Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #70937

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

Show all headers | View raw

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,

Something is already wrong here.  You say "but for negative integer" and
the then give an example of "unsigned char 83H".  unsigned char 83H is
not negative -- it's unsigned.

> 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?

I think you are talking here about integer division by two.  The article
you cite is about converting floating point values to integers so there
is no direct connection.  The rules about integer division have changed
between old C (ANSI C or ISO C from 1990) and modern C (C99 and later)
but in modern C, the result is indeed truncated (rounded towards zero).

To use your example, signed char c = -125; will give c the bit-pattern
83 in hex.  Note that I can't say c == 0x83 because that is not correct
in C -- do you know why?  Now c/2 is -125/2 which, truncated towards
zero is -62.  That will "look like" C2 in hex, but, again, c/2 == 0xC2
is not correct in C.

-- 
Ben.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

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

csiph-web