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(emacs-lisp-intro.info)multiply-by-seven in detail

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An Interactive `multiply-by-seven'
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   Let's look at the use of the special form `interactive' and then at
the function `message' in the interactive version of
`multiply-by-seven'.  You will recall that the function definition
looks like this:

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

   In this function, the expression, `(interactive "p")', is a list of
two elements.  The `"p"' tells Emacs to pass the prefix argument to the
function and use its value for the argument of the function.

   The argument will be a number.  This means that the symbol `number'
will be bound to a number in the line:

     (message "The result is %d" (* 7 number))

For example, if your prefix argument is 5, the Lisp interpreter will
evaluate the line as if it were:

     (message "The result is %d" (* 7 5))

(If you are reading this in GNU Emacs, you can evaluate this expression
yourself.)  First, the interpreter will evaluate the inner list, which
is `(* 7 5)'.  This returns a value of 35.  Next, it will evaluate the
outer list, passing the values of the second and subsequent elements of
the list to the function `message'.

   As we have seen, `message' is an Emacs Lisp function especially
designed for sending a one line message to a user.  (Note: The
`message' function.)  In summary, the `message' function
prints its first argument in the echo area as is, except for
occurrences of `%d', `%s', or `%c'.  When it sees one of these control
sequences, the function looks to the second and subsequent arguments
and prints the value of the argument in the location in the string
where the control sequence is located.

   In the interactive `multiply-by-seven' function, the control string
is `%d', which requires a number, and the value returned by evaluating
`(* 7 5)' is the number 35.  Consequently, the number 35 is printed in
place of the `%d' and the message is `The result is 35'.

   (Note that when you call the function `multiply-by-seven', the
message is printed without quotes, but when you call `message', the
text is printed in double quotes.  This is because the value returned by
`message' is what appears in the echo area when you evaluate an
expression whose first element is `message'; but when embedded in a
function, `message' prints the text as a side effect without quotes.)