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Groups > comp.lang.c > #147999
| Subject | Re: OT: improve this simple algorithm? |
|---|---|
| Newsgroups | comp.lang.c |
| References | (1 earlier) <87a790zn7z.fsf@bsb.me.uk> <_85zF.5075$II7.2317@fx17.iad> <qqj564$2m4$1@dont-email.me> <fxgzF.21520$1S4.4613@fx01.iad> <qqk7lv$op4$1@dont-email.me> |
| From | DFS <nospam@dfs.com> |
| Message-ID | <P5izF.26147$aI.67@fx44.iad> (permalink) |
| Organization | blocknews - www.blocknews.net |
| Date | 2019-11-14 14:42 -0500 |
On 11/14/2019 1:50 PM, James Kuyper wrote:
> On 11/14/19 12:56 PM, DFS wrote:
>> On 11/14/2019 4:01 AM, David Brown wrote:
>>> On 14/11/2019 05:57, DFS wrote:
> ...
>>> for(int i=0; i < (sizeof(seq) / sizeof(seq[0])); i++) {
>>>
>>>
>>> gcc has found a potential problem. It is not, in this case, a real
>>> problem in the code. It is fine to compare a signed type and an
>>> unsigned type - /if/ you know the values will be valid after the integer
>>> promotions used by C to make both sides the same type. These integer
>>> promotion rules are not always obvious, especially to someone new to the
>>> language, and it is easy to make mistakes. So many compilers can warn
>>> you when you mix signed and unsigned types in comparisons, even though
>>> it is a false positive in this case.
>>
>> Since sizeof(seq)/sizeof(seq[0]) can only be >= 0 the compiler? decides
>> the result is unsigned, is that right?
>
> No, it's much simpler than that. sizeof expressions give a result whose
> type is size_t, which is required to be an unsigned integer type. When
> you divide two values, both of them the same unsigned integer type, the
> result will have that same unsigned integer type. Exception: in the
> unlikely (but not impossible) case that SIZE_MAX <= INT_MAX, then both
> sizeof expressions will be promoted to "int", and the result of the
> division will also be an int.
>
>> Comparing int i to the (unsigned int) number of elements in the array is
>> somehow worth a warning? Not an egregious warning, but this is the kind
>> of thing that makes some people dislike C. It's like you can't write
>> working, reasonable code without being smacked on the back of the hand
>> with a ruler.
>
> If the rank of the signed type is greater than the rank of the unsigned
> type, and if the signed type is capable of representing all values of
> the unsigned type, then the unsigned value is converted to the signed
> type, and nothing confusing happens.
> In all other cases, signed/unsigned comparison results in the signed
> value being converted to an unsigned integer type, with a resulting
> change of value if the signed value is negative. This strikes me as
> something that justifies a warning. In your case, it can't have a
> negative value, but the implementation is not obligated to notice that fact.
The compiler flags certain data type comparisons, but not invalid
variable assignments that violate the range of the data type.
for(unsigned short i = -1; i < (sizeof(seq) / sizeof(seq[0])) - 1; i++)
gcc -Wall -Wextra -o seq.exe seq.c
doesn't throw any warnings
The code also doesn't output anything, or throw a runtime error.
David Brown said gcc gave this warning:
seq.c: In function ‘main’:
seq.c:8:18: warning: comparison between signed and unsigned integer
expressions [-Wsign-compare]
for(int i=0; i < (sizeof(seq) / sizeof(seq[0])); i++) {
gcc (9.1.0) on MSYS2 gives this:
seq.c: In function ‘main’:
seq.c:8:18: warning: comparison of integer expressions of different
signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
8 | for(int i=0; i < (sizeof(seq) / sizeof(seq[0])) - 1; i++) {
| ^
Unless he didn't paste the whole thing, my gcc warning is a little more
verbose and specific than his. Version difference I suppose.
>>> gcc -Wall -Wextra seq.c -o seq -Wno-sign-compare
>>>
>>> No warnings now. gcc has done all it can.
