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Re: "usual arithmetic conversions": please explain something...

From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: "usual arithmetic conversions": please explain something...
Date 2017-03-01 09:43 -0800
Organization None to speak of
Message-ID <ln8toodhad.fsf@kst-u.example.com> (permalink)
References (1 earlier) <o951vq$ch6$2@dont-email.me> <ln1suhersb.fsf@kst-u.example.com> <o96d9b$p64$1@dont-email.me> <lnshmxc6bz.fsf@kst-u.example.com> <o96tr9$u0t$1@dont-email.me>

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BartC <bc@freeuk.com> writes:
> On 01/03/2017 16:25, Keith Thompson wrote:
>> BartC <bc@freeuk.com> writes:
>>> On 01/03/2017 00:59, Keith Thompson wrote:
>>>> BartC <bc@freeuk.com> writes:
>> [...]
>>>>> Try:
>>>>>
>>>>>               printf("%u\n", c);
>>>>
>>>> No, c is an int.
>>>>
>>>> But do try:
>>>>
>>>>     printf("%llu\n", a * b);
>>
>> My mistake, I meant "%lu\n", not "%llu\n".
>>
>>> It doesn't make much difference. Trying this (on a machine where long is
>>> 32 bits):
>>
>> Sure, if you use an incorrect format you can sometimes learn
>> something about the value being passed to printf, or its
>> representation -- but what's the point?  c is of type int, and
>> happens to have a negative value; printing it as if it were an
>> unsigned int means you're lying to the compiler, and the behavior
>> is undefined.  Printing the value of a * b shows you the actual
>> value that was converted to int.
>>
>> I showed how to print the actual values with their correct types.
>
> I showed how to convert the result of a signed/signed, 
> unsigned/unsigned, signed/unsigned or unsigned/signed to a float, which 
> is a better idea.

I showed how to print the actual values without converting them.
Converting to floating-point is not helpful, and introduces
irrelevant issues.

> Otherwise you're going always going to be forcing such a result into 
> either a signed or unsigned int, so you will not know what it actually was.

No, the result is an unsigned long, which is what it actually is.

> In fact, the true result of the OP's calculation was 4294927296, with 
> the operands being 4000 and 4294967286 (the -10).

Which calculation?  There were several.

    unsigned long a = 4000;
    signed long b = -10;
    int c;

    c = a * b;

The result of the multiplication was a number of type unsigned long.
The result of the implicit conversion was a different number of type
int.  The OP's original code combined those two operations; mine split
them up so you could see the intermediate result.

[...]

-- 
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"

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Thread

"usual arithmetic conversions": please explain something... Lowell Gilbert <lgusenet@be-well.ilk.org> - 2017-02-28 18:33 -0500
  Re: "usual arithmetic conversions": please explain something... BartC <bc@freeuk.com> - 2017-02-28 23:45 +0000
    Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-02-28 16:59 -0800
      Re: "usual arithmetic conversions": please explain something... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-01 02:20 +0000
        Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-02-28 18:51 -0800
      Re: "usual arithmetic conversions": please explain something... BartC <bc@freeuk.com> - 2017-03-01 12:04 +0000
        Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 08:25 -0800
          Re: "usual arithmetic conversions": please explain something... BartC <bc@freeuk.com> - 2017-03-01 16:47 +0000
            Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 09:43 -0800
              Re: "usual arithmetic conversions": please explain something... BartC <bc@freeuk.com> - 2017-03-01 18:00 +0000
                Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 10:15 -0800
                Re: "usual arithmetic conversions": please explain something... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-01 19:08 +0000
          Re: "usual arithmetic conversions": please explain something... supercat@casperkitty.com - 2017-03-01 09:01 -0800
  Re: "usual arithmetic conversions": please explain something... jameskuyper@verizon.net - 2017-02-28 15:49 -0800
  Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-02-28 16:03 -0800
    Re: "usual arithmetic conversions": please explain something... Nick Bowler <nbowler@draconx.ca> - 2017-03-01 21:53 +0000
      Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 14:18 -0800
  Re: "usual arithmetic conversions": please explain something... Lowell Gilbert <lgusenet@be-well.ilk.org> - 2017-03-01 08:29 -0500
    Re: "usual arithmetic conversions": please explain something... Lowell Gilbert <lgusenet@be-well.ilk.org> - 2017-03-01 09:11 -0500
    Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 08:59 -0800
  Re: "usual arithmetic conversions": please explain something... scott@slp53.sl.home (Scott Lurndal) - 2017-03-01 14:00 +0000
    Re: "usual arithmetic conversions": please explain something... "James R. Kuyper" <jameskuyper@verizon.net> - 2017-03-01 10:58 -0500

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