Wish: Sign(n) function

Widget · Nov 25, 2011

I'd like to see a Sign(n) function that would return:

-1 if n is negative, 0 if n=0, and 1 if n is positive.

If it were a native function, it would be faster than using If Statements. Many programming languages already have a Sign() function.

Widget

klaus · Nov 25, 2011

The native function does exist, it's:
Signum(d)
in Android.

Best regards.

Erel · Nov 25, 2011

If it were a native function, it would be faster than using If Statements.

The performance of a solution with 'if' should be similar to the native function performance.

Widget · Nov 25, 2011

Erel said:
The performance of a solution with 'if' should be similar to the native function performance.

Erel,
Well I would create my own Sub Sign(d as double) and call that so the code is easier to read. I thought the extra overhead of the sub call and the if statements would be considerably slower than calling the Android SigNum(d) function directly.

Widget

DaOel · Jan 17, 2012

Signum function

klaus said:
The native function does exist, it's:
Signum(d)
in Android.

Best regards.

i tried it but it doesnt recognize the function "Signum()" . do i need to set a reference to an math-library?

kickaha · Jan 19, 2012

DaOel said:
i tried it but it doesnt recognize the function "Signum()" . do i need to set a reference to an math-library?

Klaus means Android native, not B4A native.

joseluis · Jan 19, 2012

I tried to access signum, and other native math functions via Reflection library, but this code is wrong.

B4X:

r.Target=r.CreateObject("java.lang.Math")
r.Target=r.RunMethod2("signum", number, "java.lang.double")

The thing is that I cannot create a Math Object. So what would be the right way to do it?

klaus · Jan 19, 2012

As a workaround you can use the code below:

B4X:

Sub Sign(d As Double) As Int
    If d = 0 Then
        Return 0
    Else
        Return(d / Abs(d))
    End If
End Sub

Best regards.

joseluis · Jan 19, 2012

Thank you. Although I was hoping to deepen my knowledge of the Reflection lib

stevel05 · Jan 20, 2012

To assist your reflection knowledge, the signum method is a static method, so you don't need to create the Math object.

Try this:

B4X:

Dim num As Float   
num = -50.0
Dim r As Reflector
result=r.RunStaticMethod("java.lang.Math","signum",Array As Object(num),Array As String("java.lang.float"))
Log(Result)

Steve

joseluis · Jan 20, 2012

Yes! That would be it! :sign0188:

agraham · Jan 20, 2012

But because of the way reflection works it's a very slow way of doing it. An if statement is optimal.

joseluis · Jan 20, 2012

It's true, agraham. Below is the code that demonstrates that the function implemented as a sub is around 100-200 times faster than the one that uses Reflection.

B4X:

Dim timestart, signtime, rsigntime As Long
Dim n As Int
   
' Test Sign function using Reflection
timestart = DateTime.Now
For n = 0 To 100000
   RSign(-20)
Next
rsigntime = DateTime.Now - timestart
   
' Test Sign function using Sub
timestart = DateTime.Now
For n = 0 To 100000
   Sign(-20)
Next
signtime = DateTime.Now - timestart

' Log the result   
Log("Sign: " & signtime & " RSign:" & rsigntime & " rel:" & Round(rsigntime/signtime))


Sub RSign(num As Float) As Int
   Return r.RunStaticMethod("java.lang.Math", "signum", _
      Array As Object(num), Array As String("java.lang.float"))   
End Sub

Sub Sign(d As Double) As Int
    If d = 0 Then
        Return 0
    Else
        Return(d / Abs(d))
    End If
End Sub

HTML:

Sign:27 RSign:4790 rel:177
Sign:27 RSign:5028 rel:186
Sign:22 RSign:2950 rel:134
Sign:19 RSign:2980 rel:157
Sign:33 RSign:3701 rel:112
Sign:21 RSign:2899 rel:138

kickaha · Jan 20, 2012

You can optimise the sign sub:

B4X:

Sub Sign(d As Double) As Int    
If d = 0 Then Return 0
Return(d / Abs(d))
End Sub

Not sure if it would be (much) faster though.

myriaddev · Jan 20, 2012

Faster still is:

Taking out divide, it returns results 1.44 times faster.

joseluis · Jan 20, 2012

Yeah, myriadev's version without division is 200-300 times faster than using reflection . Although I must say that calling the slowest one 100000 times takes less than 3 seconds for my SGII.

myriaddev · Jan 20, 2012

Great Joseluis

Hi Joseluis. I can always learn from the masters!
Could you show me your test code including my code ?
Thanks, Jerry

joseluis · Jan 20, 2012

It's pretty strightforward but for sake of completion I attach a functional example.

I can always learn from the masters too. I'm just a newbie. Enjoy

kickaha · Jan 21, 2012

myriaddev said:
Taking out divide, it returns results 1.44 times faster.

So would this be faster (taking out the ELSE)

B4X:

Sub sign(d As Double) As Int
   If d = 0 Then Return 0
   If d > 0 Then Return 1
   Return -1
End Sub

joseluis · Jan 21, 2012

I made an updated version of the Test Suite so now we can easily compare the performance of each one of the sub variations. It runs the test a number of times, showing each partial result. Then it stops and shows the total averaged results.

These are the average results from repeating 10 times a 500000 calls test.

HTML:

R:19186 signND:100 signND:52 signNDNE:54
R/sign:192 R/signND:369 R/signNDNE:355
---
R:13462 signND:100 signND:48 signNDNE:49
R/sign:135 R/signND:280 R/signNDNE:275
---
R:13402 signND:97 signND:48 signNDNE:48
R/sign:138 R/signND:279 R/signNDNE:279
---
R:14145 signND:97 signND:48 signNDNE:49
R/sign:146 R/signND:295 R/signNDNE:289
---
R:13666 signND:99 signND:47 signNDNE:48
R/sign:138 R/signND:291 R/signNDNE:285
---
R:13576 signND:97 signND:49 signNDNE:48
R/sign:140 R/signND:277 R/signNDNE:283
---
R:13561 signND:98 signND:49 signNDNE:48
R/sign:138 R/signND:277 R/signNDNE:283
---
R:13880 signND:119 signND:50 signNDNE:50
R/sign:117 R/signND:278 R/signNDNE:278
---
R:13914 signND:97 signND:48 signNDNE:48
R/sign:143 R/signND:290 R/signNDNE:290
---
R:13584 signND:100 signND:50 signNDNE:49
R/sign:136 R/signND:272 R/signNDNE:277
--------------
R:14237 sign:100 signND:48 signNDNE:49
 R/sign:142 R/signND:297 R/signNDNE:291

R: version using reflection
sign: sub version using division (klaus)
signND: sub version without using division (myriaddev)
signNDNE: sub version without using division nor elses (kickaha)

Two last ones are equally fast. The difference must be only syntactical.

Wish: Sign(n) function

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