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POC++ map question
 

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    <p>I have an integral position-based algorithm. (That is, the output of the algorithm is based on a curvilinear position, and each result is influenced by the values of the previous results).</p> <p>To avoid recalculating each time, I would like to pre-calculate at a given sample rate, and subsequently perform a lookup and either return a pre-calculated result (if I land directly on one), or interpolate between two adjacent results.</p> <p>This would be trivial for me in F# or C#, but my C++ is very rusty, (and wasn't even ever that good).</p> <p>Is map the right construct to use? And could you be so kind as to give me an example of how I'd perform the lookup? (I'm thinking of precalculating in milimetres, which means the key could be an int, the value would be a double).</p> <p><strong>UPDATE</strong> OK, maybe what I need is a sorted dictionary. (Rolls up sleeves), pseudocode:</p> <pre><code>//Initialisation fun MyFunction(int position, double previousresult) returns double {/*etc*/}; double lastresult = 0.0; for(int s = startposition to endposition by sampledist) { lastresult = MyFunction(s, lastresult); MapOrWhatever.Add(s, lastresult); } //Using for lookup fun GetValueAtPosition(int position) returns double { CheckPositionIsInRangeElseException(position); if(MapOrWhatever.ContainsKey(position)) return MapOrWhatever[position]; else { int i = 0; //or possibly something clever with position % sampledist... while(MapOrWhatever.Keys[i] &lt; position) i+=sampledist; return Interpolate(MapOrWhatever, i, i+sampledist, position); } } </code></pre> <p>Thinks... maybe if I keep a constant sampledist, I could just use an array and index it...</p>
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    1. USZyerah
    1. USBenjol
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    1. CODespite the high votes for the answer below, I'm not convinced you have given enough information about the problem. I'm not sure if you're describing a memoization problem or a string-indexing problem. If you require full context (i.e. to make a wave form), the the problem isn't a "use a map??" problem as it is you need to find a way to efficiently store and search entire strings, and merely storing values won't help you here... basically, i need more info. don't know if anybody else does.
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      1. USSan Jacinto
    2. COright now, i recommend you examine suffix trees to see if this is closer to what you need.
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    3. COAgree with San Jacinto. One of the things that could help is, if you are confident on what your solution in C# would be (C# is much closer to C++ than F#), how would you implement it. How are you going to iterate your tree (in 2D?) to determine if you have a solution for a given point? finding the closest two pre-calculated points to use for the interpolation? A map is really nice for direct searches, but it will only search in one dimension (whatever the sorting algorithm you provide uses)
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      1. USDavid Rodríguez - dribeas
 

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