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Getting 2 values in one byte

Started by V3rt3x Jun 20, 2002 at 2:40 AM 15 replies 650+ views

Original Post

Jun 20, 2002 at 2:40 AM

Hi all!! I have a byte to store 2 values (so, 4 bits per var). for exemple: int byte = 10010110; // (150) the first value would be 1001 => 9 the second 0110 => 6 for the first I can get it with opérator >> right? int var1 = (byte >> 4); // 00001001 => 9 but for the second, the best I have found is: int var2 = (byte & 1)+(byte & 2)+(byte & 4)+(byte & 8); My question : Is there a better way to extract the 4 last bits of the byte? I tried with << and then >> but there is no change

Crispy

Jun 20, 2002 at 3:09 AM

try: byte & 16 for 4 bits (255 for 8 bits etc.)

edit: nothing special...

Crispy

[edited by - crispy on June 20, 2002 4:15:56 AM]

Assassin

Jun 20, 2002 at 3:11 AM

var = byte & 0x00ff;
var2 = byte >> 4;

[edited by - Assassin on June 20, 2002 4:12:42 AM]

davidsporn

Jun 20, 2002 at 3:13 AM

quote:
Original post by Crispy
try: byte & 16 for 4 bits (255 for 8 bits etc.)

I think you mean : byte & 15
(15 = 00001111 so only the 4 low-order bit are kept)

----
David Sporn AKA Sporniket

Crispy

Jun 20, 2002 at 3:17 AM

quote:
Original post by davidsporn
I think you mean : byte & 15
(15 = 00001111 so only the 4 low-order bit are kept)

Gotta get some more sleep... zzzzzz.... 8|

Crispy

Ziphnor

Jun 20, 2002 at 3:58 AM

If you want to store 2 values in an 8-bit variable(1 byte) why are you using an int which on most machines will be 32-bit/4-bytes in memory(even though you dont use all of it)?

If you want the equivalent of a byte use a char.....

gimp

Jun 20, 2002 at 4:20 AM

  
#pragma pack( push, Packing )
#pragma pack( 1 )

struct Nibbles
{
    char x : 4;    
    char y : 4;
};

#pragma pack(pop, Packing)

The packing is to ignore the compiler set alignment. The numbers after the colon is the number of bits assigned to the variable. The type(char) what is being divided up.

If you use an int it''ll leave 3 bytes empty(I guess you could cast it away)

I''d betting this is to be transported across a network link? If so to get your data back out of a char later just cast the char from your datastream back to a (Nibble&) (Thats how I did it, probably not the best way but it worked.

This is probably a lot more readable than bit shifting, besides I''m crap as bit shifting the right bits

HTH

Chris Brodie
http:\\fourth.flipcode.com

KalvinB

Jun 20, 2002 at 5:03 AM

If you have two variables (x,y) that can have a value 0-15 getting them into a byte (byte z) is easy;

z=x*16+y;

x=z/16;
y=z%16;

Or you could do bitshifting but I think that''s a bit overkill.

Ben

IcarusIndie.com [ The Rabbit Hole | The Labyrinth | DevZone | Gang Wars | The Wall | Hosting ]

Useless Hacker

Jun 20, 2002 at 5:42 AM

quote:
Original post by KalvinB
If you have two variables (x,y) that can have a value 0-15 getting them into a byte (byte z) is easy;

z=x*16+y;

x=z/16;
y=z%16;

Or you could do bitshifting but I think that's a bit overkill.

Except that bit shifting is faster that multiplying/dividing.

z = (x << 4) | y;

x = z >> 4;
y = z & 0x0F;

[edited by - Useless Hacker on June 20, 2002 6:42:56 AM]

Dean Harding

Jun 20, 2002 at 5:52 AM

quote:
Original post by Useless Hacker
Except that bit shifting is faster that multiplying/dividing.

The compiler will probably optimize it anyway and turn it into bit shifts. In this case, it''s really just preference, but for things like this, I prefer bit shifts/masks (cause I find them to be more "natural" than multiplying/dividing)

codeka.com - Just click it.

Trugath

Jun 20, 2002 at 5:55 AM

Bitshifting is blindingly fast and is probably the best solution.

just setup a couple of macros for accessing the hi/lo (nibble? someone called it?) in the byte. same way you would access hi-lo words in a dword.

gimp

Jun 20, 2002 at 6:28 AM

The pragma I used stores the current struct packing alignment in ''Packing'' allowing me to reset it to whatever it was befor with the pop. You don''t really need it as the alignment wil reset at he end of the file anyway.

The pragma I used was VC specific. Here is my whole code segment:

  
#ifdef WIN32
	#pragma pack( push, Packing )
	#pragma pack( 1 )
#endif

#if defined(__GNUC__)
	#define PACK_STRUCT	__attribute__((packed))
#endif


struct TGA_IMAGE_DESCRIPTOR
{
	unsigned char	AlphaChannelBits	: 4;
	unsigned char	LeftToRightOrder	: 1;
	unsigned char	TopToBottomOrder	: 1;
	unsigned char	Unused				: 2;
};


struct TGA_HEADER
{
	unsigned char			IdLength;
	unsigned char			ColourMapType;
	unsigned char			ImageType;
	unsigned short			ColourMapIndex;
	unsigned short			ColourMapLength;
	unsigned char			ColourEntrySize;
	unsigned short			XOrigin;
	unsigned short			YOrigin;
	unsigned short			Width;
	unsigned short			Height;
	unsigned char			PixelDepth;
	TGA_IMAGE_DESCRIPTOR	ImageDescriptor;
	unsigned char			Data[1];
} ;


#ifdef WIN32
	#pragma pack(pop, Packing)
#endif

The GNU stuff was just copied from somewhere so I can look in to it later when porting.

I never use bitshifting. I''ve always tending to think of it as a throw back to C which I only learnt for about a week. Not to say it''s bad, probably much faster than the way I do things.

However, I like the struct metho as it''s very readable, esp when your packing 4-6 values in a byte. Each shift is then named and easy to identify. Though I''m writing a game, readability it more important to me than speed. CPU''s are fast and compilers smarter than me.

I''ll handle the high level stuff... For me ''optimisation'' is not doing stupid things like transforming more than needed or using NV_vertex array range, etc.

HTH

Chris Brodie
http:\\fourth.flipcode.com

V3rt3x

Jun 20, 2002 at 11:31 AM

Thanx for these answers guys, I could solve my pb

(using x = z >> 4; y = z & 0x0F; )

quote:
Original post by Ziphnor
If you want to store 2 values in an 8-bit variable(1 byte) why are you using an int which on most machines will be 32-bit/4-bytes in memory(even though you dont use all of it)?

If you want the equivalent of a byte use a char.....

It was for decoding a 4-bit encoded bitmap

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