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POFaster way of initializing arrays in Delphi
 

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  1. POFaster way of initializing arrays in Delphi
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    <p>I'm trying to squeeze every bit of performance in my Delphi application and now I came to a procedure which works with dynamic arrays. The slowest line in it is </p> <p>SetLength(Result, Len);</p> <p>which is used to initialize the dynamic array. When I look at the code for the SetLength procedure I see that it is far from optimal. The call sequence is as follows:</p> <p>_DynArraySetLength -> DynArraySetLength</p> <p>DynArraySetLength gets the array length (which is zero for initialization) and then uses ReallocMem which is also unnecessary for initilization.</p> <p>I was doing SetLength to initialize dynamic array all the time. Maybe I'm missing something? Is there a faster way to do this?</p> <p>EDIT: Describing the main algorithm would take a lot of space and really is unnecessary because it'm trying to optimize a small part of it. In general terms it's a veriant of Vehicle Routing Problem (<a href="http://en.wikipedia.org/wiki/Vehicle_routing_problem" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Vehicle_routing_problem</a>). I do need zillions of allocations, because I have to keep all the data, and keep it separately. Probalby it would help if I could think of some clever data structure here, but anything I can think of would greatly increase the complexity of the code. Basically I've done everything I could on the algorithmic level, so now I'm trying to get everything I can from the lowlevel things. So this is rather narrow question: is there any possibility to increase this particular call. And I think that to do this I need to write my own initialization function based on the SetLength code. And make it inline.</p>
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      1. POFaster way of initializing arrays in Delphi
      1. USAndreas Rejbrand
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    1. CO`SetLength()` is used to initialize and set the length of the array. So, I can't see how to optimize it - spliting the two features. Is it really a issue ? You should execute it only during initialization right ? Or you have it executed many times, in a loop ?
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    2. COAn array of zero length is represented by a nil pointer, FWIW - actually assigning nil to a dynamic array location is equivalent to SetLength(arr, 0). If `SetLength` is too slow, you are probably calling it too often; call it once to set a size big enough for the largest size, then keep track of the length independently, like Andreas says in his answer.
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      1. USBarry Kelly
    3. CODo you have one call to a function which does a zillion times SetLength( Result, Len ), or one zillion calls to a function which does one time SetLength( Result, Len ) ? In the first case, check Andreas answer below. In the second case, it's going to be more tricky.
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