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About function parameters, is it a bug of elisp interpreter?
From: |
Lee David |
Subject: |
About function parameters, is it a bug of elisp interpreter? |
Date: |
Tue, 22 May 2007 02:22:56 +0800 |
Hi, list
When porting some Scheme functions to Elisp, I encountered a strange
(to me) problem
as listed below.
;; compute intergral value of a math function with Simpson formula.
;; h/3 * [ Y0 + 4Y1 + 2Y2 + 4Y3 + 2Y4 + ... + 2Yn-2 + Yn ]
;; ---- [1] ----
;; (defun sum (term a next end)
;; (defun iter (a result)
;; (if (> a end)
;; result
;; (iter (funcall next a) (+ result (funcall term a)))))
;;
;; (iter a 0))
;; ---- [2] ----
(defun sum (term beg next b)
(defun iter (beg result)
(if (> beg b)
result
(iter (funcall next beg) (+ result (funcall term beg)))))
(iter beg 0))
;; ---- [3] ----
(defun simpson (f a b n)
(setq h (/ (- (float b) a) n))
(defun y (k)
(funcall f (+ a (* k h))))
(defun term (i)
(+ (y (- (* 2 i) 2))
(* 4 (y (- (* 2 i) 1)))
(y (* 2 i))))
(defun next (i)
(1+ i))
(/ (* h (sum 'term 1 'next (/ n 2))) 3))
(simpson (lambda (x) (* x x)) 0 1 100) ; 0.3333333333333333
--------------------------------------------- End Source
-------------------------------------------
The only difference between commented-out source code [1] and in-use code [2]
lies in that they chosed different parameter names, `a' for former and
`beg' for latter.
The problem is that [1] can't work when [3] choose the same name, i.e. `a'.
when both [1] and [3] choose the same name `a', I found that local function `y'
defined in `simpson' will get value `a' as 1, 2, 3, 4 ... 100 each
time it is called,
rather than constant zero as I assumed. I do not know why.
Is it a bug? Or, more probably, there are potential issues I didn't see here?
Hope someone can elaborate a little bit further, since I am newbie to elisp.
Not quite sure whether it is the right list to ask, if any better
choice, pls point out.
Tested on:
(version)
"GNU Emacs 23.0.0.1 (i386-mingw-nt5.1.2600)
of 2007-01-02 on DTOP"
and,
(version)
"GNU Emacs 21.4.1 (i486-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
of 2007-01-16 on palmer, modified by Debian"
--
Thanks,
Li Qun
- About function parameters, is it a bug of elisp interpreter?,
Lee David <=