function: how to return nil

[jesu 0]

Hello. I'm trying to create a function that can return a float or nil:

 

function myfunc(const pa_value:string): double;
begin
  if pa_value = whatever then result := -1
  else result := nil;
end;

to replace many lines like this (simplified code):

if check then
parambyname('myparam').asfloat := -1
else
parambyname('myparam').clear;

What is the right way to do that?

Thanks.

[Virgo 18]

Using variant?

function myfunc(const pa_value:string): variant;
begin
  if pa_value = whatever then result := -1
  else result := null;
end;

and then

parambyname('myparam').AsVariant := myfunc();

 

[FPiette 384]

It depends on what your function does, but just like a function returning an integer returns a special value (often -1 or 0) in case of error, your function could return a special value as a double number.

Carefully select the value so that it doesn't collide with expected values in your application. I'm think about a value like 2.12345e-308. Just define a constant to give that value a nice name easy to use and remember.

 

 

 

[Anders Melander 1795]

30 minutes ago, FPiette said:

Carefully select the value so that it doesn't collide with expected values in your application. I'm think about a value like 2.12345e-308. Just define a constant to give that value a nice name easy to use and remember.

NaN

 

 

[FPiette 384]

1 hour ago, Anders Melander said:

2 hours ago, FPiette said:

Carefully select the value so that it doesn't collide with expected values in your application. I'm think about a value like 2.12345e-308. Just define a constant to give that value a nice name easy to use and remember.

NaN

Anders, I'm not sure that NaN is a good choice because this special value is generated in other cases. Something like 2.12345e-308is a better choice and collision risk is much lower.

[Anders Melander 1795]

I'm pretty sure that anyone asking for help with something as basic as this isn't handling, or even aware of, NaN.

 

Regardless, I personally wouldn't solve the problem with a magic value.

[Der schöne Günther 316]

Jesu mentioned using Data.DB.TParams.ParamByName which returns a Data.DB.TParam object.

 

TParam has a property Value which is a Variant. As much as I loathe these things, the approach by @Virgo seems to be the most fitting.

Edited October 14 by Der schöne Günther

[Anders Melander 1795]

It's impossible to suggest a good solution without knowing more about the context. If it's just a case of either setting or clearing a TParam value then just pass the TParam to the function and let the function operate directly on that.

[Brandon Staggs 278]

You're better off doing something like

 

function myfunc(const InVal: String; out OutVal: Double): Boolean;

 

Return True if the value was set and false if not.

 

There are other ways to handle this but the worst way is a magic value in Double. There's no reason to do that when it is so easy to indicate explicitly whether or not the value is valid.

[dummzeuch 1506]

I usually prefix functions like this with "Try", so instead of

function GetTheValue(parameters ...): SomeType;

it's

function TryGetTheValue(parameters ...; out Value: SomeType): Boolean;

[Roger Cigol 103]

2 hours ago, dummzeuch said:

TryGetTheValue(

Very good suggestion by @dummzeuch Very few people give enough thought to naming of functions and variables. Developing a consistent approach can significantly help with long term program readability / long term support for large projects. Anything that hints at doing that gets my full support !

https://cigolblog.wordpress.com/2023/01/

https://cigolblog.wordpress.com/2019/10/

https://cigolblog.wordpress.com/2017/06/

[jesu 0]

Thanks for the suggestions.

Returning a dummy value would mean rewriting the stored procedures so that's a no-no.

I'm trying to avoid Variants because in the past I've had problems with variants <-> float. I've tried this instead

 

procedure ProcValorDeCombo(var pn_param: TFDParam; pt_Combo: TComboBox);
var
 vn_marca: double;
begin
  case pt_Combo.ItemIndex of
   -1: begin 
         // code ...
         pn_param.AsFloat := vn_marca;
       end;

   // code...
   else pn_param.Clear;    
  end;
end;

but when I try to call it

ProcValorDeCombo(ParamByName('myparam'), mycombo);

I get E2197 Constant object cannot be passed as var parameter

 

Edited October 18 by jesu

[Der schöne Günther 316]

Why did you mark the parameter as var in the first place?

[jesu 0]

Because I'm modifying it inside the procedure

 

[jesu 0]

ok, I'm not modifying tparam, but tparam.value

[Anders Melander 1795]

26 minutes ago, jesu said:

Because I'm modifying it inside the procedure

...

ok, I'm not modifying tparam, but tparam.value

The TFDParam parameter is an object reference (i.e. a pointer). You are not modifying the object reference; You are modifying a property on the object. So drop the var.

 

I would also change the TComboBox parameter to an integer and pass TComboBox.ItemIndex instead; There's no reason to create a dependency on TCombobox when all you need is the ItemIndex.

[jesu 0]

Right, I've removed var and it works, but the combobox style is csDropDown so I check both itemindex and text. And there are lots of them, so if I have a problem I use the name to find which one is wrong.