🔒 Locked

Parameter flags... Read on...

Started by Cyberdrek Dec 17, 2001 at 8:12 PM 6 replies 350+ views

Original Post

Dec 17, 2001 at 8:12 PM

Ok, here's something that's been on my bugging me since last night, I have a function that receives flags(ex: draw( x, y, BLAH | BLAH2 ); ) to do is get the number of bits that are set in the flag. Here's an example of how the flags are defined. const int BLAH = 128; const int BLAH2 = 64; const int BLAH3 = 32; const int BLAH4 = 16; const int BLAH5 = 8; const int BLAH6 = 4; const int BLAH7 = 2; const int BLAH8 = 1; So lets say I pass BLAH | BLAH2, I know that I have 2 bits set but lets say that I don't know the number that's going to be passed and I want to count how many bits are set, is there a way that I can do it??? "And that's the bottom line cause I said so!" ** I WANT TO BE THE MODERATOR FOR THE LINUX FORUM ** Cyberdrek Resist Windows XP's Invasive Production Activation Technology! "gitty up" -- Kramer /(bb|[^b]{2})/ that is the Question -- ThinkGeek.com Hash Bang Slash bin Slash Bash -- #!/bin/bash Edited by - cyberdrek on December 17, 2001 9:13:03 PM

krez

156

krez

Dec 17, 2001 at 8:50 PM

of course there is a way you can do that (probably there are several). unfortunately i don''t know any of them.

um, you could have a bunch of "if" statements, and increment some counter variable every time one of them is true (if(lFlags & BLAH4) numFlags++; )... that''s a lousy way to do it though...
er... your elegant solution lies in bitshifts i think... hmm... i''ll be back...

--- krez (krezisback@aol.com)

scaught

Dec 17, 2001 at 9:10 PM

There''re three ways to do it - the math way and two brute force ways:

Math way (from another post I made):

  
int countBits( int value )
{
   if( !value ) {
      return 0;
   }
   return( (int)( log10( value ) / log10( 2 ) ) + 1 );
}

Brute Force 1: if() statements as krez mentioned

Brute Force 2:

  
unsigned int countBits( unsigned int value )
{
   int count = 31;
   if( !value ) {
      return 0;
   }
   while( ( value & ( 1 << count ) ) {
      --count;
   }
   return( count + 1 );
}

(standard disclaimers apply - I didn''t test it but it looks like it should work...)

btw - I think I''d go with BF2 if I had to pick one.

-scott

scaught

Dec 17, 2001 at 9:13 PM

heh...nevermind, those count the max bits. (don''t blame me, I have the flu)

here:

  
int countBits( int value )
{
   int i, count = 0;
   for( i=0; i<32; ++i ) {
      if( value & ( 1 << i ) ) {
         ++count;
      } 
   }
   return count;
}

Sorry ''bout that...

-scott

GoofProg

Dec 17, 2001 at 9:22 PM

I would use powers of two comparison until you get the total number of bits.

Cyberdrek

Author

Cyberdrek

Dec 17, 2001 at 9:25 PM

quote:
Original post by scaught
heh...nevermind, those count the max bits. (don''t blame me, I have the flu)

here:

    
int countBits( int value )
{
   int i, count = 0;
   for( i=0; i<32; ++i ) {
      if( value & ( 1 << i ) ) {
         ++count;
      } 
   }
   return count;
}

Sorry ''bout that...

-scott

No problems, I''m just glad somebody could help me out, I''ve been at it for 2 days now...

Thanks again..

"And that''s the bottom line cause I said so!"

** I WANT TO BE THE MODERATOR FOR THE LINUX FORUM **

Cyberdrek

Resist Windows XP''s Invasive Production Activation Technology!

"gitty up" -- Kramer
/(bb|[^b]{2})/ that is the Question -- ThinkGeek.com
Hash Bang Slash bin Slash Bash -- #!/bin/bash

Cyberdrek

Author

Cyberdrek

Dec 18, 2001 at 5:48 AM

quote:
Original post by GoofProg
I would use powers of two comparison until you get the total number of bits.

if I use power of 2s, I''ll have too many "if" statements which I''m trying to avoid...

"And that''s the bottom line cause I said so!"

** I WANT TO BE THE MODERATOR FOR THE LINUX FORUM **

Cyberdrek

Resist Windows XP''s Invasive Production Activation Technology!

"gitty up" -- Kramer
/(bb|[^b]{2})/ that is the Question -- ThinkGeek.com
Hash Bang Slash bin Slash Bash -- #!/bin/bash

Stoffel

Dec 18, 2001 at 8:53 AM

Search for the thread "Let''s count bits", either in this forum or the game forum. This was an extremely active thread with some insane algorithms floating around.

Topic Locked

This topic has been locked by a moderator. New replies are not allowed.