List Class

Dears

I created a list class as

list<string> mylist

then added data
mylist.push_back("A");
mylist.push_back("B");
mylist.push_back("C-VIP");
mylist.push_back("D-VIP");


Now, I would like to create another list for keeping track of the
address of certain entries in the mylist class such as:

list<whatever??> myptr;

I would like myptr to have the addresses of the VIP entries above. Is
that possible?

How?

Thank You

HAOB BOY

The list class is designed to keep you away from raw access to pointers (That's why iterators were invented). The other problem you face is that when a list is updated the iterators become invalidated, so you can't store them easily either. Perhaps there is a way around this limitation. What is the purpose of keeping track of the addresses (pointers?) of the VIP entries?

may be the VIP example is not good but the purpose I want to keep track of certain nodes in a Linked List is that I am creating a memory-based database for a school project. There are several linked lists inmy project: one of them contains the table definitions which I will refer to whenever I need to add a record to the table linked list.

My probelm is how to know that a certain node is a definistion for certain table.

Thanx

Have you thought of using sets or multisets?

Quote:

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Originally Posted by binarybob0001

The other problem you face is that when a list is updated the iterators become invalidated,

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This is not necessarily true for lists. The iterator to an element remains valid for the lifetime of the list, unless the element is removed from the list.

Regards,

Paul McKenzie

Quote:

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Originally Posted by HAOBBOY

may be the VIP example is not good but the purpose I want to keep track of certain nodes in a Linked List

---

Then you need to keep track of the iterator to an element in the list, not pointer to an element in the list.

Basically when dealing with std::list, you cannot use pointers to address elements, only iterators. Yes, a list::iterator may be implemented as a pointer, but iterators and pointers are not the same thing.

Regards,

Paul McKenzie

Thank you all

can I do this:

string d;
d="mylist";
list<string> d.c_str();

I would like to be able to create linked list at run time?

Quote:

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Originally Posted by HAOBBOY

Thank you all

can I do this:

string d;
d="mylist";
list<string> d.c_str();

I would like to be able to create linked list at run time?

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Your linked list would be created at runtime. But why do you want to name it at runtime?

Because, I am reading a file that has names and I need to make a linked list for each name.

Have a map with the following specifications:

Code:

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std::map<std::string, std::list<whatever> > myMapOfLists;

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This will help you achieve what you want. Regards.

Quote:

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Originally Posted by HAOBBOY

Because, I am reading a file that has names and I need to make a linked list for each name.

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In that case, you can use a std::map<std::string, std::list<std::string> >
The key of the map, would be the "name", and the value, would be the list.

Quote:

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Originally Posted by Paul McKenzie

Then you need to keep track of the iterator to an element in the list, not pointer to an element in the list.

Basically when dealing with std::list, you cannot use pointers to address elements, only iterators. Yes, a list::iterator may be implemented as a pointer, but iterators and pointers are not the same thing.

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Even when adding or removing elements in a list, references (and pointers) to other objects are not at all invalidated.

For example, lib.list.modifiers says that

Quote:

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iterator erase(iterator position);
iterator erase(iterator first, iterator last);

void pop_front();
void pop_back();
void clear();.

Effects:
Invalidates only the iterators and references to the erased ele-
ments.
Throws:
Nothing.

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And, having pointers to items of any container (except std::vector<bool> which does not met container requirements), it is perfectly valid to have references to items inside the container, as long as no modifying functions are called.

Can I do nested maps

Code:

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std::map<std::string, std::map<std::string,std::string> > myMapOfLists;

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the above gives me error

Guys/Gals,

I managed to to have nested map, but how can I add data to the 2nd map then add this to the mother map?

Also, how can I reference the elements inside these maps?

How can I declare an iterator for the nested map?

Quote:

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Originally Posted by SuperKoko

Even when adding or removing elements in a list, references (and pointers) to other objects are not at all invalidated.

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I think I stated that. For lists, unless the element that the iterator is referring to is erased, iterators are still valid, regardless of any modifications done to the list.

Regards,

Paul McKenzie

Why do you need a nested map? According to what you asked for - what SuperKoko/I suggested would do the job!

Anyways, iteration would look something like this:

Code:

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#include<map>
#include<string>

using namespace std;

int main()
{
  map<string, map<string, string> > myMapOfMaps;
  for(map<string, map<string, string> >::const_iterator it = myMapOfMaps.begin();it!=myMapOfMaps.end();++it)
  {
        for(map<string, string>::const_iterator itInternal = (it->second).begin(); itInternal!=(it->second).end(); ++itInternal)
        {
            //do something
        }
  }
}

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You may also use algorithms to help you ease with this as needed. Pushing elements into a map should be easy - if you are able to deal with simple map<string, string> then I am sure you can deal with map<string, map>. Or let us know your code and what is the exact thing that you are finding difficult to express in code. Regards.

When you guys told me that it is possible to have nested containers, you solved two of my problems. So the nested map was another problem.

Anyhow, thank you.