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PODoes Linq provide a way to easily spot gaps in a sequence?
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  1. PODoes Linq provide a way to easily spot gaps in a sequence?
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    copied!<p>I am managing a directory of files. Each file will be named similarly to <code>Image_000000.png</code>, with the numeric portion being incremented for each file that is stored.</p> <p>Files can also be deleted, leaving gaps in the number sequence. The reason I am asking is because I recognize that at some point in the future, the user could use up the number sequence unless I takes steps to reuse numbers when they become available. I realize that it is a million, and that's a lot, but we have 20-plus year users, so "someday" is not out of the question.</p> <p>So, I am specifically asking whether or not there exists a way to easily determine the gaps in the sequence without simply looping. I realize that because it's a fixed range, I could simply loop over the expected range.</p> <p>And I will unless there is a better/cleaner/easier/faster alternative. If so, I'd like to know about it.</p> <p>This method is called to obtain the next available file name:</p> <pre><code>public static String GetNextImageFileName() { String retFile = null; DirectoryInfo di = new DirectoryInfo(userVars.ImageDirectory); FileInfo[] fia = di.GetFiles("*.*", SearchOption.TopDirectoryOnly); String lastFile = fia.Where(i =&gt; i.Name.StartsWith("Image_") &amp;&amp; i.Name.Substring(6, 6).ContainsOnlyDigits()).OrderBy(i =&gt; i.Name).Last().Name; if (!String.IsNullOrEmpty(lastFile)) { Int32 num; String strNum = lastFile.Substring(6, 6); String strExt = lastFile.Substring(13); if (!String.IsNullOrEmpty(strNum) &amp;&amp; !String.IsNullOrEmpty(strExt) &amp;&amp; strNum.ContainsOnlyDigits() &amp;&amp; Int32.TryParse(strNum, out num)) { num++; retFile = String.Format("Image_{0:D6}.{1}", num, strExt); while (num &lt;= 999999 &amp;&amp; File.Exists(retFile)) { num++; retFile = String.Format("Image_{0:D6}.{1}", num, strExt); } } } return retFile; } </code></pre> <p><strong>EDIT: in case it helps anyone, here is the final method, incorporating Daniel Hilgarth's answer:</strong></p> <pre><code>public static String GetNextImageFileName() { DirectoryInfo di = new DirectoryInfo(userVars.ImageDirectory); FileInfo[] fia = di.GetFiles("Image_*.*", SearchOption.TopDirectoryOnly); List&lt;Int32&gt; fileNums = new List&lt;Int32&gt;(); foreach (FileInfo fi in fia) { Int32 i; if (Int32.TryParse(fi.Name.Substring(6, 6), out i)) fileNums.Add(i); } var result = fileNums.Select((x, i) =&gt; new { Index = i, Value = x }) .Where(x =&gt; x.Index != x.Value) .Select(x =&gt; (Int32?)x.Index) .FirstOrDefault(); Int32 index; if (result == null) index = fileNums.Count - 1; else index = result.Value - 1; var nextNumber = fileNums[index] + 1; if (nextNumber &gt;= 0 &amp;&amp; nextNumber &lt;= 999999) return String.Format("Image_{0:D6}", result.Value); return null; } </code></pre>
 

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