Returns a value which represents the number of characters contained in a string.
LEN      ( string ) 
           └─┬──┘ 
     String expression
LEN(A$)
string: String expression
Returns a value which represents the number of characters contained in a string, including characters that don’t appear on the display (character codes from 00ʰ ~ 1Fʰ) and spaces.
10  INPUT "INPUT CHARACTERS"; C$
20  PRINT "LENGTH="; LEN(C$)
30  END
Determines the length of an input string.