B4J Library [B4X] Eval (expressions evaluator)

Erel · Jun 7, 2015

The attached class allows you to evaluate mathematical expressions with support for custom functions.
It is compatible with B4A, B4J and B4i.

Example:

B4X:

Sub AppStart (Args() As String)
   Dim e As B4XEval
   e.Initialize(Me, "Eval")
   Log(e.Eval("1 + Min(2, Max(-4, 1), 6)"))
   Log(e.Eval("-(2+5)*(7-3) + Sin(15 + 15) + Cos(30)"))
   Log("Error? " & e.Error)
   Log(e.Eval("-(2+5)*-(7-3)"))
   Log(e.Eval("-((-7-3))"))
   Log(1.321/-2/3.123 + (2 * 2 + 3) + 4)
   Log(e.Eval("1.321/-2/3.123 + (2 * 2 + 3) + 4"))
End Sub

'custom functions implementation
Sub Eval_Function (Name As String, Values As List) As Double
   Select Name 'it will be lower case
     Case "min"
       Dim d As Double = Values.Get(0)
       For Each n As Double In Values
         If n < d Then d = n
       Next
       Return d
     Case "max"
       Dim d As Double = Values.Get(0)
       For Each n As Double In Values
         If n > d Then d = n
       Next
       Return d
     Case "sin"
       Return SinD(Values.Get(0))
     Case Else
       Log("Invalid function: " & Name)
       Dim e As B4XEval = Sender
       e.Error = True
       Return 0
   End Select
End Sub

Note that if you are using it with B4A then call Eval after Activity_Create completes (you can add Sleep(0) instead). Otherwise the Function event will not be raised.

Updates

V2.01 - Fixes an issue with sub-expressions converted to scientific notation (which is not supported).
V2.00 - adds support for custom functions.

imbault · Oct 2, 2017

One more thing, if I may, would it be complicated to include variables in expressions like (or something to instantiate variables which can be used in this class):

B4X:

Log(e.Eval(" Max(-4, a)"))

or

B4X:

dim a as int = 10
e.SetVar("VA", a )
Log(e.Eval(" Max(-4, "VA")"))

It would be more than great

Erel · Oct 3, 2017

You can create a "get" function that returns values from an array:

B4X:

Sub Process_Globals
   Private vars(1000) As Double
End Sub

Sub AppStart (Args() As String)
   Dim e As B4XEval
   e.Initialize(Me, "Eval")
   vars(10) = 100
   Log(e.Eval("1 + Min(2, Max(-4, 1), 6)"))
   Log(e.Eval("-(2+5)*(7-3) + Sin(15 + 15) + Cos(30)"))
   Log("Error? " & e.Error)
   Log(e.Eval("-(2+5)*-(7-3)"))
   Log(e.Eval("-((-7-3))"))
   Log(1.321/-2/3.123 + (2 * 2 + 3) + 4)
   Log(e.Eval("1.321/-2/3.123 + (2 * 2 + 3) + 4"))
   Log(e.Eval("get(10) * 3")) '<-----
End Sub

Sub Eval_Function (Name As String, Values As List) As Double
   Select Name 'it will be lower case
     Case "min"
       Dim d As Double = Values.Get(0)
       For Each n As Double In Values
         If n < d Then d = n
       Next
       Return d
     Case "max"
       Dim d As Double = Values.Get(0)
       For Each n As Double In Values
         If n > d Then d = n
       Next
       Return d
     Case "sin"
       Return SinD(Values.Get(0))
     Case "get"
       Return vars(Values.Get(0)) '<-----
     Case Else
       Log("Invalid function: " & Name)
       Dim e As B4XEval = Sender
       e.Error = True
       Return 0
   End Select
End Sub

imbault · Oct 3, 2017

Erel said:

You can create a "get" function that returns values from an array:

B4X:

Sub Process_Globals
   Private vars(1000) As Double
End Sub

Sub AppStart (Args() As String)
   Dim e As B4XEval
   e.Initialize(Me, "Eval")
   vars(10) = 100
   Log(e.Eval("1 + Min(2, Max(-4, 1), 6)"))
   Log(e.Eval("-(2+5)*(7-3) + Sin(15 + 15) + Cos(30)"))
   Log("Error? " & e.Error)
   Log(e.Eval("-(2+5)*-(7-3)"))
   Log(e.Eval("-((-7-3))"))
   Log(1.321/-2/3.123 + (2 * 2 + 3) + 4)
   Log(e.Eval("1.321/-2/3.123 + (2 * 2 + 3) + 4"))
   Log(e.Eval("get(10) * 3")) '<-----
End Sub

Sub Eval_Function (Name As String, Values As List) As Double
   Select Name 'it will be lower case
     Case "min"
       Dim d As Double = Values.Get(0)
       For Each n As Double In Values
         If n < d Then d = n
       Next
       Return d
     Case "max"
       Dim d As Double = Values.Get(0)
       For Each n As Double In Values
         If n > d Then d = n
       Next
       Return d
     Case "sin"
       Return SinD(Values.Get(0))
     Case "get"
       Return vars(Values.Get(0)) '<-----
     Case Else
       Log("Invalid function: " & Name)
       Dim e As B4XEval = Sender
       e.Error = True
       Return 0
   End Select
End Sub

That's a beginning, what about a set function?

