Topic: What's wrong with "REM"?

[cjw]

1.
This is OK--------
(rem 1000 1000)
->0
(rem 1000.0 1000)
->0.0
2.My question is here:
Why???---------
(+ sum a)
->1000.0
(rem (+ sum a) 1000)
->1000.0

from the code:

(defun c:tt ()
  (setq lst '("193.8" "130" "150" "30" "110" "50" "80"))
  (setq   i      0
   sum    0
   return nil
   a      (rem 35256.2 1000)
   tmp    nil
  )
  (repeat (length lst)
    (setq b (read (nth i lst)))
    (setq sum (+ sum b))
    (setq tmp (cons (+ sum a) tmp))
    (setq return (cons (rem (+ sum a) 1000) return))
    (setq i (1+ i))
  )
  (princ (reverse tmp))
  (princ "\n")
  (princ (reverse return))
  (princ)
)

[Kerry]

RTFM

[Keith™]

The problem exhibits itself only with the result of addition in the function.

In the example provided, the value of sum is 743.8 and the value of a is 256.2, thus the value of (+ sum a) is 1000.0 and the value of (rem (+ sum a) 1000) should be 0.0 ...

To resolve the issue use this

Code: [Select]

(setq return (cons (rem (atof (rtos (+ sum a) 2 8)) 1000) return))


[CAB]

The mixing of reals and integers can be a "Got Ya" in lisp. As you can see if you use
integers for calculations you get integers BUT if you use one real your result will be a real.
This has caused more that one question here and added hours of debugging frustration
to many a programmer.

Also see FLOAT and FIX functions. There are many rounding routines here at The Swamp
if you need one.


PS I too have been guilty of reading the manual last.

[New Shooz]

The trouble I've always found with REM is their bald, skinny, whiny voiced singer - can't stand him to be honest  :pissed:

[Matt__W]

The trouble I've always found with REM is their bald, skinny, whiny voiced singer - can't stand him to be honest  :pissed:

"It's the end of the world as we know it......."

[Keith™]

The mixing of reals and integers can be a "Got Ya" in lisp. As you can see if you use
integers for calculations you get integers BUT if you use one real your result will be a real.
This has caused more that one question here and added hours of debugging frustration
to many a programmer.

Also see FLOAT and FIX functions. There are many rounding routines here at The Swamp
if you need one.


PS I too have been guilty of reading the manual last.

CAB, I don't think the problem is with real vs. integer calculations. The help clearly states that if you use a real and integer, a real is returned.

In the example provided, (rem (+ sum a) 1000) should return 0.0 as (+ sum a) = 1000.0 that would mean that (rem 1000.0 1000) = 0.0 and would be correct, except (rem (+ sum a) 1000) returns 1000.0 .. clearly an incorrect return value. It would appear as though there is a float error in the mathematics.

Try these examples
(rem 1000.0 999.99999999999999) = 0.0
(rem 999.9999999999999 1000) = 1000.0
(rem 999.99999999999999 1000) = 0.0

In the function provided, the value of a is 256.2 (the remainder of 35256.2 / 1000), the value of sum is incremented by the value of b and sum, initialized at 0

Even forcing an evaluation of the value would incorrectly show that the value (eval (+ sum a)) is less than 1000, which is clearly not true.

[CAB]

Thanks for the light bulb there Keith.
Would this also be a solution?

Code: [Select]

(defun c:tt ()
  (setq lst '("193.8" "130" "150" "30" "110" "50" "80"))
  (setq   i      0
   sum    0
   return nil
   a      (atof (rtos (rem 35256.2 1000) 2 8))  ; <-----<<
   tmp    nil
  )
  (repeat (length lst)
    (setq b (read (nth i lst)))
    (setq sum (+ sum b))
    (setq tmp (cons (+ sum a) tmp))
    (setq return (cons (rem (+ sum a) 1000) return))
    (setq i (1+ i))
  )
  (princ (reverse tmp))
  (princ "\n")
  (princ (reverse return))
  (princ)
)

[Keith™]

That does appear to be a resolution, as is the solution I put in my first post

[CAB]

Yes, i just took your solution & placed it outside the loop.

[cjw]

Thank you all! genius!
I just don't know how to express my gratitude！
Thanks.

[Keith™]

I just don't know how to express my gratitude！

Perhaps at some point in the future you might consider dropping a nickle in the offering plate .. although it is not required ...

incidently, glad to be of help

[tjr]

RTFM

No matter how you slice it this is pretty much a "DH" response. Sometimes people need a foot to backside push in the right direction to find things. Happens to everyone. However the elitist "RTFM" stuff is a little unwarranted.

[Kerry]

..  elitist ...

I THINK NOT !



I don't expect other people to do my homework , or study for me .. ( nor the reciprocal)  If that makes me elitist by your definition, I can live with it. 

[MP]

I THINK NOT !

It's curious why a man of your intelligence would be daft to the notion that this kind of response, in particular to a noob to the forum, would be somewhere between rude and off putting.