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3.2.1 Change a Function Definition

If you want to change the code in multiply-by-seven, just rewrite it. To install the new version in place of the old one, evaluate the function definition again. This is how you modify code in Emacs. It is very simple.

As an example, you can change the multiply-by-seven function to add the number to itself seven times instead of multiplying the number by seven. It produces the same answer, but by a different path. At the same time, we will add a comment to the code; a comment is text that the Lisp interpreter ignores, but that a human reader may find useful or enlightening. The comment is that this is the “second version”.

     (defun multiply-by-seven (number)       ; Second version.
       "Multiply NUMBER by seven."
       (+ number number number number number number number))

The comment follows a semicolon, ‘;’. In Lisp, everything on a line that follows a semicolon is a comment. The end of the line is the end of the comment. To stretch a comment over two or more lines, begin each line with a semicolon.

See Beginning a .emacs File, and Comments, for more about comments.

You can install this version of the multiply-by-seven function by evaluating it in the same way you evaluated the first function: place the cursor after the last parenthesis and type C-x C-e.

In summary, this is how you write code in Emacs Lisp: you write a function; install it; test it; and then make fixes or enhancements and install it again.