Ripping a single bit out of a Long

Sariel · 2012-07-02 10:28

I am trying to take just the MSB of a long (one bit of info), and depending on if it is a 1 or a 0 do some other program trickery with if/else statements. Is there an easy way of doing this? I initially tried this:

  NetTemp := L[BasePin]   
  repeat 32
      if NetTemp => 00_0000_0000_0000_0000_0000_0000_0000  
        .....
      else
        ...
    NetTemp <-= 1

(L[BasePin] is a 128 long buffer that is loaded from an SD card earlier in the program, just to get that clarified.)

but that really gets confusing since that is actually a negative number when converted to decimal, so there is no way to base a greater than/less than off of this method when taking into consideration that the rest of the bits in the var "NetTemp" could be anything, making it a positive or negative number. I also tried:

NetTemp := ||L[BasePin]   
if NetTemp => 00_0000_0000_0000_0000_0000_0000_0000
  .....

and I was still getting odd results. anyone know of some way of isolating just the MSB, even if it is to another variable so that I can do some true/false work?

lonesock · 2012-07-02 10:33

Spin uses signed integers, and a nifty thing to note is all negative numbers have the MSB set, so...

if x < 0

Jonathan

Sariel · 2012-07-02 10:37

Just noticed a silly type-o in my code above... it should have read..

if NetTemp => %1000_0000_0000_0000_0000_0000_0000_0000

I assume that does not matter, and lonesock is still correct?

Sariel · 2012-07-02 10:43

AHHHA! I see what you mean. MSB designates the + or - status of the number then.. very clever indeed. thanks a bunch buddy. program is acting normal now.

JonnyMac · 2012-07-02 11:02

I have this method in my standard template:

get_bit(value, pos)

  return (value >> pos) & 1

Sariel · 2012-07-02 11:11

Yet another reason to love you folks. Find a few ways to skin a cat, and learn something every time I am here. Thanks for the snip Jon. I'm sure I will be using that one.

HShanko · 2012-07-02 11:30

Sounds cruel to RIP a bit out of a long. Then you have a 31 bit 'almost' long. They gave you good advice; just test the bit. None of that mean ripping.

Vega256 · 2012-07-02 16:56

HShanko wrote: »

Sounds cruel to RIP a bit out of a long. Then you have a 31 bit 'almost' long. They gave you good advice; just test the bit. None of that mean ripping.

I suppose, though, that you can never destroy a piece of digital information; the long lives forever.

Duane C. Johnson · 2012-07-02 18:21

You could kind of sneak up and peak at the bit and then leave it there.

Duane J

nohab · 2012-07-03 02:25

Jon, do you by any chance also have such a handy "set_bit" method?

Bits · 2012-07-03 10:27

nohab wrote: »

Jon, do you by any chance also have such a handy "set_bit" method?

I do

Pub Bits (In,bit,value)
     Result  := (In & !(1 << bit)) | (value << bit)

Where...

value = 1 or 0
in = the original number
bit = what bit in the long.

photomankc · 2012-07-03 10:38

Nice. Took me a sec to get the (In & !(1<<bit)) thing but I get it. I need to add that to the parts bin.

nohab · 2012-07-04 07:14

Bits wrote: »

I do

Thanks !
Clever code :-)

JonnyMac · 2012-07-04 10:38

nohab wrote: »

Jon, do you by any chance also have such a handy "set_bit" method?

This is my version from a user-friendly programming template for the EFX-TEK HC-8+

pub set_bit(target, pos, newbit)

  if ((pos => 0) and (pos =< 31))                               ' if good bit position
    if (newbit)                                                 '  if true/1
      target |= (1 << pos)                                      '   set the bit                                     
    else                                                        '  if false/0
      target &= !(1 << pos)                                     '   clear the bit

  return target

My version is a little less clever than the neat one-liner that Bits showed you; again, mine is built into a programming template that is intended to be easy on Spin newbies. Mine will also set the bit for any non-zero value in the newbit parameter. This allows me to use true and false results from other operations, for example, as in this program I was working on yesterday that looks for specific numbered files on an SD card

repeat idx from 0 to 15
    make_filename(idx)
    inventory := set_bit(inventory, idx, open_file(@FileName, "r")) 
    close_file

nohab · 2012-07-05 13:18

Thanks Jon,

Your solution is pretty close to my first thoughts, it's very easily understood and has indeed an advantage with non-zero-or-one values.
Bits solution seems efficient (faster ?), so I guess it's still true that there are pros and cons with every design.

/Nick

Ripping a single bit out of a Long

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