Getting user input for the deletion of the desired linked list element

Question

NodePointer deleteNode(NodePointer head,int number)
{
    NodePointer ptrNode;
    NodePointer ptrNodePrev;
    int count;
    if(head == NULL)
    {
        printf("There is no element at list");
    }
    if(number == 1)
    {
       ptrNodePrev=head;
       head=head->next;
       free(ptrNodePrev);
    }
    else{
        ptrNode=head;
        while(ptrNode->next != NULL && count+1 != number){
            count++;
            ptrNodePrev=ptrNode;
            ptrNode=ptrNode->next;
        }
        ptrNode=ptrNode->next;
        printf("Node: %d-%d-%d - %s",ptrNodePrev->next->x.year,
                                      ptrNodePrev->next->x.month,
                                      ptrNodePrev->next->x.day,
                                      ptrNodePrev->next->x.event);
        printf("Successfully deleted");
        free(ptrNodePrev->next);
        ptrNodePrev->next=ptrNode;
    }
    return head;
}

Hi I've been learning single linked lists and wanted to do a example about it.Basically i ask user to input how many nodes does he want and then he fills them. After that i ask them to delete a node then they will enter a number which will supposedly deletes the according node.Its fine if the number inputed is not 1 but if it is then i would basically get random numbers. I dont get why it happens i looked through most of the questions about this topic here but couldnt find reason.Can anyone explain my mistake please ?

Answer 1

Actually when number is equal to 1 then the function works correctly.

NodePointer deleteNode(NodePointer head,int number)
{
    NodePointer ptrNode;
    NodePointer ptrNodePrev;
    int count;
    if(head == NULL)
    {
        printf("There is no element at list");
    }
    if(number == 1)
    {
       ptrNodePrev=head;
       head=head->next;
       free(ptrNodePrev);
    }
    else
    {
        //...
    }
    return head;
}

Maybe in main you forgot to assign the return value of the function to the pointer to the head node like

head = deleteNode( head, 1 );

Otherwise the function has undefined behavior because 1) the variable count is not initialized and 2) you do not check that after the while loop count + 1 is equal to number

    while(ptrNode->next != NULL && count+1 != number){
        count++;
        ptrNodePrev=ptrNode;
        ptrNode=ptrNode->next;
    }
    ptrNode=ptrNode->next;
    //...

Pay attention to that in C indices start from 0. And the second parameter of the function should have an unsigned integer type as for example size_t. Otherwise the user is allowed to pass to the function a negative value.

Also the function should not issue any message.

The function can be declared and defined the following way.

int deleteNode( NodePointer *head, size_t n )
{
    while ( n-- && *head != NULL ) head = &( *head )->next;

    int success = *head != NULL;

    if ( success )
    {
         NodePointer current = *head;
         *head = ( *head )->next;
         free( current );
    }

    return success;
}

And if in main you have a declaration of the pointer like

NodePointer head = NULL;
//...

then the function is called like

deleteNode( &head, n );

where n is some value specifying an index in the list starting form 0. That is if you want to delete the first node then you should write

deleteNode( &head, 0 );