Thanks a lot

Erel · Oct 3, 2017

Very simple:

B4X:

Case "set"
 vars(Values.Get(0)) = Values.Get(1)
 Return 0

The expression will look like: "Set(10, 200)"

Erel · Mar 27, 2019

V2.01 - Fixes an issue with sub-expressions converted to scientific notation (which is not supported).

Gabino A. de la Gala · Jun 9, 2019

Would it be very difficult to adapt it so that it would also work with functions that returned other types of data such as strings or dates?

Erel · Jun 10, 2019

There is no types system here. It can only handle pure mathematical formulas.

agraham · Mar 7, 2022

ezpos said:
Do calculations with these values, is it a problem??

No, this is normal and expected. The results are Double values which are binary values that cannot represent accurately every decimal value. If you need exact decimal results you need to use a special library designed to handle decimals like my

BigNumbers library

If you need to do very accurate claculations using decimal numbers then the normal Java numeric types are not appropriate because, being finite length binary numbers, they cannot accurately represent every possible decimal number. The Java libraries contain a BigDecimal class that is optimised...

www.b4x.com

Erel · Mar 8, 2022

Use NumberFormat / B4XFormatter to convert a number to string.

Diego Roman · Jul 21, 2022

Excellent, works perfect, i needed it, thank you very much...

Ben10 · Nov 10, 2024

This is my first time attempting to use a B4X library with B4A. I've created a program in B4J using this library which I would like to run on an Android device using B4A. When attempting to run in B4A the code execution fails and asks if the B4XEval library is missing. Looking at the library in the "AdditionalLibraries/B4X" folder the file type designations are ".b4j". What do i need to do to have this library run successfully in B4A?

Erel · Nov 11, 2024

A b4xlib works exactly the same as xml/jar libraries. Copy it to the additional libraries folder. You can put it inside the B4X folder. Both options are good. Refresh the libraries list and check it.

Ben10 · Nov 11, 2024

Thanks, I just did that but now the follow error comes up: "Cannot find: C:\Program Files\Anywhere Software\B4A\libraries\jcore.jar"

Is this because the B4XEval library is dependent on this B4J library?

Erel · Nov 12, 2024

Sorry, I was mistaken. B4XEval isn't distributed as a b4xlib. You need to add B4XEval.bas with Project - Add Existing Module.

mjcoon · Thursday at 8:06 PM

Erel said:
Sorry, I was mistaken. B4XEval isn't distributed as a b4xlib. You need to add B4XEval.bas with Project - Add Existing Module.

Thanks I got that to work. But there is no list of operators (in the source) and I easily crash it by using "^", which I thought was quite ordinary.
What I am looking for, unsuccessfully, is

ExpressionEvaluator

Written by Andrew Graham

in B4X module form or library...

agraham · 2024-11-22T08:48:37+0000

It's here

ExpressionEvaluator library

I've been meaning to do this for my own use for a while and I have finally got a round tuit that allowed me to get on with it. Everyone should have a round tuit - they are so useful! :) This is the recursive descent parser from my BasicLib library stripped down to deal with arithmetic...

www.b4x.com

mjcoon · 2024-11-22T11:34:10+0000

Thank you Andrew, I had already found file ExpressionEvaluator1.1.zip. Co-incidentally I have for years used your B4PPC library which says: "Overview
This library contains an Evaluator object that is an arithmetic expression evaluator. It supports the arithmetic sub-set of the full Basic4ppc language with a few additions taken from Basic4android. This library requires .NET 2.0 or later and will run on both the desktop and device. This help file describes version 1.1 of the library."
(My B4PPC program correctly shows my age for "3^4"...)
But the content of the zip files does not look like B4X
There is a ExpressionEvaluator1.1source.zip file too which does also not look like B4A
Am I going mad as well as old (a similar age to you, I think...)?

agraham · 2024-11-22T11:47:32+0000

Have you downloaded the one from the link in my post above? I have and it's definitely the B4A version

mjcoon · 2024-11-22T14:19:22+0000

agraham said:
Have you downloaded the one from the link in my post above? I have and it's definitely the B4A version

Thank you Andrew, yesterday's copy, still in my Downloads folder, is wrong but today's (1) and (2) versions are fine, as you say, and working for me, to evaluate "3^4" correctly!

agraham · 2024-11-22T14:30:56+0000

It is unfortunate that both the Basic4ppc library and the B4A library ended up having the same version number and file name.

B4J Library [B4X] Eval (expressions evaluator)

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