simplest way is the following. This prepends new items, so printing the list will yield the data in the reverse order that it was added.

Code:

#include <stdio.h>
#include <stdlib.h>
 
typedef struct Item {
    int data;
    struct Item *next;
} Item; // typedef allows us to say Item instead of struct Item
 
Item *listNewItem(int data, Item *next) {
    Item *item = malloc(sizeof *item);
    if (!item) { perror("listNewItem"); exit(EXIT_FAILURE); }
    item->data = data;
    item->next = next;
    return item;
}
 
Item *listAdd(Item *list, int data) {
    return listNewItem(data, list);
}
 
Item *listClear(Item *list) {
    for (Item *next = NULL; list; list = next) {
        next = list->next;
        free(list);
    }
    return NULL;
}
 
int main() {
    Item *list = NULL; // init list pointer to NULL
 
    for (int i = 0; i < 10; ++i)
        list = listAdd(list, i);
 
    for (Item *item = list; item; item = item->next)
        printf("%d ", item->data);
    printf("\n");
 
    list = listClear(list);
    return 0;
}

If you want to add the data to the end of the list, one possibility is to search for the end each time you add an item.

Code:

#include <stdio.h>
#include <stdlib.h>
 
typedef struct Item {
    int data;
    struct Item *next;
} Item;
 
Item *listNewItem(int data, Item *next) {
    Item *item = malloc(sizeof *item);
    if (!item) { perror("listNewItem"); exit(EXIT_FAILURE); }
    item->data = data;
    item->next = next;
    return item;
}
 
Item *listAdd(Item *list, int data) {
    Item *item = listNewItem(data, NULL);
 
    // If the list is empty, return item
    if (!list) return item;
 
    // Otherwise find the end of the list and append item there
    Item *p = list;
    for ( ; p->next; p = p->next) ;
    p->next = item;
 
    return list;
}
 
Item *listClear(Item *list) {
    for (Item *next = NULL; list; list = next) {
        next = list->next;
        free(list);
    }
    return NULL;
}
 
int main() {
    Item *list = NULL; // init list pointer to NULL
 
    for (int i = 0; i < 10; ++i)
        list = listAdd(list, i);
 
    for (Item *item = list; item; item = item->next)
        printf("%d ", item->data);
    printf("\n");
 
    list = listClear(list);
    return 0;
}

However, searching for the end of the list every time you add an item is not very efficient. Instead, you can keep track of the "tail" of the list.

Code:

#include <stdio.h>
#include <stdlib.h>
 
typedef struct Item {
    int data;
    struct Item *next;
} Item;
 
typedef struct List {
    Item *head;
    Item *tail;
} List;
 
Item *listNewItem(int data, Item *next) {
    Item *item = malloc(sizeof *item);
    if (!item) { perror("listNewItem"); exit(EXIT_FAILURE); }
    item->data = data;
    item->next = next;
    return item;
}
 
void listAdd(List *list, int data) {
    Item *item = listNewItem(data, NULL);
    if (list->tail)
        list->tail->next = item;
    else
        list->head = item;
    list->tail = item;
}
 
void listClear(List *list) {
    for (Item *item = list->head, *next = NULL; item; item = next) {
        next = item->next;
        free(item);
    }
    list->head = list->tail = NULL;
}
 
int main() {
    List list = {NULL, NULL}; // init head and tail pointers to NULL
 
    for (int i = 0; i < 10; ++i)
        listAdd(&list, i);
 
    for (Item *item = list.head; item; item = item->next)
        printf("%d ", item->data);
    printf("\n");

    listClear(&list); 
    return 0;
}

**SolderGuy** · 11-28-2021

Than you aghast and john.c for your answers. They are highly appreciated.

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