string — for char, a string; for schar, a simple string.

index — a valid array index for the string.

character, new-character — a character.

char and schar access the element of string specified by index.

char ignores fill pointers when accessing elements.

 (setq my-simple-string (make-string 6 :initial-element #\A)) → "AAAAAA" 
 (schar my-simple-string 4) → #\A 
 (setf (schar my-simple-string 4) #\B) → #\B 
 my-simple-string → "AAAABA" 
 (setq my-filled-string 
       (make-array 6 :element-type 'character 
                     :fill-pointer 5 
                     :initial-contents my-simple-string)) 
→ "AAAAB" 
 (char my-filled-string 4) → #\B 
 (char my-filled-string 5) → #\A 
 (setf (char my-filled-string 3) #\C) → #\C 
 (setf (char my-filled-string 5) #\D) → #\D 
 (setf (fill-pointer my-filled-string) 6) → 6 
 my-filled-string → "AAACBD"

aref, elt, Section 3.2.1 (Compiler Terminology)

(char s j) ≡ (aref (the string s) j)