>>>
>>> Or has it?
>>
>>> Remember I told you to enable optimisation for better static analysis?
>>>
>>> gcc -Wall -Wextra seq.c -o seq -Wno-sign-compare -O2
>>>
>>> seq.c: In function ‘main’:
>>> seq.c:9:11: warning: iteration 8 invokes undefined behavior
>>> [-Waggressive-loop-optimizations]
>>> if(seq[i + 1] - seq[i] > 1) {
>>> ~~~^~~~~~~
>>> seq.c:8:3: note: within this loop
>>> for(int i=0; i < (sizeof(seq) / sizeof(seq[0])); i++) {
>>> ^~~
>>
>>
>> Bingo!
>>
>> I've just been replaced by 3 letters in the compiler instruction.
>
> Surely those three letters have not replaced you completely?
They taught me to stop at 8... but they can't paint my guest bath... so
I guess I'm good for now.
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OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-12 15:55 -0500
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-12 21:20 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-12 23:28 -0500
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 10:52 +0100
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-13 10:16 -0500
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 16:53 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 11:18 +0000
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-12 23:30 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 05:12 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 16:34 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 10:07 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 22:25 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 12:02 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 23:27 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 19:06 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 10:45 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 13:28 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 12:58 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 15:39 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 15:04 +0000
Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-15 12:19 -0500
Re: OT: improve this simple algorithm? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-15 09:46 -0800
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 18:59 +0000
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-16 11:59 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-16 09:02 +1300
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 11:17 +0100
Re: OT: improve this simple algorithm? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-11-15 21:38 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 11:44 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-14 18:11 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 13:33 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 01:49 +0000
Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-14 10:38 -0800
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 12:56 -0500
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-14 13:50 -0500
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 14:42 -0500
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-14 15:25 -0500
Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-14 11:03 -0800
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 16:08 -0500
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 11:30 +0100
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 11:17 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-15 17:35 -0500
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-15 17:39 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 23:19 +0000
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-15 18:53 -0500
Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-15 19:08 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-16 01:30 +0000
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-16 07:39 -0500
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-16 07:37 -0500
Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-15 09:46 -0500
Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-15 12:38 -0500
Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 23:07 +0000
Re: OT: improve this simple algorithm? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 16:54 -0800
Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-28 20:17 -0500
Re: OT: improve this simple algorithm? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-29 06:08 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 20:45 +0100
Re: OT: improve this simple algorithm? Keith Thompson <kst-u@mib.org> - 2019-11-14 10:10 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 20:19 +0100
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-12 23:57 -0500
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 10:34 +0100
Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-13 23:57 -0500
Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-13 23:00 -0800
Re: OT: improve this simple algorithm? Ike Naar <ike@sdf.lonestar.org> - 2019-11-14 07:53 +0000
Re: OT: improve this simple algorithm? guinness.tony@gmail.com - 2019-11-14 02:34 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 10:01 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 11:27 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 12:46 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 12:01 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 14:05 +0100
Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-12 18:34 -0800
Re: OT: improve this simple algorithm? Keith Thompson <kst-u@mib.org> - 2019-11-12 19:01 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 17:08 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 17:06 +0000
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 19:11 +0000
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 05:15 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 18:13 +0000
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 20:16 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 20:03 +0000
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-15 04:51 +0100
Re: OT: improve this simple algorithm? Keith Thompson <kst-u@mib.org> - 2019-11-14 10:15 -0800
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 20:52 +0100
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 10:57 +0100
Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-13 23:23 +1300
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 11:53 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 14:02 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 13:40 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 17:01 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 16:28 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 22:49 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 22:38 +0000
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 23:06 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 10:10 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 10:42 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 12:53 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 12:28 +0000
Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 14:21 +0100
Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 14:28 +0000
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 15:38 +0100
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 13:37 +0100
Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 13:38 +0100
Re: OT: improve this simple algorithm? Thiago Adams <thiago.adams@gmail.com> - 2019-11-13 05:21 -0800
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