Simplest and fasted way to do this is:

FUNCTION F_ContainsString : BOOL
VAR_IN_OUT
	StringArray : ARRAY [*] OF T_MaxString;
END_VAR
VAR_IN_OUT CONSTANT
	Value : T_MaxString;
END_VAR
VAR
	i : DINT;
END_VAR

=============================================================

FOR i := LOWER_BOUND(StringArray, 1) TO UPPER_BOUND(StringArray,1) DO 
	F_ContainString := StringArray[i] = Value;
	IF F_ContainString THEN RETURN; END_IF
	END_FOR

I have also library called TwinCAT Dynamic Collections.

There's a data structure called a Set, it only stores unique values. You can use the hash set (FB_Hash_Set) or tree set (FB_Tree_Set) to store existing QR codes then check of they exist in the set. It's dynamic so you can store as many QR codes as you need for your application. Heck, you can use it to store all the QR codes in your inventory (as long as you have less than 2 billion items) at run-time and find then extremely fast.

Here's some example code:

PROGRAM MAIN
VAR CONSTANT
	QR_CODE_LENGTH : __UXINT := 4;
END_VAR
VAR
	eState : (E_INIT, E_RUN);
	bDoesQrCodeExist : BOOL;
	sNewQrCode : STRING(QR_CODE_LENGTH) := '0011';
	fbQrCodeSet : TcDynCollections.FB_Hash_Set(0);
	arsQrCodes : ARRAY[0..4] OF STRING(QR_CODE_LENGTH) := ['1001', '0110', '1101', '1110', '0101'];
	i : TcDynCollections.T_Position;
END_VAR
=============================================================
CASE eState OF
	E_INIT: // Add existing QR codes to set.
		FOR i := 0 TO 4 DO fbQrCodeSet.Insert(arsQrCodes[i]); END_FOR
		eState := E_RUN;
	E_RUN: // Check if QR Code is new by checking of it's contained in the set.
		bDoesQrCodeExist := fbQrCodeSet.Contains(sNewQrCode);	
END_CASE

You can even take advantage of a Map/Dictionary/Associative Array (FB_Hash_Map or FB_Tree_Map) in the library to store metadata along with your QR code in the sense that Key = QR Code and Value = Struct/Function Block containing metadata.

@runtimevictor Thanks, sounds better....