In IDL, the semicolon is the comment character. When IDL encounters the semicolon, it ignores the remainder of the line. It is good programming practice to fully annotate programs with comments. There are no execution-time or space penalties for comments in IDL.
A comment can exist on a line by itself, or can follow another IDL statement, as shown below:
; This is a comment COUNT = 5 ; Set the variable COUNT equal to 5.