What is Free Pascal Error 3208 Illegal assignment to for-loop variable

Question

Tell me about Free Pascal compiler error 3208 "Illegal assignment to for-loop variable."

Answer 1

We cannot change the value of the indexer variable in a For-In loop.

For demonstration, copy this code into a new Project of type Simple Program, and compile it. Then, add the '$' character to the "Define symDemo" comment in the first line, to convert it to a compiler directive, and try to compile.

For discussion see https://forum.lazarus.freepascal.org/index.php?topic=17488.0

program ForIn; {$AppType Console} { Define symDemo}
var
  strChrLst              , // list of  characters, without separators
  idxChrOne : string     ; // only one character; indexer in For-In loop
  strChrUpr : string = ''; // uppercase version of the current character, init-ed to empty
begin
  strChrLst := 'a[z'                            ; // initialize list of characters to work on
  for idxChrOne in strChrLst do begin           ; // scan all characters in the list
    {$IfNDef symDemo}                           ; // when "symDemo" is not defined
    strChrUpr := UpCase( idxChrOne )            ; // convert the current character to upcase
    {$Else}                                     ; // when "symDemo" is     defined
    idxChrOne := UpCase( idxChrOne )            ; // gives Error 3208 on "idxChrOne"
    {$EndIf}                                    ; // done with "symDemo"
    if ( strChrUpr < 'A' )                        // when the current character is
    Or ( strChrUpr > 'Z' ) then begin           ; //    outside the range of alphabet letters
      writeln(StdErr,'Not a letter: ',idxChrOne); // emit the non-letter character on StdErr
    end                                         ; // done with the non-letter character
  end                                           ; // done scanning characters in the list
end.