Emacs Interactive functions with arguments

[Christopher Dimech <dimech@gmx.com>]

Dear Compeers,

I understand a few things about Emacs Functions.

For instance, the following function uses simply
an (interactive) declaration.  This means that
IntchgW will not accept the passing of arguments
to it.

( defun IntchgW ()
    "Interchanges words using (transpose-words arg)."
    (interactive)
    (transpose-words -2)
)

Have also constructed the function IntchgW to interchange
words.

( defun IntchgW (arg)
    "Interchanges words using (transpose-words arg)."
    (interactive "NType Arg to transpose-words: ")
    (transpose-words arg)
)

I can call the function using two methods
(1) Using  "M-x IntchgW <Ret> -2 <Ret>"
    Arg is inputted through the Mini-Buffer.
(2) Using "C-u -2 M-x IntchgW"
    Here Arg is passed as a prefix to IntchgW.

And if I use (interactive "nType Arg to transpose-words: "),
I can only pass the argument Arg using the Min-Buffer
"M-x IntchgW <Ret> -2 <Ret>".  I cannot pass Arg as a prefix
to  IntchgW using "C-u -2 M-x IntchgW"

But then I have the following functions.  And the effect of the
(interactive "p") and (interactive "pType Arg to transpose-words: ")
is confusing to me.

Besides general comments, I have some specific question.
(1) The two constructs (interactive "p") and (interactive "pMessage: ")
are valid, but are they equivalent?
(2) How is (interactive "p") different from  (interactive "N").
They both represent numeric arguments, but what is different?

Reagrds

( defun IntchgWp (arg)
    "TODO"
    (interactive "p")
    (transpose-words arg)
)

( defun IntchgW--pmesg (arg)
    "TODO"
    (interactive "pType Arg to transpose-words: ")
    (transpose-words arg)
)