Check if a string constant is contained in a set of strings

Question

What is the best and most SIMPLE way to do this in modern Delphi (Sydney):

var ThisExtension: string;
if ThisExtension in ['.jpeg', '.jpg', '.jpe', '.jif', '.jfif'] then

The above code gives me this compiler error:

E2015 Operator not applicable to this operand type

The answer in the proposed SO question "Set Of String??!!" does not nearly meet the required criteria of simplicity as much as MatchText does: A `type` must be declared AND a `constant` must be declared AND a `variable` must be declared etc. On the other hand, `System.StrUtils.MatchText` is a simple and fast one-liner. — user1580348, Jun 17 '21 at 06:30

score 2 · Accepted Answer · answered Jun 15 '21 at 23:42

2

The System.StrUtils unit has a simple function for this:

if System.StrUtils.MatchText(ThisExtension, ['.jpeg', '.jpg', '.jpe', '.jif', '.jfif']) then

I don't know whether this is the most modern approach. (Maybe Generics has a better approach?). But it is fast and simple.

answered Jun 15 '21 at 23:42

user1580348

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Note that `MatchText()` is case-insensitive. If you want a case-sensitive comparison, use [`MatchStr()`](http://docwiki.embarcadero.com/Libraries/en/System.StrUtils.MatchStr) instead. – Remy Lebeau Jun 15 '21 at 23:57
Who needs case sensitivity when comparing file extensions? – user1580348 Jun 15 '21 at 23:59
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Not all file systems are case-insensitive. – Remy Lebeau Jun 15 '21 at 23:59
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Yes, this is the best modern way IMHO. You can also declare a constant `const PictureExts: TArray = ['.bmp', '.png', '.jpg', '.gif'];` and use that. – Andreas Rejbrand Jun 16 '21 at 08:52
The possibility of declaring an expression like `TArray = ['.bmp', '.png', '.jpg', '.gif']` as a constant is an ingenious simplification in `Generics`. – user1580348 Jun 17 '21 at 07:06
@AndreasRejbrand Could you please write an answer with an `if` condition using `TArray = ['.bmp', '.png', '.jpg', '.gif']` so I can accept it? – user1580348 Jun 17 '21 at 10:03
@user1580348: I could, but I wouldn't feel good about it because that answer would essentially be the same as your answer but with a tiny difference! (Essentially: `A(12, 13)` vs. `C := 13; A(12, C)`.) You can accept your own answer, and anyone can read my comment about the fairly new Delphi possibility to create dynamic array constants. – Andreas Rejbrand Jun 17 '21 at 10:15
@AndreasRejbrand I can accept my own answer only 10 hours from now. Meanwhile, some unknown person has closed my question (!) although I have clearly stated in a comment that the proposed alternative answer does not meet the criteria of my question! This is very strange! There are strange people here on SO... – user1580348 Jun 17 '21 at 12:30
@AndreasRejbrand When I try to declare a constant like in your example: `const PictureExts: TArray = ['.bmp', '.png', '.jpg', '.gif'];` then I get a compiler error: `E2086 Type 'TArray' is not yet completely defined` – user1580348 Jun 17 '21 at 20:20
@user1580348: Which Delphi version are you using? – Andreas Rejbrand Jun 17 '21 at 21:04
@AndreasRejbrand Delphi 10.4.2. However, when I declare `array of string` instead then it works. – user1580348 Jun 17 '21 at 22:53
@user1580348: I assume you are declaring the `const` as a member of a class or record type? Because the syntax works for (1) global constants, (2) local constants, and (3) inline-local constants, but not for member constants. I consider this a bug in the Delphi compiler. – Andreas Rejbrand Jun 18 '21 at 07:13
@AndreasRejbrand You are right: When I declare it as an inline/local constant then it works. However, meanwhile, I solved the problem by using `array of string` as a class member instead. – user1580348 Jun 18 '21 at 21:41
[System.StrUtils.IndexText] (http://docwiki.embarcadero.com/Libraries/Sydney/en/System.StrUtils.IndexText) `IndexText(ThisExtension, ['.jpeg', '.jpg', '.jpe', '.jif', '.jfif']) >= 0;` – USauter Jun 29 '21 at 18:28

Remy Lebeau · Answer 2 · 2021-06-16T00:05:29.327

0

Maybe not the simplest or most elegant, but you can use a TStringList, eg:

var
  Exts: TStringList;

procedure InitExtensions;
begin
  Exts := TStringList.Create;
  Exts.CaseSensitive := ...; // false by default
  Exts.Add('.jpeg');
  Exts.Add('.jpg');
  Exts.Add('.jpe');
  Exts.Add('.jif');
  Exts.Add('.jfif');
end;

...

var
  ThisExtension: string;
begin
  ...
  if Exts.IndexOf(ThisExtension) <> -1 then
  begin
    ...
  end;
  ...
end;

...

initialization
  InitExtensions;
finalization
  Exts.Free;
end.

edited Jun 16 '21 at 00:05

answered Jun 15 '21 at 23:50

Remy Lebeau

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That does not meet the criteria of simplicity as much as `MatchText` does. – user1580348 Jun 15 '21 at 23:53
I would use `try-end-finally`. – user1580348 Jun 16 '21 at 00:03
@user1580348 Sure, if the `TStringList` is a local variable, but that is actually not what I intended. I have updated my example – Remy Lebeau Jun 16 '21 at 00:05

Check if a string constant is contained in a set of strings

2 Answers2