Topic: Determining Even or Odd

[Hangman]

Hey guys, I'm in a bit of a pickle.  I'm about to get swamped (no pun intended     ) with a "need it yesterday" type of project.  This thing will be an ongoing project for the next 6 months.  It's a nightmare, long story.  So I'm trying to put together a couple routines that will help me for the onslaught come Monday morning.

Is there a way to determine a real number be either Even or Odd without having to type in every single number from 1 to 50 ??

Code: [Select]

...
  (setq pt-dist_12 (distance pt1 pt2))
  (setq plate-ctr (/ pt-dist_12 3))
  (setq plate-ctr (fix plate-ctr))
  (progn
    (cond  ( (  ... ;;If plate-ctr is Odd then do something ...
              ( (  ... ;;If plate-ctr is Even then do something else ...
  ))))

Is there a possibility of that or do I need to do something like this ...

Code: [Select]

...
  (setq pt-dist_12 (distance pt1 pt2))
  (setq plate-ctr (/ pt-dist_12 3))
  (setq plate-ctr (fix plate-ctr))
  (progn
    (cond  ( (or (= plate-ctr 1) (= plate-ctr 3) (= plate-ctr 5) (= plate-ctr 7) (= plate-ctr 9)) ;; all the way to 49
                ( ...  do something here ... ))
              ( (or (= plate-ctr 2) (= plate-ctr 4) (= plate-ctr 6) (= plate-ctr 8)  (= plate-ctr 10)) ;; all the way to 50
                ( ...  do something here ... ))
  ))))

Thank you for your thoughts & help.

[MickD]

is there a modulo operator in lisp??

if num%2 > 0 answer is odd. (where '%' = modulo operator)

[MickD]

lisp arithmetic function 'rem' looks like it should do the trick

[JohnK]

Another spin on the topic:
If the last number is a one, its odd. If its not, then its even.

1: 00000001
2: 00000010
3: 00000011
4: 00000100
...

[JohnK]

(defun odd? (i)
  ;; odd?
  ;; Returns 0 if even and 1 if odd
   (logand i 1))

[MP]

Code: [Select]

(defun IsEven ( x )
    (zerop (rem x 2))
)

Code: [Select]

...
(setq
    pt-dist_12 (distance pt1 pt2)
    plate-ctr  (fix (/ pt-dist_12 3))
)

(if (IsEven plate-ctr)
    (DoStuffForEvenCondition)
    (DoStuffForOddCondition)
)
...


[MickD]

logand would be by far the quickest given the fact you don't need the remainder and division is a slow(er) operation for a cpu, another practical use for 0's and 1's

[Hangman]

Hey guys, thanks for the quick responses.  Looking over these, I'm think'n MickD is correct, I'd prefer to use the logand function.  I like the post Se7en posted,

(defun odd? (i)
  ;; odd?
  ;; Returns 0 if even and 1 if odd
   (logand i 1))

But I'm not understanding exactly how it's to work.  I've tried it with several different numbers, but it keeps coming back as Odd.  I don't think I'm working it properly and the examples in the help files don't give me much.  Could you guys post a couple examples for me. ??

Nice post MP, I like the way you did that.  Thanks for the input & help.

[MP]

logand would be by far the quickest given the fact you don't need the remainder and division is a slow(er) operation for a cpu, another practical use for 0's and 1's

... but no practical speed difference is realized in lisp --

Elapsed milliseconds / relative speed for 32768 iteration(s):

    (zerop (rem 1 2)) ............... 1438 / 1.01 <fastest>
    (zerop (logand 1 1)) ............ 1453 / 1.00 <slowest>

    (zerop (rem 255 2)) ............. 1437 / 1.01 <fastest>
    (zerop (logand 255 1)) .......... 1453 / 1.00 <slowest>

    (zerop (rem 65535 2)) ........... 1469 / 1.01 <fastest>
    (zerop (logand 65535 1)) ........ 1485 / 1.00 <slowest>

    (zerop (rem 1073741824 2)) ...... 1468 / 1.02 <fastest>
    (zerop (logand 1073741824 1)) ... 1500 / 1.00 <slowest>

[MickD]

layer upon layer upon layer.....

oh well, it was food for thought though.

in technical terms, rem/modulo returns the remainder from a division eg 12/10 = 2 and 12/2 = 0.

[Hangman]

Yeah, well, my head feels like it's going to burst into flames.

K, so, 'rem' might be faster.  I don't think a couple milliseconds is going to make too much of a difference here.  But I'm learn'n someth'n new.

I just found the Logand info  (couldn't find it online, had to revert to an old book written for release 12).
So, the code Se7en posted:

Code: [Select]

(defun odd? (i)
  ;; odd?
  ;; Returns 0 if even and 1 if odd
   (logand i 1))
The 'i', to my understanding, is the integer to be tested ?.
So, one would setq i to an integer, and the 'logand' would test i for an odd bitcode ?.

So I could set my code something like:

Code: [Select]

...
(setq
    pt-dist_12 (distance pt1 pt2)
    plate-ctr  (fix (/ pt-dist_12 3))
)
(setq Odd-Even (logand plate-ctr 1))
(progn
(if (= Odd-Even 0)
    (DoStuffForEvenCondition)
    (DoStuffForOddCondition)
))
...

Hey look, it worked !!!  Too cool.  Thanks guys.

[CAB]

Code: [Select]

;; returns t of num is odd
(defun odd? (num)
   (= (logand num 1) 1)
)

Code: [Select]

(print (odd? 1))
(print (odd? 2))
(print (odd? 3))
(print (odd? 4))

Code: [Select]

;; returns t of num is even
(defun even? (num)
   (zerop (logand num 1))
)

Code: [Select]

(print (even? 1))
(print (even? 2))
(print (even? 3))
(print (even? 4))

[JohnK]

<snip>
    (zerop (rem 65535 2)) ........... 1469 / 1.01 <fastest>
    (zerop (logand 65535 1)) ........ 1485 / 1.00 <slowest>

    (zerop (rem 1073741824 2)) ...... 1468 / 1.02 <fastest>
</snip>

Hey, wait a min. Well that scenario has got to be slower. Your pinning one boolean test up against two (Native mathematics is the fastest hands down so that can be negated from the test.).

If we pin my one Boole up against your math&Boole i bet i could do better then that.

[JohnK]

<snip>

Code: [Select]

;; returns t of num is even
(defun even? (num)
   (zerop (logand num 1))
)</snip>

One question: Why have a test for both odd and even? ``If its not one...''

[JohnK]

<snip>
The 'i', to my understanding, is the integer to be tested ?.
So, one would setq i to an integer, and the 'logand' would test i for an odd bitcode ?.
</snip>

And returns true if and oly if both tests are true. In the case of `logand ... 1' we are testing to see if the first bit is a one in an int.  (Does that make sence?)

1 = 00000001
2 = 00000010

So

(logand 2 1) returns what?