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Re: convert unsigned long long into char[4]

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Malcolm McLean <malcolm.mclean5@btinternet.com> writes:

> On Tuesday, November 24, 2015 at 11:40:48 PM UTC, bilsch01 wrote:
>> See the code below.  The function d2hx takes a 4 digit hex number 0xfffe 
>> and converts it into ascii bytes returned in unsigned long long 
>> 'answer'.  The contents of answer is 0x46464645
>> where 0x46 = 'F' and 0x45 = 'E'
<snip>
> unsigned long long d2hx(int dc)
> {
>   char hex[5];
>   int i;
>   unsigned long long answer = 0;
>   assert(dc >= 0 && dc <= 0xFFFF);
>   
>   for(i=0;i<4;i++)
>   {
>     hex[i] = "0123456789abcdef"[(dc >> ((3-i) * 4)) & 0xF];

(The OP wants "ABCDEF".)

>   }
>   hex[i] = 0;
>   memcpy(&answer, hex, 5);

That might not produce the value the OP wants.  By writing 0x46464645
there is an indication that the result should be a specific integer
value.

If that's the case, I'd do it by arithmetic rather than memcpy:

  answer = (answer << 8) + "0123456789ABCDEF"[(dc >> (3-i) * 4) & 0xF];

>   return answer;
> }
>
>  int main(){
>   int dc = 0xfffe;
>   unsigned long long answer;
>
>  answer = d2hx(dc);
>  // bug here, add the &
>  printf("answer = %s\n", &answer);

It's not clear what the bug is.  It might be that the op intended to
print a hex value rather than a string.  Also, though it's rare to care
about portability in code like this, you should a cast to that argument
or all bets are, technically, off.  It's undefined behaviour as it is.

>  return 0;
> }

-- 
Ben.

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Thread

convert unsigned long long into char[4] bilsch01 <king621@comcast.net> - 2015-11-24 15:40 -0800
  Re: convert unsigned long long into char[4] Barry Schwarz <schwarzb@dqel.com> - 2015-11-24 17:21 -0800
    Re: convert unsigned long long into char[4] bilsch01 <king621@comcast.net> - 2015-11-24 18:10 -0800
      Re: convert unsigned long long into char[4] Keith Thompson <kst-u@mib.org> - 2015-11-24 18:18 -0800
  Re: convert unsigned long long into char[4] Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-25 00:23 -0800
    Re: convert unsigned long long into char[4] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-25 11:54 +0000
    Re: convert unsigned long long into char[4] bilsch01 <king621@comcast.net> - 2015-11-25 11:35 -0800
  Re: convert unsigned long long into char[4] Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-28 22:06 +0000

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