Algorithm to create a singly linked list

Question

I have an algorithm that should create a singly linked list from a textbook. It barely touched on any examples, so I would need some help figuring it out (still new to C.)

Essentially, the algorithm runs as follows:

Algorithm: CREATE (HEAD, ITEM)
1. [Create NEW node]
a) Allocate memory for NEW node.
b) IF NEW = NULL then Print: “Memory not Available” and Return
c) Set NEW→DATA = ITEM
d) Set NEW→LINK = NULL
2. [Whether List is empty, head is the content of HEADER]
If HEAD = NULL then Set HEAD = NEW
3. Else
a) Set Temp = HEAD
b) While Temp→LINK ≠ NULL do
Set Temp = Temp→LINK
[End of while]
c) Set Temp→LINK = NEW
[End of IF]
4. Return

Here is what I have tried so far, but I could not understand the arrow mappings in the algorithm. Are these existing C features?

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

typedef struct node {
    int DATA;
    struct node * LINK;
} node_t;


node_t *create(int head, int item){
    node_t* new =  NULL;
    new = (node_t *)malloc(5*sizeof(node_t));
    if(new == NULL){
        prtinf("Memory not available");
        return -1;
    }
}

Don't skip chapters in your course book. It's nonsensical to try to implement a linked list if you don't understand how struct members are accessed. — Cheatah
– Cheatah, Commented Dec 4, 2022 at 19:17
@Cheatah it does not go into much detail about structs, so I guess i'll have to look these up online. The books objective is really to provide a bunch of algorithms — Dollar X
– Dollar X, Commented Dec 4, 2022 at 19:24
The algorithm notation is entertaining, though! :) One of the best examples for "obfuscation by documentation". The source text of an implementation is less confusing than the text version. :) — BitTickler
– BitTickler, Commented Dec 4, 2022 at 19:33

Bhaskar13 · Accepted Answer · 2022-12-04 19:25:07Z

The arrow operator (->) in the algorithm is used to access a member of a struct using a pointer to that struct. It is equivalent to using the dot operator (.) to access a member of a struct, but with the added step of dereferencing the pointer to the struct. For example, if p is a pointer to a struct node, then p->DATA is equivalent to (*p).DATA.

In your code, you are using the node_t typedef to define the struct node type. This means that you can use node_t * to refer to a pointer to a struct node, and you can use node_t->DATA to access the DATA member of a struct node pointed to by node_t.

It looks like you are trying to implement the first step of the algorithm, which is to allocate memory for the new node and store the ITEM value in the DATA member of the node. You are using malloc to allocate memory for the new node, but you need to initialize the DATA member and the LINK member of the node. You can do this by using the arrow operator to access the members of the node:

node_t *create(int head, int item){
    node_t* new =  NULL;
    new = (node_t *)malloc(sizeof(node_t));
    if(new == NULL){
        printf("Memory not available");
        return -1;
    }

    new->DATA = item;  // Set the DATA member of the new node
    new->LINK = NULL;  // Set the LINK member of the new node to NULL
}

You also need to return the new node from the create function so that you can use it later in the algorithm.

node_t *create(int head, int item){
    node_t* new =  NULL;
    new = (node_t *)malloc(sizeof(node_t));
    if(new == NULL){
        printf("Memory not available");
        return -1;
    }

    new->DATA = item;  // Set the DATA member of the new node
    new->LINK = NULL;  // Set the LINK member of the new node to NULL

    return new;  // Return the new node
}

You can then use the create function to create a new node and store it in a local variable in your main program. This will allow you to continue implementing the rest of the algorithm.

int main() {
    int head = 0;  // The head of the linked list
    int item = 5;  // The data to store in the new node

    node_t *new_node = create(head, item);  // Create a new node

    // Continue implementing the rest of the algorithm...

    return 0;
}

Thorough explanation! Many thanks for sharing, I understood this and it was simple to digest.

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