c# - Efficiently find nearest dictionary key

Question

I have a bunch of pairs of dates and monetary values in a SortedDictionary<DateTime, decimal>, corresponding to loan balances calculated into the future at contract-defined compounding dates. Is there an efficient way to find a date key that is nearest to a given value? (Specifically, the nearest key less than or equal to the target). The point is to store only the data at the points when the value changed, but efficiently answer the question "what was the balance on x date?" for any date in range.

A similar question was asked ( What .NET dictionary supports a "find nearest key" operation? ) and the answer was "no" at the time, at least from the people who responded, but that was almost 3 years ago.

The question How to find point between two keys in sorted dictionary presents the obvious solution of naively iterating through all keys. I am wondering if any built-in framework function exists to take advantage of the fact that the keys are already indexed and sorted in memory -- or alternatively a built-in Framework collection class that would lend itself better to this kind of query.

Answer 1

Since SortedDictionary is sorted on the key, you can create a sorted list of keys with

var keys = new List<DateTime>(dictionary.Keys);

and then efficiently perform binary search on it:

var index = keys.BinarySearch(key);

As the documentation says, if index is positive or zero then the key exists; if it is negative, then ~index is the index where key would be found at if it existed. Therefore the index of the "immediately smaller" existing key is ~index - 1. Make sure you handle correctly the edge case where key is smaller than any of the existing keys and ~index - 1 == -1.

Of course the above approach really only makes sense if keys is built up once and then queried repeatedly; since it involves iterating over the whole sequence of keys and doing a binary search on top of that there's no point in trying this if you are only going to search once. In that case even naive iteration would be better.

Update

As digEmAll correctly points out, you could also switch to SortedList<DateTime, decimal> so that the Keys collection implements IList<T> (which SortedDictionary.Keys does not). That interface provides enough functionality to perform a binary search on it manually, so you could take e.g. this code and make it an extension method on IList<T>.

You should also keep in mind that SortedList performs worse than SortedDictionary during construction if the items are not inserted in already-sorted order, although in this particular case it is highly likely that dates are inserted in chronological (sorted) order which would be perfect.