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Groups > comp.os.linux.advocacy > #465458
| From | owl <owl@rooftop.invalid> |
|---|---|
| Newsgroups | comp.os.linux.advocacy |
| Subject | Re: The substantiation that -highhorse asked for |
| Date | 2018-08-31 17:53 +0000 |
| Organization | O.W.L. |
| Message-ID | <ab80ga0hga.hou4@rooftop.invalid> (permalink) |
| References | (7 earlier) <f045ebee-7115-4ccc-8573-89e47422d3d0@googlegroups.com> <z8b003a.ba9o3g@rooftop.invalid> <40348344-ce24-48a0-ad42-30aee4f5b7d9@googlegroups.com> <sz9b3l3ga.h3i@rooftop.invalid> <92fb76f2-9a2e-4605-b681-237bef31911f@googlegroups.com> |
Steve Carroll <fretwizzer@gmail.com> wrote:
> On Friday, August 31, 2018 at 10:53:35 AM UTC-6, owl wrote:
>> Steve Carroll <fretwizzer@gmail.com> wrote:
>> > On Friday, August 31, 2018 at 6:33:13 AM UTC-6, owl wrote:
>> >> Steve Carroll <fretwizzer@gmail.com> wrote:
>> >> > On Wednesday, August 29, 2018 at 8:31:19 AM UTC-6, owl wrote:
>> >> >> DFS <nospam@nospam.com> wrote:
>> >> >> > On 8/29/2018 2:14 AM, owl wrote:
>> >> >> >> DFS <nospam@nospam.com> wrote:
>> >> >> >>>
>> >> >> >>> import random
>> >> >> >>>
>> >> >> >>> print "Group 1: r<0"
>> >> >> >>> print "Group 2: r==0"
>> >> >> >>> print "Group 3: r>0 && r<=10"
>> >> >> >>> print "Group 4: r>10 && r<=100"
>> >> >> >>> print "Group 5: r>100 && r<=1000"
>> >> >> >>> print "Group 6: r>1000 && r<=10000"
>> >> >> >>> print "Group 7: r>10000 && r<=100000"
>> >> >> >>> print "Group 8: r>100000 && r<=1000000"
>> >> >> >>> print "Group 9: r>1000000"
>> >> >> >>>
>> >> >> >>> def printgroup(grp):
>> >> >> >>> print "Found random number " + str(r) + " in Group " + str(grp)
>> >> >> >>>
>> >> >> >>> for j in range(20):
>> >> >> >>> r = random.randint(-1000,1001)
>> >> >> >>> if r < 0: printgroup(1); exit
>> >> >> >>> if r > 1000000: printgroup(9); exit
>> >> >> >>> comps =
>> >> >> >>> [(-1,0),(1,10),(11,100),(101,1000),(1001,10000),(10001,100000),(100001,1000000)]
>> >> >> >>> for i, x in enumerate(range(len(comps))):
>> >> >> >>> if r > comps[i][0] and r <= comps[i][1]: printgroup(i+2); exit
>> >> >> >>> i+=1
>> >> >> >>>
>> >> >> >>>
>> >> >> >>>
>> >> >> >>> That's 3 if-then-exits to handle 9 comparisons
>> >> >> >>
>> >> >> >> If you call that 3 ifs, then I get to call this zero ifs:
>> >> >> >>
>> >> >> >> anon@lowtide:~/code/ifs$ cat goo.c
>> >> >> >> #include <stdio.h>
>> >> >> >> #include <stdlib.h>
>> >> >> >>
>> >> >> >> void flz(void){printf("a<0\n");}
>> >> >> >> void fgm(void){printf("a>1000000\n");}
>> >> >> >> void f0(void){printf("a==0\n");}
>> >> >> >> void f1(void){printf("a>0 && a<=10\n");}
>> >> >> >> void f2(void){printf("a>10 && a<=100\n");}
>> >> >> >> void f3(void){printf("a>100 && a<=1000\n");}
>> >> >> >> void f4(void){printf("a>1000 && a<=10000\n");}
>> >> >> >> void f5(void){printf("a>10000 && a<=100000\n");}
>> >> >> >> void f6(void){printf("a>100000 && a<=1000000\n");}
>> >> >> >>
>> >> >> >> int main(int argc, char *argv[])
>> >> >> >> {
>> >> >> >> int a=0;
>> >> >> >> int i=0;
>> >> >> >> void (*fp[1000001])(void)={0};
>> >> >> >> fp[0]=f0;
>> >> >> >> for(i=1;i<=10;i++) fp[i]=f1;
>> >> >> >> for(i=11;i<=100;i++) fp[i]=f2;
>> >> >> >> for(i=101;i<=1000;i++) fp[i]=f3;
>> >> >> >> for(i=1001;i<=10000;i++) fp[i]=f4;
>> >> >> >> for(i=10001;i<=100000;i++) fp[i]=f5;
>> >> >> >> for(i=100001;i<=1000000;i++) fp[i]=f6;
>> >> >> >> a=atoi(argv[1]);
>> >> >> >> (a<0?flz:a>1000000?fgm:fp[a])();
>> >> >> >> return 0;
>> >> >> >> }
>> >> >> >> anon@lowtide:~/code/ifs$
>> >> >> >
>> >> >> > affirmative
>> >> >> >
>> >> >>
>> >> >> But who would write it that way, and why? Sure, there's no if statements,
>> >> >> but that comes at the expense of 1,000,001 function pointers. And the
>> >> >> nested ternaries constitute conditional tests anyway. Not to mention
>> >> >> less readability. It would run faster, use less stack, and be much more
>> >> >> readable and faster to design and write without error, by just using a
>> >> >> list of ifs.
