Re: What is truncation for (singed) integer in C ?

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: What is truncation for (singed) integer in C ?
Date	2015-09-19 13:45 -0700
Organization	None to speak of
Message-ID	<lna8sidtol.fsf@kst-u.example.com> (permalink)
References	<6f66f235-1b11-4b0c-b2d8-c49ac3eac78e@googlegroups.com>

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fl <rxjwg98@gmail.com> writes:
> I see the following on wikipedia and doubt about negative integer when
>  truncating.
>
> https://en.wikipedia.org/wiki/Type_conversion
> ///////
>  Data can be lost when floating-point representations are converted to
>  integer representations as the fractional components of the
>  floating-point values will be truncated (rounded towards zero)
> .......
>
> I agree for positive integer, truncation in C is towards 0. But for
> negative integer, it looks like away from '0'. For example, for a
> unsigned char 83H, one bit (LSB) truncation result will be 41H, whose
> non-truncation value (8-bit) is equal to 82H. The real value of 82H
> plus '1' is 83H. That is, 82H < 83H. For these negative numbers, it is
> not rounded towards zero.
>
> Is the above link not talking the same thing with my understanding?

The link is talking about conversion of *floating-point* values to
integers.

For example, (int)3.7 = 3, and (int)-3.7 == -3.

I'm not entirely clear on what your example is supposed to be.  For
starters, C expresses a hexadecimal value as 0x83, not 83H.  You appear
to be talking about truncating the result of dividing 0x83 by 2 (you
didn't mention the division).  0x83 / 2 == 0x41; for unsigned values,
integer division truncates.  There are other rules for negative values
-- but 0x83 is positive, so those rules don't apply.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

What is truncation for (singed) integer in C ? fl <rxjwg98@gmail.com> - 2015-09-19 12:25 -0700
  Re: What is truncation for (singed) integer in C ? fir <profesor.fir@gmail.com> - 2015-09-19 12:55 -0700
    Re: What is truncation for (singed) integer in C ? fir <profesor.fir@gmail.com> - 2015-09-19 13:20 -0700
  Re: What is truncation for (singed) integer in C ? Geoff <geoff@invalid.invalid> - 2015-09-19 13:05 -0700
  Re: What is truncation for (singed) integer in C ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-19 21:28 +0100
    Re: What is truncation for (singed) integer in C ? Tim Prince <tprince@computer.org> - 2015-09-19 16:38 -0400
      Re: What is truncation for (singed) integer in C ? James Kuyper <jameskuyper@verizon.net> - 2015-09-19 16:52 -0400
  Re: What is truncation for (singed) integer in C ? Keith Thompson <kst-u@mib.org> - 2015-09-19 13:45 -0700
  Re: What is truncation for (singed) integer in C ? James Kuyper <jameskuyper@verizon.net> - 2015-09-19 16:46 -0400
  Re: What is truncation for (singed) integer in C ? Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-09-19 16:47 -0400
  Re: What is truncation for (singed) integer in C ? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-09-19 22:55 -0600
  Re: What is truncation for (singed) integer in C ? James Kuyper <jameskuyper@verizon.net> - 2015-09-20 10:54 -0400
    Re: What is truncation for (singed) integer in C ? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-09-20 10:11 -0600

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