This might sound silly, but is it possible to somehow specify a variable with a string containing the name of the variable?

For example:

Var
   MyVariable1 : Word;
   MyVariableName : String;
Procedure ShowVariable(Variablename);
Begin
Writeln(Variablename,' = $', inttohex(    (Somehow get the value of the variable specified by the name here ) ,4));
End;
Begin
MyVariableName:= 'MyVariable1';
ShowVariable(MyVariableName);
End.

Is such a thing possible?

James

Hi all! It's my first answer to the mailing list, hopefully I won't screw anything up.

PM James Richters wrote

This might sound silly, but is it possible to somehow specify a variable with a string containing the name of the variable?

You might want to try using generic dictionaries. E.g. like this:

program Project1;
uses
  SysUtils, Generics.Collections;
type
  TVariableDictionary = specialize TDictionary<String, PWord>;
var
  MyDictionary: TVariableDictionary;
  MyVariable1: Word;
begin
  MyDictionary := TVariableDictionary.Create;
  MyDictionary.Add('MyVariable1', @MyVariable1);
  MyDictionary['MyVariable1']^ := 100;
  WriteLn(MyVariable1);
  WriteLn(MyDictionary['MyVariable1']^);
  FreeAndNil(MyDictionary);
end.

If you need to address variables of different type, try Variants ( https://wiki.freepascal.org/Variant ).

associative array?

On Jul 7, 2019, at 3:58 PM, James Richters wrote: This might sound silly, but is it possible to somehow specify a variable with a string containing the name of the variable?

Does you mean like RTTI so you can get a pointer to a local variable by name? I was actually curious about this recently also, does FPC have local variable RTTI? That would be useful to have actually. I know it exists for classes but not sure about locals.

Regards,
Ryan Joseph

No.
If you want that you need to declare them as published properties of a class.

Michael.

Is there an example of how declare them this way? I only have about 50 variables I wish to do this with.

Thank you for the answer and the example... I didn't know there was such a thing as a variable dictionary, but It looks like that may work for me, I'll give it a try!

James

Generics.Collections seems to be something that was added after 3.0.4, I've downloaded the current FPC source code, but I’m not sure how to build it.. can someone point me in the right direction on how to compile the current source code?

James

I'm on windows....

Generics.Collections seems to be something that was added after 3.0.4, I've downloaded the current FPC source code, but I’m not sure how to build it.. can someone point me in the right direction on how to compile the current source code?

James

James Richters wrote

I'm on windows.... Generics.Collections seems to be something that was added after 3.0.4, I've downloaded the current FPC source code, but I’m not sure how to build it.. can someone point me in the right direction on how to compile the current source code?

If it is in 3.2-beta, you can get a 3.2-beta from here: https://sourceforge.net/projects/lazarus-snapshots/files/ You can install as 2ndary install.

Otherwise fpcupdeluxe

James Richters wrote

I'm on windows.... Generics.Collections seems to be something that was added after 3.0.4, I've downloaded the current FPC source code, but I’m not sure how to build it.. can someone point me in the right direction on how to compile the current source code?

If it is in 3.2-beta, you can get a 3.2-beta from here: https://sourceforge.net/projects/lazarus-snapshots/files/ You can install as 2ndary install.

Otherwise fpcupdeluxe

You can use the TFPGMap class in fgl unit to implement the dictionary. fgl should be available in 3.0.4. Try to replace the Generics.Collections with fgl and the type definition in the example code from zh loza. The rest should be identical.

[...]

uses fgl;
type
TVariableDictionary = specialize TFPGMap<String, PWord>;

[...]

Thank you, using fgl; did work.

Am I correct in assuming that if I want other types of Variables I would change PWord in specialize TFPGMap<String, PWord>;
to something else, maybe PString, PDouble, PLongint.... etc?

James

This might sound silly, but is it possible to somehow specify a variable with a string containing the name of the variable?

Yes it certainly is. I wrote a unit that can handle arbitrary variable types for my project pmake. Pmake it a build tool for pascal.

https://github.com/daar/pmake/blob/master/pmake/pmake_variables.pas

To set a variable, simply use: set_(“myvar”, 3.141). To read it, use: writeln(val_(“myvar”)).

Rgds, Darius

sure, but you end up with a separate dictionary for each type. Or, you can try variants, as originally suggested, but then you need to declare all your variables as a Variant type.

uses fgl, variants;
type
   TVariableDictionary = specialize TFPGMap<String, PVariant>;

What is it for?

What about:

procedure ShowVariable(const Variablename:string; const value:word);
begin
  Writeln(Variablename,' = $'+inttohex(value ,4));
end;
ShowVariable('myVar',myVar);

And as far as I understand, you want to call
ShowVariable('myVar',myVar), but not wasting time writing the name enclosed in quotes, let the program get the identifier name in runtime, or the other way around, let the program get the value from a name passed in runtime.

