How do I get previous key from SortedDictionary?

Question

I have dictionary containing key value pairs.

SortedDictionary dictionary=new SortedDictionary();
dictionary.Add(1,33);
diction         


        

Answer 1

KeyValuePair<int, int> lookingForThis = dictionary
  .Reverse()
  .SkipWhile(kvp => kvp.Key != 4)
  .Skip(1)
  .FirstOrDefault();

Answer 2

I was also looking for an answer to this problem, and I thought a better solution than all of the answers here is to use the TreeDictionary<K, V> from the C5 Collections (GitHub/NuGet), which is an implementation of a red-black tree.

It has Predecessor/TryPredecessor and WeakPredessor/TryWeakPredecessor methods (as well as equivalent methods for successors) which does exactly what you want.

For example:

TreeDictionary<int,int> dictionary = new TreeDictionary<int,int>();
dictionary.Add(1,33);
dictionary.Add(2,20);
dictionary.Add(4,35);

// applied to the dictionary itself, returns KeyValuePair<int,int>
var previousValue = dictionary.Predecessor(4);
Assert.Equals(previousValue.Key, 2);
Assert.Equals(previousValue.Value, 20);

// applied to the keys of the dictionary, returns key only
var previousKey = dictionary.Keys.Predecessor(4);
Assert.Equals(previousKey, 2);

// it is also possible to specify keys not in the dictionary
previousKey = dictionary.Keys.Predecessor(3);
Assert.Equals(previousKey, 2);

Answer 3

You could loop through the dictionary and keep track of values, I guess. Something like this:

public int GetPreviousKey(int currentKey, SortedDictionary<int, int> dictionary)
{
    int previousKey = int.MinValue;
    foreach(KeyValuePair<int,int> pair in dictionary)
    {
        if(pair.Key == currentKey)
        {
            if(previousKey == int.MinValue)
            {
                throw new InvalidOperationException("There is no previous key.");
            }
            return previousKey;
        }
        else
        {
            previousKey = pair.Key;
        }
    }
}

However, this is a pretty odd operation to require. The fact that you need it might be pointing at a problem with your design.

Answer 4

I would prefer to use linq if that's the case... try this it will surely work

SortedDictionary<int,int> dictionary = new SortedDictionary<int,int>();
dictionary.add(1, 33);
dictionary.add(2, 20);
dictionary.add(4, 35);

int SelectedKey = 4;

var ResutValue = ( from n in dictionary    
               where n.Key < TheSelectedKey
               select n.Value).Last();

this.txtResult.Text = ResultValue.ToString();

Answer 5

Dictionary<TKey,TValue> is unsorted, so there is no previous thing for some item. Instead you can use SortedDictionary<TKey,TValue>.

EDIT: Sorry I read the title only LOL.

If you have to use LINQ:

int givenKey = 4;
var previousItem = dict.Where((pair, index) => 
                                   index == dict.Count || 
                                   dict.ElementAt(index + 1).Key == givenKey)
                       .FirstOrDefault();

Answer 6

How about this? I havent tested it though, but should give something to start think in this direction. Hope it helps.

        int input = 4;
        List<int> lKeys = dictionary.Keys.ToList();
        int reqIndex = lKeys.IndexOf(input) - 1;
        int reqAnswer = dictionary[reqIndex];

test for other conditions like if (reqIndex != -1) etc..