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1. There is nothing that I can see with or without it.
    
2. Returns with all lower case as it was defined.
    
3. I think you mean c instead s.

#include <hmg.ch>

Function Main()
LOCAL c := "alles", s := "abcdefghijk"

FOR EACH c IN s
    IF c $ "aei"
       c := UPPER( c )
       MsgDebug(c) // AEI
    ENDIF
NEXT
MsgDebug(s)      // abcdEfghIjk
MsgDebug(c)      // alles
Return Nil

1. First at all, I wish to clear up when we use this routine is coming from Harbour not from HMG. It's our dialect language xBase that empires and it's nothing with HMG.
    
2. When you use $ its means that is be contained at any string.
    
3. The c var in your example it's in trying to be re-assigned but it has momentainly assigned because it's previous been taken the var in the address memory (let's say as reference, often used with @) but it can't when back at begining of FOR EACH. You can see the added MsgDebug(c) that I putted in.
    
4. The power of FOR EACH with IN and the $ operator is a binary relational. Nothing else.

bluebird wrote: ↑Mon Mar 13, 2017 5:49 am Dear Mentor

The following excerpt from HMG for beginners does not realy explain what seems to be a powerful loop device
Can anyone provide a better example and explanation?

Does "c IN @s" mean "character in both strings"? What is the at sign for
Thanks

Local c := "alles", s := "abcdefghijk"
FOR EACH c IN @s
   IF c $ "aei"
      c := UPPER( c )
   ENDIF
NEXT
? s      // AbcdEfghIjk
? s      // alles

bluebird wrote: ↑Mon Mar 13, 2017 5:49 am Does "c IN @s" mean "character in both strings"?

   FOR EACH c IN @s 

1. take a character of string <s> (starting from the first one)
2. store it in variable <c>
3. use this value of <c> as argument for code found inside the loop.
4. repeat steps 1..3 until all characters of <s> have been passed to <c> and processed

bluebird wrote: ↑Mon Mar 13, 2017 5:49 amWhat is the at sign for

PeteWG wrote: ↑Mon Mar 13, 2017 3:08 pm The "at" <@> symbol means that members of <s> are passed to <c> and therefore to the code inside loop, by reference.
this means that any change of value of <c> inside the loop, will reflect back to string <s>, permanently changing its content.
If you don't use the <@> the string <s> will remain unaltered no matter what happens with <c> inside loop.
Note that if an array (instead of string) or hash is used as IN <expression>, their elements are always passed by reference.