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    <p>A nested for-loop is overkill, and in particular, the "remove" call itself represents yet another for-loop. All in all, your second optimized version has a worse time-complexity than the first solution, in particular when there are many hits.</p> <p>The fastest solution is likely to be the following:</p> <pre><code>foreach(var item in listItems) item.IsHit = false; foreach (int hitIndex in hits) listItems[hitIndex - 1].IsHit = true; </code></pre> <p>This avoids the inefficient nested for-loops, and it avoids the overhead of the delegate based <code>.ForEach</code> method (which is a fine method, but not in performance critical code). It involves setting IsHit slightly more frequently, but most property setters are trivial and thus this is probably not a bottleneck. A quick micro-benchmark serves as a fine sanity check in any case.</p> <p><strong>Only if <code>IsHit</code> is truly slow</strong>, the following will be quicker:</p> <pre><code>bool[] isHit = new bool[listItems.Count]; //default:false. //BitArray isHit = new BitArray(listItems.Count); //BitArray is potentially faster for very large lists. foreach (int hitIndex in hits) isHit [hitIndex - 1] = true; for(int i=0; i &lt; listItems.Count; i++) listItems[i].IsHit = isHit[i]; </code></pre> <p>Finally, consider using an array rather than a <code>List&lt;&gt;</code>. Arrays are generally faster if you can avoid needing the <code>List&lt;&gt;</code> type's insertion/removal methods.</p> <p>The <code>var</code> keyword is C# 3.5 but can be used in .NET 2.0 (new language features don't require newer library versions, in general - it's just that they're most useful with those newer libs). Of course, you know the type with which <code>List&lt;&gt;</code> is specialized, and can explicitly specify it.</p>
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    1. COMinor note on the complexity of the OP's second version: the Remove call will iterate precisely over those elements of hits *not* iterated over by the explicit inner for loop, so the overal complexity of that solution is on the order of `listItems.Count` × `hits.Count`
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