STD Lists doubt

[lautarox]

I need to store multiple classes in a list in order to access the data by a number, but the list is in continuous modification, so I can't access the data using the position of the data in the list. I think the better way is to scan for the data number, but in that case I need to return an iterator, how can I return an iterator from a function? Do I have an alternative to this?
Thanks in advance

[Daved]

It's easy to return an iterator, just return it like you would any other type:

std::list<MyClass>::iterator foo();

Are you sure that's the best way, though? If you're accessing the data by a number, perhaps a map would be more appropriate? Instead of scanning the list for that number, which is O(n), you would be looking it up in a map which is O(log n). You could also use a hashed map (like unordered_map) which if setup correctly would give you O(1) lookup.

If the order you keep the item in the container is important, then that might not be the best solution, though. In that case you'd have two keys, one being the ordering in the container and the other being the number you referred to.

[lautarox]

I've read about maps

Classname tanto;

map<int,Classname> first;

first.insert(1, tanto);

Now, to access the class data, is it first[1].function(); or first->function(); ?

[Daved]

map is a standard container just like list or vector. It maps a key to a value. So if your class had some type of unique ID, you could map that ID to the class object. Then, when you want to get the object from the container, you just lookup the key instead of scanning each element trying to find it.

#include <iostream>
#include <map>
#include <string>

class my_data
{
public:
  std::string text;
  my_data(const char* t = "") : text(t) { }
};

int main()
{
    std::map<int, my_data> data_map;

    // insert new elements
    data_map.insert(std::make_pair(1, my_data("one")));
    data_map.insert(std::make_pair(1200, my_data("twelve hundred")));
    data_map.insert(std::make_pair(5, my_data("five")));

    // insert or change existing elements
    data_map[10] = my_data("ten");  // insert
    data_map[1200] = my_data("one thousand two hundred");  // change
    data_map[77] = my_data("seventy seven");  // insert

    // Look up a single element:
    int lookup = 10;
    std::map<int, my_data>::const_iterator one_item = data_map.find(lookup);
    if (one_item != data_map.end())
        std::cout << one_item->second.text << " (" << lookup << ")\n";
    else
        std::cout << lookup <<  " not found.\n";

    lookup = 100;
    one_item = data_map.find(lookup);
    if (one_item != data_map.end())
        std::cout << one_item->second.text << " (" << lookup << ")\n";
    else
        std::cout << lookup <<  " not found.\n";

    // Lookup a single element (but automatically create a new one if it isn't there):
    lookup = 5;
    std::cout << data_map[lookup].text << " (" << lookup << ")\n";
    lookup = 50;
    std::cout << data_map[lookup].text << " (" << lookup << ")\n";

    // Show everything in the map:
    std::map<int, my_data>::const_iterator map_end = data_map.end();
    for (one_item = data_map.begin(); one_item != map_end; ++one_item)
    {
        std::cout << "Key: " << one_item->first;
        std::cout << " Value: \"" << one_item->second.text << "\"\n";
    }

    // done
    std::cin.get();
}

(Note: I just typed this up, I didn't test it.) Notice how you can insert in any order, but the loop at the end outputs them in sorted order. Also notice how you can use operator[] or iterators and member functions to access the map contents. The lookups in the map are faster algorithmically than list lookups.

[lautarox]

I can't thank you enough! I really needed this