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copied!<p>This can be solved elegantly using Linq:</p> <pre><code>public static void Main(string[] args) { List<int> list = new List<int> { 2, 3, 4, 5, 2, 4, 6, 2, 4, 7, 3, 8, 2 }; var grouping = list .GroupBy(x => x) .Select(x => new { Item = x.Key, Count = x.Count()}); foreach (var item in grouping) Console.WriteLine("Item {0} has count {1}", item.Item, item.Count); } </code></pre> <p>Internally it probably uses hashing to partition the list, but the code hides the internal details - here we are only telling it <em>what</em> to calculate. The compiler / runtime is free to choose <em>how</em> to calculate it, and optimize as it sees fit. Thanks to Linq this same code will run efficiently whether run an a list in memory, or if the list is in a database. In real code you should use this, but I guess you want to know how internally it works.</p> <p>A more imperative approach that demonstrates the actual algorithm is as follows:</p> <pre><code> List<int> list = new List<int> { 2, 3, 4, 5, 2, 4, 6, 2, 4, 7, 3, 8, 2 }; Dictionary<int, int> counts = new Dictionary<int, int>(); foreach (int item in list) { if (!counts.ContainsKey(item)) { counts[item] = 1; } else { counts[item]++; } } foreach (KeyValuePair<int, int> item in counts) Console.WriteLine("Item {0} has count {1}", item.Key, item.Value); </code></pre> <p>Here you can see that we iterate over the list only once, keeping a count for each item we see on the way. This would be a bad idea if the items were in a database though, so for real code, prefer to use the Linq method.</p>

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