Odd or Even

[fearless]

Use AND and BT. Odd numbers expressed in binary always have the rightmost (2^0) bit = 1 and even numbers always have the 2^0 bit = 0.

Code Select Expand

IsEven MACRO val1:REQ
push eax
mov eax, val1
and eax, 1
bt  eax, 0 ; test lowest bit
jnc Even ; jmp if bit is 0

:Odd
mov edx, FALSE
jmp OddEvenEnd

:Even
mov edx, TRUE

OddEvenEnd:

pop eax
exitm <edx>
ENDM

[NightWare]

 :eek why AND ?

EDIT :
you can reduce your macro to something like :

Code Select Expand

IsEven MACRO val1:REQ
xor edx,edx ; edx = 0
bt val1,0 ; test lowest bit
adc edx,edx ; edx = 0+carry
btc edx,0 ; invert bit 0
ENDM

and deduce this macro :

Code Select Expand

IsOdd MACRO val1:REQ
xor edx,edx ; edx = 0
bt val1,0 ; test lowest bit
adc edx,edx ; edx = 0+carry
ENDM


[jj2007]

you can reduce your macro to something like :

Code Select Expand

IsEven MACRO val1:REQ
xor edx,edx ; edx = 0
bt val1,0 ; test lowest bit
adc edx,edx ; edx = 0+carry
btc edx,0 ; invert bit 0
ENDM


EVEN that is much too ODD for my taste. Since there is a small risk that noobs read this thread, here is the final solution (see herge's reply #4):

      mov ebx, 123
      test ebx, 1
      .if zero?
         print str$(ebx), " is even", 13, 10
      .else
         print str$(ebx), " is odd", 13, 10
      .endif

For the noobs: Some of the earlier posts were written with the agenda "how can I perform simple things in the most complicated way".
For the moderators: Please introduce a smiley saying "Attention, I am pulling your leg"  :lol

[sinsi]

wow...just...wow
I can't believe the effort that has gone into this topic, all for a "test ebx,1".
The best was introducing edx, but second prize has to go to using all these bt/btc opcodes...
My offering

Code Select Expand


    test ebx,1  ;test bl,1
    jnz is_oddly_an_odd_number ;you know, 1,11,101,111,1001 ad infinitum
    ;code here for even number


Hey, this is like the "destructive AND vs non-destructive TEST" thing, isn't it. :bg

[jj2007]

The best was introducing edx, but second prize has to go to using all these bt/btc opcodes...

I am deeply disappointed that you chose to ignore my simple FPU solution, #7.

Hey, this is like the "destructive AND vs non-destructive TEST" thing, isn't it. :bg

How odd that you even thought of that thread :green

[jdoe]

wow...just...wow
I can't believe the effort that has gone into this topic, all for a "test ebx,1".
The best was introducing edx, but second prize has to go to using all these bt/btc opcodes...
My offering

Code Select Expand


    test ebx,1  ;test bl,1
    jnz is_oddly_an_odd_number ;you know, 1,11,101,111,1001 ad infinitum
    ;code here for even number


Hey, this is like the "destructive AND vs non-destructive TEST" thing, isn't it. :bg

For sure, some of this board topics are just fascinating.   :cheekygreen:

[sinsi]

Still waiting for the sse2 version though.

[jdoe]

Still waiting for the sse2 version though.

And the code timing because we are still not sure by how many cpu clocks "test ebx,1" will beat them all   :green2

[zooba]

I think I've got the slowest one. Usually takes a few seconds, but can occasionally take a number of hours, depending on how complicated the number is:

Code Select Expand

IsOdd PROC number:DWORD
    LOCAL buffer:BYTE[256]

    lea  eax, buffer
    mov  BYTE PTR [eax], 0
    mov  eax, cat$(eax, "Is the number ", ustr$(number), " an odd number?")

    invoke MessageBoxA, 0, eax, chr$("Odd or even?"), MB_YESNO OR MB_ICONQUESTION
   
    .if(eax == IDYES)
        mov  eax, TRUE
    .else
        mov  eax, FALSE
    .endif
   
    ret
IsOdd ENDP

Cheers,

Zooba :U

_{I haven't actually tested that code, and I don't normally use MASM32's string macros, so apologies if it is all wrong :bg}

[MichaelW]

Considering that this is the Laboratory, you should create a speed-optimized version that displays only the value of the last bit. It would still run slow on some systems, but for most it should be much faster.

[jj2007]

I think I've got the slowest one. Usually takes a few seconds, but can occasionally take a number of hours, depending on how complicated the number is:

I did some timings, and the results look ODD, it EVEN seems there is some quadratic element:

Code Select Expand

number       time
123          5'
1234         7'30''
4321         9'00''
12345678     3h30'00''
123454321 --- exception #5


Besides, your code failed miserably for the last number. Zooba, could you please post the complete source?

[eaOn]

The thread is getting ODD, EVENthough we are only dealing with a simple algo here. :lol

Here's my little contribution. It's a good exercise for a  masm32 starter like me. :P

Code Select Expand

main proc
    LOCAL num1:DWORD
    mov num1, sval(input("Enter a number : "))
    mov ebx,num1
    and al,1b                          ;logical operation.
    cmp al,1
    jne even1
    je odd
   

odd:
    print str$(ebx)," is odd"
    jmp done

even1:
    print str$(ebx)," is even"


done:

    ret

main endp
It's hard to technically explain it but whenever your value is in odd bit-0 is always set to 1.
For example:
ODD
7d  =0111b
15d=1111b
9d  =1001b


EVEN
10d=1010b
6d  = 0110b

This holds true with large values too. I guess this a matter of common sense nowadays.  :U



All solution is accepted by the way. It's up to the user how to deal with them. If speed is a factor, then use bitwise operators.

[drizz]

here is an experienced coder contribution :))

Code Select Expand


.data
szeven db "EVEN",0
szodd db "ODD",0
align 4
msgs dd offset szeven,offset szodd
.code
mov eax,number
and eax,1
mov eax,[msgs+eax*4]
invoke MessageBox,0,eax,0,0


Code Select Expand


.data?
msgs db 8 dup (?)
.code
mov eax,number
and eax,1
cmp eax,1
sbb eax,eax; 0 if eax >= 1 else -1
and eax,"NEVE"-"DDO"
add eax,"DDO"
mov dword ptr msgs,eax
invoke MessageBox,0,addr msgs,0,0

[NightWare]

    and al,1b                          ;logical operation.

:lol

Considering that this is the Laboratory, you should create a speed-optimized version that displays only the value of the last bit. It would still run slow on some systems, but for most it should be much faster.

:lol i have one !!! here => and eax,1
beside bogdan has said it's difficult to do it faster !

[DoomyD]

How about...
shr ebx,1
jc _odd_number_