From: Joel VanderWerf <vjoel@...>
Date: 2006-11-30T07:53:09+09:00
Subject: Re: One field maintaining (boolean) state of multiple items

Paul Lutus wrote:
> El Gato wrote:
> 
>> I have a friend who posed a question to me about how to maintain the
>> state of a few boolean values (radio buttons to be specific) in one
>> (integer) database column.  It really is more of a curious question than
>> anything else, as he could simply add more columns to the table to avoid
>> even thinking about it, but for the sake of fun he didn't.  So, let's
>> say you have three boolean values that you need to store in one field,
>> and you must be able to determine what the state is (true/false) of any
>> of the 3 values at any time.  How would you do this?  My immediate
>> thought went to bit shifting or some other bit operations, but I've
>> never really used anything like that before, so I'm a little stuck on
>> this.  Thoughts?
> 
> Given an integer that you want to disassemble into binary bits:
> 
> v = 15
> 
> Let's create an array that will contain the values of the bits:
> 
> array = []
> 
> Now let's scan the integer's bits:
> 
> 1.upto(8) do
>   array << (v & 1 == 1)
>   v >>= 1
> end
> 
> Now the array contains 8 boolean (true/false) values corresponding to the
> low 8 bits in the original integer.
> 

irb(main):003:0> v = 15; (0..7).map {|i| v[i]}
=> [1, 1, 1, 1, 0, 0, 0, 0]

For more info, ri 'Fixnum#[]'. But there is no #[]= (makes sense if you 
think about it).

Another idea, storing the data in a string of length 1:

require 'bit-struct' # http://redshift.sourceforge.net/bit-struct

class Bits < BitStruct
   unsigned    :bit0,     1
   unsigned    :bit1,     1
   unsigned    :bit2,     1
   unsigned    :bit3,     1
   unsigned    :bit4,     1
   unsigned    :bit5,     1
   unsigned    :bit6,     1
   unsigned    :bit7,     1
end

b = Bits.new

b.bit3 = 1
p b.bit3 # ==> 1
p b.bit4 # ==> 0

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407