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    <p>The problem is with the <code>Select</code> method. In order to sum up all the digits (as described in the algorithm) you would need to return <code>1</code> and <code>0</code> instead of <code>10</code>, <code>1</code> and <code>4</code> instead of <code>14</code> (as in your example). </p> <p>The <strong>easiest</strong> (<strong>but it does not have to be the most optimal</strong>) way to do that is to conert number from <code>Select</code> to <code>string</code> (14 -> "14") and then split the string characters using <code>SelectMany</code>. </p> <p>So your code should look as follows:</p> <pre><code>int mod10sum = number.Reverse() .SelectMany((c, i) =&gt; ((c - '0') &lt;&lt; ((i + 1) &amp; 1)).ToString()) .Sum(c =&gt; c - '0') % 10; checkDigit = (mod10sum == 0 ? 0 : 10 - mod10sum).ToString("0"); Console.WriteLine(checkDigit); </code></pre> <p><strong>A bit of theory</strong></p> <p>LINQ <a href="http://msdn.microsoft.com/en-us/library/bb534336.aspx" rel="nofollow">SelectMany</a> returns <code>IEnumerable&lt;&gt;</code>. When you return <code>string</code> (which is IEnumerable) then that's why <code>SelectMany</code> "splits" returned string into characters. </p> <p>Microsoft has very nice page (<a href="http://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b" rel="nofollow">101 LINQ Samples</a>) with different LINQ samples which should help you out.</p> <p><strong>EDIT</strong></p> <p>I would also recommend working on that conversion from <code>int</code> to <code>string</code>. I was working on similar project literally yesterday and in my case that conversion is a bit problematic from performance point of view as we call that method millions of times. If you have to calculate lots of mod10's then it might be not the best solution.</p>
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    1. CODamn you type fast
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    2. CONot possible to type this (and add links etc) in just one minute. So i assume that the OP and Tom are linked somehow ;)
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      1. USTim Schmelter
    3. CO@Tom, I really like your solution but will accept Rawling's one as it does not have mentioned performance problem. And thanks for the link to 101 LINQ.
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