From: Joel VanderWerf <vjoel@...>
Date: 2008-01-13T14:43:32+09:00
Subject: Re: Question about Hex => Signed Int

Ken Bloom wrote:
> On Thu, 10 Jan 2008 13:19:46 -0500, Joel VanderWerf wrote:
>> Tim Conner wrote:
>>> Thanks for that.  Is there a ruby function which I can pass a hex
>>> number and it works out if it is +/- and then returns the correct
>>> decimal value?
>> Convert to unsigned first, like above, then do something like this:
>>
>>      length = 32 # in bits; you have to choose this
>>
>>      mid = 2**(length-1)
>>      max_unsigned = 2**length
>>      to_signed = proc {|n| (n>=mid) ? n - max_unsigned : n}
>>
>>      p to_signed["0xFF467A7B".to_i(16)]
>>      p to_signed["0x38CFEF".to_i(16)]
>>
>> See http://en.wikipedia.org/wiki/Twos_complement.
> 
> Why go through all the trouble to make this a proc?

Cut and paste from some code that was using #define_method with this 
proc, and thereby saved performing the exponentiations on each call. 
That's all.

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