deleting all specific keys found in linked list - c++

Question

I am trying to delete all specific key element in my linked list in one function .
that is if linked list has 1 2 2 3 4 4 5 5 8 2 6 32 4 6 7 7 then if I pass the function 2 that function delete all the 2's in linked list
my linked list is here
class float_list
{
struct node
{
double data;
struct node *next;
};
node *head;
public:
float_list(void)
{
head = nullptr;
};
void appendNode(double);
void print_list();
void deleteNode(double);
};
now my deleteNode(double is here)
void float_list::deleteNode(double num)
{
node *nextptr, *previousptr = nullptr;
nextptr=head;
if(!head->data){return;}
if(head->data==num)
{
nextptr= head->next;
delete head;
head = nextptr;
}
else
while(nextptr)
{
previousptr= nextptr;
if(nextptr->data==num)
{
previousptr->next = nextptr->next;
delete nextptr;
cout<<"I Found the --> "<<num<<" is going to be deleted"<<endl;
nextptr = previousptr;
//nextptr = nextptr->next;
}
nextptr = nextptr->next;
}
delete nextptr;
delete previousptr;
}
I tried in all different ways but always get accesss violation error . Please give me conceptual and code hints if possible . thanks
the code is in win32 Vs2010 app

Answer 1

What happens after the while loop end. Well, nextptr == NULL. delete NULL == problem.
Try this:
node *previous = nullptr, *current = head, *temp;
while(current){
temp = current->next;
if(abs(current->data - num) < MARGIN_OF_ERROR){
if (previous){
previous->next = current->next;
} else {
head = current->next;
}
delete current;
} else{
previous = current;
}
current = temp;
}

Answer 2

something like this (pseudo-code)
public void removeData( double data )
{
if ( this.node == null ){ return; }
if ( this.node->data == data ){
this.node = this.node.node;
}
this.node.removeData( data );
}

Answer 3

Two issues I can trace:
You should never use operator== to check equality of two floating
point numbers, there is an issue with floating point arithmetic -
they are not exact, and result might not be as you expect [not a solution to your problem but definetly an issue]
Your previousptr and nextptr are the same thing [they are both pointing at the same address!]. You should modify previousptr before the current iteration. [right before nextptr = nextptr->next;]. Because of it, when you delete nextptr and later set:
nextptr = previousptr;
nextptr = nextptr->next;
You are actually accessing the element you just deleted, which causes your illegal access.

Answer 4

Probably this can be done through recursion, but here is a moving pointer solution:
void removenumber(node **head, int value)
{
node * current;
node * prev;
node * tmp;
current = (*head);
if ( current == NULL) return;
prev = current;
current = current->next;
while( current!= NULL && current->next != NULL)
{
if ( current->data == value)
{ //remove current
tmp = current->next;
prev->next = current->next;
free(current);
current = tmp;
//previous is the same
continue;
}
current = current->next;
prev = prev->next;
}
// now current->next != NULL check it
if(current!=NULL) //what is current was deleted?
{
if(current->data == value )
{
prev->next = NULL;
free(current);
}
}
if ( (*head)->data == value) //remove head
{
tmp = (*head)->next;
free(*head);
*head = tmp;
}
}