What is the fastest way substraction of long array items?

Question

3.00/5 (2 votes)

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I have a long array

Dim LongArray As Long() = {1595741111714190885, 2681354977222731062, 3897343441765742102, 1813043105221908777, 2970433094069856569, ...}

I want subtract each item from next item.

Dim DiffArray As New List(Of Long)

For i As Integer = 0 To LongArray.Count - 1
      Dim Diff As Long = LongArray(i + 1) - LongArray(i)
      DiffArray.Add(Diff)
Next

But this way too slow.

What I have tried:

FOR NEXT

Dim LongArray As Long() = {1595741111714190885, 2681354977222731062, 3897343441765742102, 1813043105221908777, 2970433094069856569}

Dim DiffArray As New List(Of Long)

For i As Integer = 0 To LongArray.Count - 1
      Dim Diff As Long = LongArray(i + 1) - LongArray(i)
      DiffArray.Add(Diff)
Next

LINQ

Dim DiffArray2 = (From x In LongArray Let nextindex = LongArray.ToList.IndexOf(x) + 1 Let nextelement = LongArray.ToList.ElementAt(If(nextindex = LongArray.ToList.Count, nextindex - 1, nextindex)) Select nextelement - x).ToList()

Posted 23-Aug-18 11:52am

gacar

Updated 23-Aug-18 18:07pm

v2

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Comments

Patrice T 23-Aug-18 18:43pm

What exactly are you trying to do ?
Your code is not only doing subtractions.

2 solutions

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Graeme_Grant · Accepted Answer · 2018-08-23T18:08:00

Dave is spot on... To give you an example, I ran a benchmark on 10,000 items using 1. Dave's code; 2. a Linq version; 3. a PLinq version. Below is the code:

VB

<Benchmark(Baseline:=True)>
Public Sub Execute()
    Dim diffArray = New Long(maxSize - 1 - 1) {}

    For i As Integer = 0 To diffArray.Length - 1
        diffArray(i) = LongArray(i + 1) - LongArray(i)
    Next
End Sub

<Benchmark>
Public Sub ExecuteLinq()
    Dim diffArray As Long() = LongArray.Skip(1).[Select](Function(x, i) LongArray(i) - x).ToArray()
End Sub

<Benchmark>
Public Sub ExecutePLinq()
    Dim diffArray As Long() = LongArray.Skip(1).[Select](Function(x, i) LongArray(i) - x).AsParallel().AsOrdered().ToArray()
End Sub

And here are the benchmark results:

// * Summary *

BenchmarkDotNet=v0.11.1, OS=Windows 10.0.17134.228 (1803/April2018Update/Redstone4)
Intel Core i7-4980HQ CPU 2.80GHz (Haswell), 1 CPU, 8 logical and 4 physical cores
  [Host]     : .NET Framework 4.7.2 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3132.0
  DefaultJob : .NET Framework 4.7.2 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3132.0


       Method |        Mean |     Error |    StdDev | Scaled | ScaledSD |
------------- |------------:|----------:|----------:|-------:|---------:|
      Execute |    22.02 us | 0.2142 us | 0.2003 us |   1.00 |     0.00 |
  ExecuteLinq |   297.39 us | 1.6327 us | 1.5272 us |  13.51 |     0.14 |
 ExecutePLinq | 1,210.37 us | 6.4961 us | 5.7586 us |  54.98 |     0.54 |

// * Legends *
  Mean     : Arithmetic mean of all measurements
  Error    : Half of 99.9% confidence interval
  StdDev   : Standard deviation of all measurements
  Scaled   : Mean(CurrentBenchmark) / Mean(BaselineBenchmark)
  ScaledSD : Standard deviation of ratio of distribution of [CurrentBenchmark] and [BaselineBenchmark]
  1 us     : 1 Microsecond (0.000001 sec)

Hope this helps! :)

Dave Kreskowiak · Accepted Answer · 2018-08-23T14:29:00

Solution 1

Lists are going to slow things down a bit.

The quickest you're going to get this to run is your Fox/Next loop, but first, preallocate your DiffArray to the size required to hold all the values you're going to calculate, which should be the size of your LongArray - 1.

VB.NET

Dim DiffArray(LongArray.Count - 1) As Long

For i As Integer = 0 To LongArray.Count - 1
    Dim Diff As Long = LongArray(i + 1) - LongArray(i)
    DiffArray(i) = Diff
Next

Posted 23-Aug-18 14:29pm

Dave Kreskowiak

Comments

Jochen Arndt 24-Aug-18 4:49am

Should be
For i As Integer = 0 To DiffArray.Count - 1
or
For i As Integer = 0 To LongArray.Count - 2

Countered the 1 vote anyway.

gacar 24-Aug-18 4:56am

Yes, right.

Dave Kreskowiak 24-Aug-18 8:44am

Thanks! It was late, and my brain was turned off. :)

gacar 24-Aug-18 5:02am

Dave Kreskowiak, thaks for solution. Yes, very speed now.