Bad luck, Pascal is not a script language, it is not aware of identifier names in runtime there is no "eval". Thanks to that it gets rids of all the burden of having to keep in runtime structures to know the name, type and value of variable. It generate fast native programs.

Nevertheless, there are ways, you can program that in Freepascal using classes, properties, RTII etc. It is more flexible and powerful. But obviously it has a price in complexity. Instead or using the standard operators and reference a simple variable, you must use a more convoluted syntax.

As far as I see, you don't intend to let the program create new vars in runtime, you just want to write the identifiers of the vars you have hardcoded.  Is it worth?

(I hope it is not just for debugging)

Well what it’s for, is not just debugging, I’m trying to manage outputs to ports that are defined by port numbers and pins in an ini file, the ports and pins can really be anything but I need to keep track of the output states of all the pins by specific port value because I can only output all 8 bits of the port at one time. I can just read back the port and then use AND or OR with what I want and write it back out, but that’s slow, and it also is not necessarily accurate because some ports will give what’s on the pin itself, not what output it is trying to send out… you can be outputting a 1 and the user can force it to 0 and it will read in a 0 now when you write to it, it’s outputting a 0 and if the user removes the external condition forcing it to 0 it will not go back to 1.. the ports are 1/O ports and are designed to be operated in this manner. Anyway I made an array of ports and the data each should contain, and then I also have an array of each I/O pin (defined with a string of the name of the variable that holds the pin number) and the port it is looking for… and then I have procedures and functions that use these arrays to set the stored value of each port correctly, then I just output the entire 8 bits from the storage array. It’s working well, but I’m tired of doing things like this:

SetStoragePinByName('Ini_Opto_Enable_Positive_Pin', Ini_Opto_Enable_Positive_Pin,True); Which calls a procedure like this:

Procedure SetStoragePinbyname(SSPBN_Var_Name:String; setpinnum:Byte; Pin_State:Boolean);

So in quotes I have the name of the variable that contains the name of the variable in the array, so it can figure out the position in the port array, then the actual contents of the actual variable being used… so I know what pin number I am outputting to, and then the state I want to set… So I have a TON of exactly what you describe, calls to functions and procedures with the name of the variable in quotes AND the variable itself. Variable names are not assigned at runtime, they are pre-defined. I did not need to use the name of the variable in my pin-list array, but it makes it less confusing if the variable name is what I search for in the array than some kind of cryptic code that would be difficult to maintain.

So yes, the only reason to want to do this is so I could hopefully shorted the call to just

               SetStoragePinByName('Ini_Opto_Enable_Positive_Pin', True) ;

And it would somehow get the value of 'Ini_Opto_Enable_Positive_Pin'

Hmmmm now that I’m typing this all out, I think I can just put the value of the pin in another array also referenced by the same string and then I could make my calls like that.

James.

James Richters schrieb am Mo., 8. Juli
2019, 22:44:

SetStoragePinByName('Ini_Opto_Enable_Positive_Pin', True) ; And it would somehow get the value of 'Ini_Opto_Enable_Positive_Pin

That indeed sounds like something where you could use classes together with published properties. At least if you're willing to spend a bit of effort.

Regards,
Sven

I’m curious how use classes together with published properties would work, is there a sample of how to use this somewhere?

James

Haev a look at rtti and the functionality available in the typinfo unit. The tests included in fpc are a good resource. Have a look at the trtti*.pp tests:
https://svn.freepascal.org/cgi-bin/viewvc.cgi/trunk/tests/test/

Start with trtti4.pp and work your way up to more complex examples.

James Richters wrote

ports that are defined by port numbers and pins in an ini file,

Well, then they are dynamic. They are not hardcoded.

I would use something like:

TPortIdentifier=(Pin_1, pin_2, Port_x, Port_y...);
TIOPort=object
        private
           FPortPin:boolean;
           FPortByte:boolean;
           procedure SetPortPin(value:boolean);
           procedure SetPortByte(value:byte);
        public

          OtherData:... // pin, port etc or other data

           PortAdress:PWord; // pointer to memory
           Name:string;
           constructor Init(aName:name;aPword:pWord);
           property  PortPin:boolean read fPortPin write SetPortPin;
           property  PortByte:byte read fPortByte write SetPortByte;
         end;
   TPortList=array[TPortIdentifier] of TIOPort;
var
  PortList:TPortList;
begin
   // fill the array with name and values from Ini file
    PorList[Pin_1].init('Pin-1',@xxxx);
    PorList[Pin_2].init('Pin-2',@xxxx);
   // call
   PortList[pin_1].PortPin:=true;
   PortList[pin_2].PortPin:=false;
end.

Or you can use a TFPGMap from the standard library fgl.