CODEWITHSUNDEEP
cstructlinked-listsingly-linked-list
SnorLaxatives
SnorLaxatives

Reputation: 5

Inserting at tail of linked list

I am writing a program to become familiar with linked lists and when attempting to insert a new link at the end of the list the value is seemingly not stored. I assume it has to do with a local variable not saving to the variable passed into the function.

The function in question:

int insertAtTail(int intBeingAdded, List *userList){
    while(userList->next != NULL){
        userList = userList->next;
    }
    List *newLink = (List *)malloc(sizeof(List));
    userList->next = newLink;
    newLink->next = NULL;
    newLink->value = intBeingAdded;
    return 1;
}

The entirety of the file:

#include "stdio.h"
#include "stdlib.h"

typedef struct list{
    int value;
    struct list * next;
} List;

List * initIntegerList(void);
int insertAtHead(int intBeingAdded, List *userList);
int insertAtTail(int intBeingAdded, List *userList);
void printList(List *userList);

int main(void){
    List *myList;
    myList = initIntegerList();
    insertAtHead(2, myList);
    insertAtHead(1, myList);
    insertAtTail(6, myList);
    printList(myList);
    freeList(myList);
}

List * initIntegerList(void){
    List * listPointer = (List *)malloc(sizeof(List));
    if(listPointer != NULL){
        listPointer->next = NULL;
        return listPointer;
    }else{
        printf("Memory not available for allocation\n");
        return listPointer;
    }
}

int insertAtHead(int intBeingAdded, List *userList){
    List *previousHead = (List *)malloc(sizeof(List));
    if(previousHead != NULL){
        previousHead->value = intBeingAdded;
        previousHead->next = userList->next;
        userList->next = previousHead;
        return 1;
    }
    return 0;
}

int insertAtTail(int intBeingAdded, List *userList){
    while(userList->next != NULL){
        userList = userList->next;
    }
    List *newLink = (List *)malloc(sizeof(List));
    userList->next = newLink;
    newLink->next = NULL;
    newLink->value = intBeingAdded;
    return 1;
}

void printList(List *userList){
    printf("Values in list: ");
    List *currentLink = userList;
    while(currentLink->next != NULL){
        printf(" %d", currentLink->value);
        currentLink = currentLink->next;
    }
    printf("\n");
}

The output I am seeing is only 0,1,2 are stored and the 6 is not making it.

Upvotes: 0

Views: 187

Answers (2)

Vlad from Moscow
Vlad from Moscow

Reputation: 311068

Your list is built such a way that the head node does not contain a value. It is a dummy node. On the other hand, the last node always contains the data member next equal to NULL.

So this condition in the function printList

while(currentLink->next != NULL){

does not output the value of the last node.

The function can be written like

void printList( const List *userList )
{
    printf("Values in list: ");

    if ( userList != NULL )
    {
        for ( const List *currentLink = userList->next; 
              currentLink != NULL;
              currentLink = currentLink->next )
        {
            printf( " %d", currentLink->value );
        }
    }

    printf("\n");
}

Pay attention to that it is a bad idea to have a dummy node as a head node. The head node should be just set to NULL.

Upvotes: 0

dbush
dbush

Reputation: 224387

When you print the list:

while(currentLink->next != NULL){

You stop when you reach the last node and don't print its contents. You instead want:

while(currentLink != NULL){

Also, your list has a dummy entry at the start of the list. When you print the list, you want to skip that one.

List *currentLink = userList->next;

Upvotes: 1

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