StackOverflow2013

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POHow to find the top several values from an array?
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copied!<p>I have an array of float values and want the value and more importantly the position of the maximum four values.</p> <p>I built the system originally to walk through the array and find the max the usual way, by comparing the value at the current position to a recorded max-so-far, and updating a position variable when the max-so-far changes. This worked well, an O(n) algo that was very simple. I later learned that I need to keep not only the top value, but the top three or four. I extended the same procedure and complicated the max-so-far into an array of four max-so-fars and now the code is ugly. </p> <p>It still works and is still sufficiently fast because only a trivial amount of computations have been added to the procedure. it still effectively walks across the array and checks each value once.</p> <p>I do this in MATLAB with a sort function that returns two arrays, the sorted list and the accompanying original position list. By looking at the first few values I have exactly what I need. I am replicating this functionality into a C# .NET 2.0 program. </p> <p>I know that I could do something similar with a List object, and that the List object has a built in sort routine, but I do not believe that it can tell me the original positions, and those are really what I am after. </p> <p>It has been working well, but now I find myself wanting the fifth max value and see that rewriting the max-so-far checker that is currently an ugly mess of if statements would only compound the ugliness. It would work fine and be no slower to add a fifth level, but I want to ask the SO community if there is a better way. </p> <p>Sorting the entire list takes many more computations than my current method, but I don't think it would be a problem, as the list is 'only' one or two thousand floats; so if there is a sort routine that can give back the original positions, that would be ideal.</p> <p>As background, this array is the result of a Fourier Transform on a kilobyte of wave file, so the max values' positions correspond to the sample data's peak frequencies. I had been content with the top four, but see a need to really gather the top five or six for more accurate sample classification.</p>

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