Syntax for Returning One Character of String by Index - plc

Question

I am attempting to compare one character of a string to see if it is my delimiter character. However, when I execute the following code the value that gets placed in the variable valstring is a number that represents the byte that was converted to a string and not a character itself. For Example the value may be the string '58'.
Through my testing in CoDeSys using the debugging features I know that the string sReadLine contains a valid string of characters. I'm just not sure of the syntax to single only one of them out; the sReadLine[valPos + i] part is what I don't understand.
sReadLine : STRING;
valstring : STRING;
i : INT;
valPos : INT;
FOR i := 0 TO 20 DO
IF BYTE_TO_STRING(sReadLine[valPos + i]) = '"' THEN
EXIT;
END_IF
valstring := CONCAT(STR1 := valstring, STR2 := BYTE_TO_STRING(sReadLine[valPos + i]));
END_FOR

Answer 1

I think you have multiple choises.
1) Use built-in string functions instead. You can use MID function get get part of a string. So in your case something like "get one character from valPos + 1 from sReadLine.
FOR i := 0 TO 20 DO
IF MID(sReadLine, 1, valPos + i) = '"' THEN
EXIT;
END_IF
valstring := CONCAT(STR1 := valstring, STR2 := MID(sReadLine, 1, valPos + i));
END_FOR
2) Convert the ASCII byte to string. In TwinCAT systems, there is a function F_ToCHR. It takes a ASCII byte in and returns the character as string. I can't find something like that for Codesys, but i'm sure there would be a solution in some library. So please note that this won't work in Codesys without modifications:
FOR i := 0 TO 20 DO
IF F_ToCHR(sReadLine[valPos + i]) = '"' THEN
EXIT;
END_IF
valstring := CONCAT(STR1 := valstring, STR2 := F_ToCHR(sReadLine[valPos + i]));
END_FOR
3) The OSCAT library seems to have a CHR_TO_STRING function. You could use this instead of F_ToCHR in step 2.
4) You can use pointers to copy the ASCII byte to a string array (MemCpy) and add a string end character. This needs some knowledge of pointers etc. See Codesys forum for some example.
5) You can write a helper function similar to step 2 youself. Check the example from Codesys forums. That example doesn't include all characters so it needs to be updated. It's not quite elegant.

Answer 2

When you convert a byte to a string, what is beeing converted is the digital representation of the byte.
This means you are interpreting that byte as an ascii character (The ascii decimal value of : is 58).
So if you want to Concat chars instead of their ascii decimal representation, you need another function:
valstring := CONCAT(STR1 := valstring, STR2 := F_ToCHR(sReadLine[valPos + i]));
EDIT:
As Quirzo, I couldn't find a similar F_ToCHR function for Codesys, but you could easily build one yourself.
For example:
Declaration Part:
FUNCTION F_ASCII_TO_STRING : STRING
VAR_INPUT
input : BYTE;
END_VAR
VAR
ascii : ARRAY[0..255] OF STRING(1):=
[
33(' '),'!','"','#',
'$$' ,'%' ,'&' ,'´',
'(' ,')' ,'*' ,'+' ,
',' ,'-' ,'.' ,'/' ,
'0' ,'1' ,'2' ,'3' ,
'4' ,'5' ,'6' ,'7' ,
'8' ,'9' ,':' ,';' ,
'<' ,'=' ,'>' ,'?' ,
'#' ,'A' ,'B' ,'C' ,
'D' ,'E' ,'F' ,'G' ,
'H' ,'I' ,'J' ,'K' ,
'L' ,'M' ,'N' ,'O' ,
'P' ,'Q' ,'R' ,'S' ,
'T' ,'U' ,'V' ,'W' ,
'X' ,'Y' ,'Z' ,'[' ,
'\' ,']' ,'^' ,'_' ,
'`' ,'a' ,'b' ,'c' ,
'd' ,'e' ,'f' ,'g' ,
'h' ,'i' ,'j' ,'k' ,
'l' ,'m' ,'n' ,'o' ,
'p' ,'q' ,'r' ,'s' ,
't' ,'u' ,'v' ,'w' ,
'x' ,'y' ,'z' ,'{' ,
'|' ,'}' ,'~'
];
END_VAR
Implementation part:
F_ASCII_TO_STRING := ascii[input];

Answer 3

As Sergey said, this might not be an optimal solution to your problem. It seems like you want to extract the longest substring not containing any character " from initial input sReadLine to valstring, starting from position valPos.
In your implementation, for each valid input character, CONCAT() needs to search for the end of valstring, before appending only 1 character to it.
You should rather decompose your problem and use two standard functions to be optimal:
FIND() --> to get the position of the next character " (or to know if there is none),
MID() --> to create a string from initial position up to before the first character " (or the end of the input string).
That way, there remains only 2 loops; each one is hidden in these functions.