>> >> >>
>> >> >> anon@lowtide:~/code/ifs$ time ./blah 500
>> >> >> a>100 && a<=1000
>> >> >>
>> >> >> real 0m0.002s
>> >> >> user 0m0.000s
>> >> >> sys 0m0.000s
>> >> >> anon@lowtide:~/code/ifs$ time ./goo 500
>> >> >> a>100 && a<=1000
>> >> >>
>> >> >> real 0m0.020s
>> >> >> user 0m0.008s
>> >> >> sys 0m0.008s
>> >> >> anon@lowtide:~/code/ifs$
>> >> >
>> >> > What are those times representing, your if/else vs the above (or his code)?
>> >> >
>> >>
>> >> if/else vs the above.
>> >> But you can combine the above with DFS's loop and it's just as fast.
>> >
>> > Because what Feeb said about the complier is correct?
>> >
>>
>> I don't remember what fabtard said, but there is no performance advantage
>> to putting the ifs in a loop. There may coding time advantage if the
>> the number of conditions is extreme, but a sequence of ifs would still
>> make what is happening more obvious, and for a relatively small number of
>> conditions the amount of time spent coding the loop might be greater --
>> just from care taken to get the array values right, off-by-one issues,
>> etc -- than a simple sequence of ifs, especially with an editor like
>> vim which makes code block skeletons so easy to duplicate. Also, not
>> all if sequences are going to lend themselves to looping in a way that
>> would reduce the number of written conditions.
>
> "For a REAL language, a COMPILED language, the compiler often
> makes optimizing choices such as transforming your for-loop
> into a sequence of if-then statements. The user can also specify
> that certain loops should be "unrolled" which leads to the
> same behavior.
>
> In short, there is no great difference between a for-loop and
> an "unrolled" loop sequence." - Feeb
>
>> The code for the earlier doo.c, which was designed to remove all if
>> statements, would not even need the ternary conditional except for the
>> a<0 and a>1000000 tests.
>
> From what very little I've read on if vs ternary in C, there isn't much of
> a performance diff (apparently there can be some in certain conditions).
>
>> The rest of it just uses the number passed in
>> as an index into the function pointer array. But that array is huge,
>> and there is a big performance hit, so minimizing the number of ifs is
>> not necessarily a good thing.
>
> What's worse are the if/else if statements written inside of loops (like I
> had in my earliest example in this thread). But hey, it's JavaScript, no
> one expects it to run fast anyway ;) I was actually surprised by FF, it
> was doing 100 million elements in 3-4 seconds *in* a browser (so
> we're talking DOM elements here, not just array elements)... a billion
> in ~34.3 secs. What the best time you've gotten in C for such a task?
Just looping, it looks like about 400 million per second:
anon@lowtide:~/code/ifs$ cat loop.c
#include <stdio.h>
int main(void)
{
int i=0;
printf("start\n");
for(i=0;i<1000000000;i++)
{
;
}
printf("done\n");
return 0;
}
anon@lowtide:~/code/ifs$ time ./loop
start
done
real 0m2.501s
user 0m2.496s
sys 0m0.004s
anon@lowtide:~/code/ifs$
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Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-28 08:28 -0400
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-28 09:20 -0700
Re: The substantiation that -highhorse asked for fr314159@gmail.com - 2018-08-28 11:27 -0700
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-28 11:39 -0700
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-29 09:46 -0400
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-28 14:52 -0400
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-28 12:20 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-28 17:21 +0000
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-28 14:46 -0400
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-28 19:14 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-28 12:30 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-28 20:09 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-28 13:58 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-28 21:16 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-28 14:31 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-28 21:43 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-28 15:06 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-28 22:26 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-28 15:35 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-28 23:14 +0000
Re: The substantiation that -highhorse asked for Snit <usenet@gallopinginsanity.com> - 2018-08-28 14:57 -0700
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-28 23:11 -0400
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-29 06:14 +0000
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-29 09:34 -0400
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-29 14:31 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-29 11:01 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-31 12:33 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-31 09:16 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-31 16:53 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-31 10:22 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-31 17:53 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-08-31 11:36 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-09-01 13:45 +0000
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-09-01 13:59 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-09-01 08:15 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-09-01 16:00 +0000
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-09-01 11:20 -0700
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-31 15:00 -0400
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-31 19:40 +0000
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-09-01 10:15 -0400
Re: The substantiation that -highhorse asked for Steve Carroll <fretwizzer@gmail.com> - 2018-09-01 08:22 -0700
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-09-01 16:28 +0000
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-09-01 16:39 -0400
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-09-01 22:00 +0000
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-09-01 18:27 -0400
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-09-01 22:40 +0000
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-09-01 19:09 -0400
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-09-01 23:59 +0000
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-28 23:01 -0400
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-29 06:26 +0000
Re: The substantiation that -highhorse asked for "F. Russell" <fr@random.info> - 2018-08-28 19:25 +0000
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-29 09:32 -0400
Re: The substantiation that -highhorse asked for fr314159@gmail.com - 2018-08-29 06:46 -0700
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-08-29 09:55 -0400
Re: The substantiation that -highhorse asked for fr314159@gmail.com - 2018-08-29 08:02 -0700
Re: The substantiation that -highhorse asked for DFS <nospam@nospam.com> - 2018-09-01 10:14 -0400
Re: The substantiation that -highhorse asked for owl <owl@rooftop.invalid> - 2018-08-29 15:12 +0000
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