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(emacs-lisp-intro.info)Interactive multiply-by-seven

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An Interactive `multiply-by-seven', An Overview
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   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.)

   (Note: Some Keybindings, 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).