How to convert an IEnumerable> to a IEnumerable

Question

I have an IEnumerable<IEnumerable<T>> collection that I want to convert to a single dimension collection. Is it possible to achieve this with a generic extension method? Right now I'm doing this to achieve it.

List<string> filteredCombinations = new List<string>();

//For each collection in the combinated results collection
foreach (var combinatedValues in combinatedResults)
{
    List<string> subCombinations = new List<string>();
    //For each value in the combination collection
    foreach (var value in combinatedValues)
    {

        if (value > 0)
        {
            subCombinations.Add(value.ToString());
        }
    }
    if (subCombinations.Count > 0)
    {
       filteredCombinations.Add(String.Join(",",subCombinations.ToArray()));
    }
}

If it's not possible to get a generic solution, how can I optimize this in an elegant fashioned way.

the question in the title is answered here: http://stackoverflow.com/questions/1590723/flatten-list-in-linq — Matt Ellen, Feb 10 '10 at 22:34

driis · Answer 1 · 2010-02-10T22:11:54.967

18

You can use the Enumerable.SelectMany extension method for this.

If I read your code correctly, the code for that would be:

var filteredCombinations = combinatedResults.SelectMany(o => o)
    .Where(value => value > 0)
    .Select(v => v.ToString());

Edit: As commented, the above code is not joining each element of the subsets to a string, as the original code does. Using the built-in methods, you can do that using:

var filteredCombinations = combinatedResults
     .Where(resultSet => resultSet.Any(value => value > 0)
     .Select(resultSet => String.Join(",",
         resultSet.Where(value => value > 0)
                  .Select(v => v.ToString()).ToArray()));

edited Feb 10 '10 at 22:11

answered Feb 10 '10 at 21:08

driis

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What if you want every subset to be one element of the new collection? That's what I do with the String.Join... – John Doe Feb 10 '10 at 21:30
@Earwicker, I fail to see how I will get empty strings in the result. The resultSet must start out as an enumerable of a numeric type, since the elements is being compared to zero in the orignal code. (I admit I typed the code without Visual Studio, so feel free to correct me :-) – driis Feb 10 '10 at 22:06
There are two `if` statements in the original code. You only have one `Where` clause. You've kept the one that filters out the zero values, but you have missed the one that filters out empty sub-lists. A list containing all zeros will become an empty array, which you pass to `Join`. The original code doesn't do that. – Daniel Earwicker Feb 10 '10 at 22:09
Hmm, now you're querying every `resultSet` twice with the same expression. Sorry to be picky... you could just copy my answer to save time! :) – Daniel Earwicker Feb 10 '10 at 22:14

score 3 · Answer 2 · edited May 23 '17 at 12:01

3

I would personally use Enumerable.SelectMany, as suggested by driis.

However, if you wanted to implement this yourself, it would be much cleaner to do:

IEnumerable<T> MakeSingleEnumerable<T>(IEnumerable<IEnumerable<T>> combinatedResults)
{
    foreach (var combinatedValues in combinatedResults) {
         foreach (var value in combinatedValues)
              yield return value;
    }
}

edited May 23 '17 at 12:01

Community

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1

answered Feb 10 '10 at 21:13

Reed Copsey

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Matt Howells · Accepted Answer · 2010-11-25T13:42:47.413

3

Here you go:

var strings = combinedResults.Select
    (
        c => c.Where(i => i > 0)
        .Select(i => i.ToString())
    ).Where(s => s.Any())
    .Select(s => String.Join(",", s.ToArray());

edited Nov 25 '10 at 13:42

answered Feb 10 '10 at 21:49

Matt Howells

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1

This whole question has been bizarre! The wording asked one question, the example asked another. And you've answered a third, different question altogether, and got accepted... I've already posted the correct answer, and have no votes (not that I mind, it's just amusing.) – Daniel Earwicker Feb 10 '10 at 22:05
I think your `Implode` would be more elegant if it just operated on `IEnumerable`. If you want a conversion on each item of some other type of sequence, use `Select` first. – Daniel Earwicker Feb 10 '10 at 22:17
Your implementation of Implode would probably be more efficient and certainly more concise if it simply used `string.Join` rather than StringBuilder. i.e. `return string.Join(separator,sequence.ToArray())` – Eamon Nerbonne Nov 17 '10 at 10:35
@Eamon: Having done a bit of digging into the String.Join implementation, I think you are dead right. Fixed. – Matt Howells Nov 25 '10 at 13:43

score 2 · Answer 4 · answered Feb 10 '10 at 21:54

You asked two different questions. The one you described in the title is already answered by drilis.

But your example code is a different problem. We can refactor it in stages. Step 1, build the subCombinations list using some Linq:

List<string> filteredCombinations = new List<string>();

//For each collection in the combinated results collection
foreach (var combinatedValues in combinatedResults)
{
    var subCombinations = combinatedValues.Where(v => v > 0)
                                          .Select(v => v.ToString())
                                          .ToList();

    if (subCombinations.Count > 0)
       filteredCombinations.Add(string.Join(",",subCombinations.ToArray()));
}

Now the outer loop, leaving us with just this:

var filteredCombinations = combinatedResults
    .Select(values => values.Where(v => v > 0)
                            .Select(v => v.ToString())
                            .ToArray())
    .Where(a => a.Count > 0)
    .Select(a => string.Join(",", a));

score 0 · Answer 5 · answered Feb 10 '10 at 21:10

0

use linq SelectMany

answered Feb 10 '10 at 21:10

Preet Sangha

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How to convert an IEnumerable> to a IEnumerable

5 Answers5