An Interactive multiply-by-seven, An Overview

Both the use of the special form interactive and one way to display a value in the echo area can be illustrated by creating an interactive version of multiply-by-seven.

Here is the code:

(defun multiply-by-seven (number)       ; Interactive version.
  "Multiply NUMBER by seven."
  (interactive "p")
  (message "The result is %d" (* 7 number)))

You can install this code by placing your cursor after it and typing C-x C-e. The name of the function will appear in your echo area. Then, you can use this code by typing C-u and a number and then typing M-x multiply-by-seven and pressing RET. The phrase ‘The result is …’ followed by the product will appear in the echo area.

Speaking more generally, you invoke a function like this in either of two ways:

1. By typing a prefix argument that contains the number to be passed, and then typing M-x and the name of the function, as with C-u 3 M-x forward-sentence; or,
2. By typing whatever key or keychord the function is bound to, as with C-u 3 M-e.

Both the examples just mentioned work identically to move point forward three sentences. (Since multiply-by-seven is not bound to a key, it could not be used as an example of key binding.)

(See Some Key Bindings, to learn how to bind a command to a key.)

A prefix argument is passed to an interactive function by typing the META key followed by a number, for example, M-3 M-e, or by typing C-u and then a number, for example, C-u 3 M-e (if you type C-u without a number, it defaults to 4).