Question

I’m interested in speed, not good looking code, that is why I’m using array and not list(of integer).

I have an array looking like: 0,1,0,1,1,0,1,0,1,1,1,0,0,1

I’m interesting in the position of each number so I can later pick one randomly.

so what I do is looping through the array to take position number of each 1 then creating a new array looking like this: 2,4,5,7,9,10,11,14

is bitwise could be used here? I have no idea

code look like:

Private Function theThing() As Integer()     Dim x As Integer     'arIn() would be a parameter     Dim arIn() As Integer = {0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 1}     Dim ar() As Integer = Nothing     Dim arCount As Integer = -1      For x = 1 To arIn.GetUpperBound(0)         If arIn(x) = 1 Then             arCount += 1         End If     Next      If arCount > -1 Then         'using redim preseve is slower than the loop above         ReDim ar(arCount)          arCount = 0         For x = 1 To arIn.GetUpperBound(0)             If arIn(x) = 1 Then                 ar(arCount) = x                 arCount += 1             End If         Next     End If      Return ar End Function 

* EDIT *

current solution(10% to 15% faster) is now

Private Function theThing() As Integer     Dim ar() As Integer = {0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 1}     Dim arLenght As Integer = ar.GetUpperBound(0)     Dim arCount As Integer = 0     Dim x As Integer      For x = 1 To arLenght         If ar(x) = 1 Then             ar(arCount) = x             arCount += 1         End If     Next      dim r As New Random()      Return ar(r.Next(arCount)) End Function 

I don’t think it can be optimized more than that, unless someone find a way to do exactly what the solution does but way faster

Before this question, my whole thing was able to do about 25500 run each 10 seconds.

Now, it can do over 32250 all the time, a 21% increase, thanks!

Answer 1

Few tips on the original algorithm:

1. Try storing the results of arIn.GetUpperBound(0) in a variable. I don’t know how VB makes it’s loops, but there is a chance that the function gets called once every iteration. You should check that though.
2. That If arCount > -1 is always going to be true. Remove it.

If you wish to keep the same inputs/outputs, then I don’t think there is much else that can be improved.

Now if you wanted a function that does the random selection too, then it might be a bit better. I’ll write in C# since I know it better. You should be able to understand:

public static int GetRandomSetBit(int[] AllBits) {     // Perhaps check here if AllBits is null/empty. I'll skip that for now.      int L = AllBits.Length;     int SetBitCount = 0;      // No point to save a few bytes here. Also - you can make a global array     // large enough for all occasions and skip allocating it every time.     // In that case consider automatic resizing and watch out for     // multithreading issues (if you use several threads).     int[] GoodPositions = new int[L];      for ( int i = 0; i < L; i++ )         if ( AllBits[i] != 0 )         {             GoodPositions[SetBitCount] = i;             SetBitCount++;         }      Random r = new Random(); // Should use one global instance      return GoodPositions[r.Next(SetBitCount)]; } 

I’m afraid it won’t get any better than that. Not unless you can somehow change the inputs/outputs or requirements.