CODEWITHSUNDEEP
cdata-structureslinked-list
aug2uag
aug2uag

Reputation: 3445

Adding element to front of linked list in C

I'm following the Stanford CS Ed Library tutorial on Linked Lists. I am trying to add a new list to the front of my linked list, and it's not working based on the printout I get from the Length function defined below.

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

//build new struct for node
//node has value and points to next node
struct node{
    int value;
    struct node *next;
};

//declare a new struct node that contains 3 nodes (head, middle, tail)
struct node *Build123(){
    struct node *head, *middle, *tail = NULL;

    head = malloc(sizeof(struct node));
    middle = malloc(sizeof(struct node));
    tail = malloc(sizeof(struct node));

    head->value = 3;
    head->next = middle;

    middle->value = 5;
    middle->next = tail;

    tail->value = 9;
    tail->next = NULL;

    return head;
};

//declare a function Length and variable counter to calculate size of list
int Length(struct node *head) {
    int count = 0;
    struct node *iterator = head;
    while (iterator != NULL) {
        count++;
        iterator = iterator->next;
    }
    return count;
}

//declare function Push to add new lists that would be added to the front
void Push (struct node **headRef, int value){
    struct node *newNode;
    newNode = malloc(sizeof(struct node));
    newNode->value = value;
    newNode->next = *headRef;
}

int main(){
    //instantiate the 3 element linked list named beast
    struct node *beast = Build123();

    //add 2 elements to the front of the linked list via pass by reference
    Push(&beast, 6);
    Push(&beast, 12);

    //calculate length of linked list after elements have been added
    int len = Length(beast);

    //print length of linked list to screen 
    printf("%d\n",len);
    return 0;
}

I get 3, when I expect to receive 5. Would you please assist me to find the error in the code that prevents me from obtaining the value I expect? I could not figure out why despite much tinkering. Thank you!

Upvotes: 6

Views: 8333

Answers (3)

Mani
Mani

Reputation: 17585

At the end of the push() method, you have to add:

*headRef = newNode

This is because headRef should always point to the first node in your linked list.

Upvotes: 1

Patashu
Patashu

Reputation: 21773

The problem is that when you do something like Push(&beast, 6); what beast points to is unchanged by the function Push. Even though Push adds more elements to the linked list, when you call Length on beast later it calls it on the same node that beast originally had at the start - so it is completely unknowing of the extra, added nodes.

At the end of Push(), you need to do this:

*headRef = newNode;

so that beast will correctly point to the new start of the list.

Upvotes: 4

Carl Norum
Carl Norum

Reputation: 224864

You don't modify headRef in your Push function, so your list's head never actually changes. beast always stays pointing to the original node it was created to point to. Add this line:

*headRef = newNode;

In Push(), and you'll be set.

Upvotes: 3

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