How to find point between two keys in sorted dictionary

Question

I have a sorted dictionary that contains measured data points as key/value pairs. To determine the value for a non-measured data point I want to extrapolate the value between t

Answer 1

regular loop should be ok here:

IEnumerable keys = ...; //ordered sequence of keys
double interpolatedKey = ...;

// I'm considering here that keys collection doesn't contain interpolatedKey

double? lowerFoundKey = null;
double? upperFoundKey = null;

foreach (double key in keys)
{
    if (key > interpolatedKey)
    {
        upperFoundKey = key;
        break;
    }
    else
        lowerFoundKey = key;
}

You can do it in C# with LINQ with shorter but less effective code:

double lowerFoundKey = key.LastOrDefault(k => k < interpolatedKey);
double upperFoundKey = key.FirstOrDefault(k => k > interpolatedKey);

In order to it efficiently with LINQ it should have a method which is called windowed in F# with parameter 2. It will return an IEnumerable of adjacent pairs in keys collection. While this function is missing in LINQ regular foreach loop should be ok.