Copy subarray based on index array

Question

There are a bunch of answers on how to get a subarray with start and end index or start index and length. But I m looking for a way to get an subarray based on index array. Here is what I have (which works fine but seems clunky).

//sample arrays (could be unordered)
double[] expiry= { 0.99, 0.9, 0.75, 0.60, 0.5, 0.4, ...};
double[] values = { 0.245, 0.24, 0.235, 0.22, 0.21, 0.20, ... };


//find index of all elements meeting criteria in array expiry
int[] expind = expiry
  .Select((b, i) => b == 0.75? i : -1)
  .Where(i => i != -1).ToArray();

//create new arrays of appropriate size
double[] newvalues = new double[expind.Length];

//populate new arrays based on index
for (int i = 0; i < expind.Length; i++)
  newvalues[i] = values[expind[i]];

//check values
foreach (var item in newvalues)
   Console.WriteLine(item);

Is there a more efficient and general way of doing this please?

UPDATE

next attempt (Still not super efficient, but at least loopless):

Array.Sort(expiry, values);     
double criteria = 0.75;
int start = Array.IndexOf(expiry, criteria);
int last = Array.LastIndexOf(expiry, criteria);
int length = last - start + 1;
double[] newvalues2 = new double[length];
Array.Copy(values, start, newvalues2, 0, length);

Answer 1

Hi you can find the values in this way using lambda expression:

double[] newVals = values.Where((t, i) => expiry[i] == 0.75).ToArray();

Answer 2

This is a bit more concise. no need to actually put the indexes into an expind array; just use the indices directly with the Where() overload that takes an index:

double[] newpoints = points.Where((p, i) => (expiry[i] == 0.75)).ToArray();
double[] newvalues = values.Where((v, i) => (expiry[i] == 0.75)).ToArray();

See deeper discussion.

Now, if for some reason you already have an array of expind indices, but not the original array of expiry it came from, you can do this:

double[] newpoints = expind.Select(ind => values[ind]).ToArray();

Answer 3

Depending on the circumstances, this might work for you.

private static IEnumerable<double> GetByCondition(List<double> expiry, List<double> value)
{
  for(int i = 0; i < expiry.Count; i++)
    if(expiry[i] == 0.75)
      yield return value[i];
}

Furthermore, I'd put it as a extension method, if frequently used in your arrays/lists.

public static IEnumerable<double> GetValuesByExpiry(
  this List<double> self, List<double> values)
{
  return GetByCondition(self, values);
}

As @Corak mentioned, the problem might be eliminated all together if you merge those two arrays into a single one consisting of touples. If appropriate in your case, of course. You can probably zip them together.