I think all of us know the how works COALESCE SQL function. True? In a simple version, evaluates two arguments and if the first is NULL, returns the second.

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I think all of us know the how works COALESCE SQL function. True? In a simple version, evaluates two arguments and if the first is NULL, returns the second.

I need this behaviour in Delphi.

Anyone know a standard function like this.
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The problem becomes because I have a function that receives two parameters of type char and I need to send to it as the argument the first character of a TField and this TField can be null.

ReorganizeCoordinatesControls(
FEditComponent.LAYOUT.AsString[1],
FEditComponent.ORIENTATION.AsString[1]);

when is null, taking the first character causes an AVE. Apart, a variable of type char, can't receive an empty string.

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Of course, I know how to create the function. The curiosity is about if a standard function like this exists in the Delphi libraries.

Of course, I've searched on google, in forums, in stackoverflow and so on.

Anyone knows?
https://docs.microsoft.com/es-es/sql/t-sql/language-elements/coalesce-transact-sql?view=sql-server-2017

Comments

  1. Uwe RaabeSeptember 20, 2018 at 4:17 AM

    Not quite sure what you are looking for, but a simple
    (FEditComponent.LAYOUT.AsString + #20)[1] should do it. Just change the to whatever default you need.

    ReplyDelete
  2. Achim KalwaSeptember 20, 2018 at 4:22 AM

    There is "IfThen" from StrUtils.pas, which comes close to Coalesce:

    function IfThen(
    AValue: Boolean;
    const ATrue: string;
    AFalse: string = ''): string;

    To use it, pass the result of IfThen() to ReorganizeCoordinatesControls():

    IfThen(FEditComponent.LAYOUT.IsNull, #0, FEditComponent.LAYOUT.AsString[1])

    ReplyDelete
  3. Uwe RaabeSeptember 20, 2018 at 4:51 AM

    Achim Kalwa Unfortunately that won't work. IfThen is a function and all parameters are evaluated for the call, which will give the same AV.

    ReplyDelete
  4. Achim KalwaSeptember 20, 2018 at 4:57 AM

    Uwe Raabe You are right.

    ReplyDelete
  5. Jeroen Wiert PluimersSeptember 20, 2018 at 6:10 AM

    Why not this:

    ReorganizeCoordinatesControls(
    FEditComponent.LAYOUT,
    FEditComponent.ORIENTATION);

    Then have ReorganizeCoordinatesControls determine which TField is assigned and take the first character from the .AsString.

    ReplyDelete
  6. Juan C. CillerueloSeptember 20, 2018 at 7:55 AM

    Finally, I did this:

    this is the call
    ReorganizeCoordinatesControls(
    Coalesce(FEditComponent.LAYOUT, '#'),
    Coalesce(FEditComponent.ORIENTATION, '#'));

    This is the method

    function TConfigurationsController.Coalesce(
    AField :TStringField; IfEmpty :char):char;
    begin
    if AField.IsNull then Result := IfEmpty
    else Result := AField.AsString[1];
    end;


    Uwe Raabe
    Your solution is good. But I think is more like a trick, not a technique.

    And more, it assumes that the dataset that owns the TField is in Edit mode, and maybe not.

    I hate the tricks. They complicate the maintenance of the code.

    I know. ...a good comment, near it... but... As Kernighan and Plauger emphasize, "Don't document bad code, rewrite it"

    Anyway, thanks to all.

    ReplyDelete
  7. Juan C. CillerueloSeptember 20, 2018 at 9:17 AM

    Attila Kovacs

    You have reason about the bug.

    Here is the fixing:

    function TConfigurationsController.Coalesce(
    AField :TStringField; IfEmpty :char):char;
    begin
    if (AField.IsNull) or (AField.AsString.Trim = '') then Result := IfEmpty
    else Result := AField.AsString[1];
    end;


    And you have reason about the other solution:

    It's a solid trick. At least if the DataSet is in any edit Mode




    Jokes apart. Thanks for pointing me the bug.

    ReplyDelete
  8. Uwe RaabeSeptember 20, 2018 at 2:19 PM

    Juan C. Cilleruelo What makes you think that the dataset has to be in edit mode?

    ReplyDelete

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