How to insert element in first index in dictionary?

Question

Is there a method or technique that allows you to insert an element into a Dictionary<TKey, TValue> guaranteeing that the item is in the first index of that dictionary's KeyCollection.

For example:

Dictionary<String, String> dic = foo.GetOutput(); 

// `dic` is something like:

// {"foo", "baa"},
// {"a", "b"}

I need something like:

dic.Add("key", "value", 0);
// where `0` is the index that `key` to be inserted.

foreach(KeyValuePair<String, String> key in dic) 
{
     Console.WriteLine("{0} = {1}", key.Key, key.Value);
}

Output:

key = value
foo = baa
a = b

Answer 1

By not using a dictionary.

Dictionary<TKey, TValue> is implemented as a hash-table. The position of keys internal to the dictionary depends upon the hash-code, the means by which that hash-code was reduced further to provide an index into its internal structure, and the order of insertion in an entirely implementation-dependant way.

This isn't the only way to implement a dictionary. SortedDictionary<TKey, TValue> uses a tree structure internally and so always keeps keys in an order. In this case we still can't insert something in the beginning, rather we insert something and it gets put in the appropriate place.

If ordering is what you care about most, then you don't want a puredictionary at all. Rather you want either a List<KeyValuePair<TKey, TValue>> or you want a structure that offers both the functionality of a list and of a dictionary, which is provided by OrderedDictionary. This isn't generic, but you can easily create a generic wrapper around it (doesn't give the performance benefits of internally using generics, but does give type-safety in use).

Answer 2

I know it is a three years old question. But found a workaround of this problem. It may help someone

Dictionary<String, String> dic = foo.GetOutput();

dic = (new Dictionary<string, string> {{"key","value"}}).Concat(dic).ToDictionary(k => k.Key, v => v.Value);

This will insert the element in the beginning of dictionary :)

Answer 3

Dictionaries are unordered; elements are meant to be retrieved with a key, whose hash points to its value's location.

What you might want is a List <KeyValuePair>, whose elements can be inserted into a specific index.

List<KeyValuePair<string, string>> list = dic.ToList();
list.Insert(0, new KeyValuePair<string, string>("a", "b"));

foreach(KeyValuePair<string, string> pair in list)
    Console.WriteLine("{0} = {1}", pair.Key, pair.Value);

Answer 4

This is not possible with Dictionary<TKey, TValue> as it presents it's values in an unordered fashion when enumerated. There is SortedDictionary<TKey, TValue> which provides ordering but it does so by using an IComparer<TKey> against the key value directly. Here you want the key to be a String and have ordering based on an int. That is not possible with either of these types.

I think you'll need to implement a new type with these very specific semantics in them. For example.

class OrderedMap<TKey, TValue> {
  private readonly Dictionary<TKey, TValue> _map = new Dictionary<TKey, TValue>();
  private readonly List<TKey> _list = new List<TKey>();

  public void Add(TKey key, TValue value) {
    if (!_map.ContainsKey(key)) {
      _list.Add(key);
    }
    _map[key] = value;
  }

  public void Add(TKey key, TValue value, int index) {
    if (_map.ContainsKey(key)) {
      _list.Remove(key);
    }
    _map[key] = value;
    _list.Insert(index, key);
  }

  public TValue GetValue(TKey key) {
    return _map[key];
  }

  public IEnumerabe<KeyValuePair<TKey, TValue>> GetItems() {
    foreach (var key in _list) { 
      var value = _map[key];
      yield return new KeyValuePair<TKey, TValue>(key, value);
    }
  }
}

Note this does come with some non-trivial performance differences over a traditional Dictionary<TKey, TValue>. For example Add and Remove are slower.

Answer 5

this is my solution, maybe not the best solution but it works. =)

public static ComboBox FillDropDownList(Dictionary<String, String> dictionary, ComboBox dropDown, String selecione)
{
    var d = new SortedDictionary<String, String>();

    d.Add("0", selecione);

    foreach (KeyValuePair<string, string> pair in dictionary)
    {
        d.Add(pair.Key, pair.Value);
    }

    dropDown.DataSource = new BindingSource(d, null);
    dropDown.DisplayMember = "Value";
    dropDown.ValueMember = "Key";

    dropDown.SelectedIndex = 0;

    return dropDown;
}

Answer 6

Dictionary<TKey, TValue> is inherently unordered (or rather, the ordering is unpredictable and shouldn't be relied upon). If you want some sort of ordering, you need to use a different type. It's hard to recommend any particular type without knowing more about your requirements.

Answer 7

The Dictionary<TKey,TValue> class does not hold items in an ordered manner, so there is no "first" item.

There is a SortedDictionary<Tkey,TValue> (.NET 4.0+), which sorts by the key, but again, this is a very vague idea of "first".

Answer 8

The Dictionary<TKey, TValue> can't be ordered.

You can try SortedDictionary<TKey, TValue> instead, but that one is ordered by the Key, not by a separate index.

Answer 9

A Dictionary is an un-ordered collection. You could try OrderedDictionary - http://msdn.microsoft.com/en-us/library/system.collections.specialized.ordereddictionary.aspx - which has an Insert() method which is what you're after.