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POTransform sequence in Linq while being aware of each Select/SelectMany result
 

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  1. POTransform sequence in Linq while being aware of each Select/SelectMany result
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    <p>Here's my problem. I have one specific list, which I'll present as a <code>int[]</code> for simplicity's sake.</p> <pre><code>int[] a = {1,2,3,4,5}; </code></pre> <p>Suppose I need to transform each item on this list, but depending on the situation, I may return an int or an array of ints.</p> <p>As an example, suppose I need to return <code>{v}</code> if the value is <strong>odd</strong>, and <code>{v,v+1}</code> if the value is <strong>even</strong>. I've done this:</p> <pre><code>int[] b = a.SelectMany(v =&gt; v % 2 == 0 ? new int[] { v, v+1 } : new int[] { v }) .ToArray(); </code></pre> <p>So if I run this, I'll get the expected response:</p> <pre><code>{1,2,3,3,4,5,5} </code></pre> <p>See that I have repeating numbers, right? <code>3</code> and <code>5</code>. I don't want those repeating numbers. Now, you may tell me that I can just call <code>.Distinct()</code> after processing the array.</p> <p>This is the problem. The <code>SelectMany</code> clause is fairly complex (I just made up a simpler example), and I definitely don't want to process <code>3</code> if it's already present in the list.</p> <p>I could check if <code>3</code> is present in the original list. But if I got <code>3</code> in the <code>SelectMany</code> clause, I don't want to get it again. For instance, if I had this list:</p> <pre><code>int[] a = {1,2,3,4,5,2}; </code></pre> <p>I would get this:</p> <pre><code>{1,2,3,3,4,5,5,2,3} </code></pre> <p>Thus returning <code>v</code> (my original value) and <code>v+1</code> again at the end. Just so you can understand it better <code>v+1</code> represents some processing I want to avoid.</p> <p>Summarizing, this is what I want:</p> <ol> <li>I have a list of objects. <strong>(Check)</strong></li> <li>I need to filter them, and depending on the result, I may need to return more than one object. <strong>(Check, used <code>SelectMany</code>)</strong></li> <li>I need them to be distinct, but I can't do that at the end of the process. I should be able to return just <code>{v}</code> if <code>{v+1}</code> already exists. <strong>(Clueless...)</strong></li> </ol> <p>One thing I thought about is writing a custom <code>SelectMany</code> which may suit my needs, but I want to be sure there's no built-in way to do this.</p> <p><strong>EDIT</strong>: I believe I may have mislead you guys with my example. I know how to figure out if <code>v+1</code> is in a list. To be clear, I have one object which has 2 int properties, Id and IdParent. I need to "yield return" all the objects <strong>and</strong> their parents. But I just have the ParentId, which comes from the objects themselves. I'm able to know if <code>v+1</code> is in the list because I can check if any object there has the same Id as the ParentId I'm checking.</p> <p><strong>ANSWER</strong>: I ended up using <code>Aggregate</code>, which can be used to do exactly what I'm looking for.</p>
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    1. CODoes the order of the resulting array matter?
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    2. CO@TimSchmelter Not at all :)
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    3. COJust a question, but since linq uses deferred execution, would you be able to run a second linq statement without the first executing? I don't know if the fist executes when you do the second. aka get rid of the `.ToArray` which executes the commands and then run another linq to filter out the data you want
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