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Groups > comp.lang.c++ > #120340
| From | Ben Bacarisse <ben@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: thread about the pros and cons of lambdas, but more about cons |
| Date | 2024-09-25 23:13 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <871q17idqr.fsf@bsb.me.uk> (permalink) |
| References | <20240925195423.00007ecc@yahoo.com> |
Michael S <already5chosen@yahoo.com> writes:
> On Wed, 25 Sep 2024 17:14:09 +0200
> David Brown <david.brown@hesbynett.no> wrote:
>
>> On 25/09/2024 16:14, Michael S wrote:
>> > On Wed, 25 Sep 2024 15:04:27 +0200
>> > David Brown <david.brown@hesbynett.no> wrote:
>> >
>> >> On 25/09/2024 14:39, Bonita Montero wrote:
>> >>> Am 25.09.2024 um 14:36 schrieb David Brown:
>> >>>
>> >>>> That's reasonable if you have something useful to say. Telling
>> >>>> people that they can use lambdas here is /not/ useful. We all
>> >>>> know we could use lambdas, but that is totally missing the point
>> >>>> of the discussion.
>> >>>
>> >>> I guess Michael S doesn't know this, otherwrite he would have used
>> >>> a lambda himself. That's pre-C++11-style and maybe his knowledge
>> >>> is from then.
>> >>>
>> >>
>> >> Seriously? Sometimes I wonder if you ever bother reading other
>> >> people's posts.
>> >>
>> >> Of /course/ Michael knows he could use a lambda there.
>> >
>> > [O.T.]
>> > I know that they can be used, but I never use lambdas myself.
>> > Because of that my knowledge is theoretical and I am not fluent with
>> > syntax.
>> >
>> > Why I am not using lambdas myself? Because I think that lambda with
>> > captures makes code harder to follow and to understand (that's my
>> > 1st hand experience from reading big corpus of Ada83 code. Ada83
>> > does not have lambdas, but it has nested procedures that can access
>> > parent's variable, similarly to lambda with [&] default capture).
>> > And lambda without captures does not provide enough of advantage
>> > over named functions to bother with mastering new concept.
>> >
>>
>> It's maybe worth having another thread about the pros and cons of
>> lambdas, but that really should be a new thread.
>>
>
>
> Some time ago on comp.lang.c we had very long thread about floodfill4
> algorithm (that both myself and TimR took more seriously than an
> issue deserves, but that's off topic).
>
> Today I coded two implementations of original brute-force recursive
> algorithm.
>
> // floodfill_recursive_nested.
> // Implementation is in none-tricky C++
> // Very similar to what can be done in C
>
> #include <cstddef>
>
> int floodfill4(
> unsigned char *grey,
> int width, int height,
> int x, int y,
> unsigned char target, unsigned char dest)
> {
> if (width < 1 || height < 1)
> return 0;
> if (x < 0 || x >= width || y < 0 || y >= height)
> return 0;
>
> size_t w = width, h = height;
> if (grey[y*w+x] != target)
> return 0;
>
> struct {
> unsigned char *grey;
> size_t width, height;
> unsigned char target, dest;
> void core(size_t x, size_t y) const
> {
> if (x < width && y < height) {
> auto idx = y*width+x;
> if (grey[idx] == target) {
> grey[idx] = dest;
> core(x - 1, y);
> core(x + 1, y);
> core(x, y - 1);
> core(x, y + 1);
> }
> }
> }
> } context = {
> .grey = grey,
> .width = w, .height = h,
> .target = target, .dest = dest,
> };
> context.core(x, y);
> return 1;
> }
>
> // end of floodfill_recursive_nested.
>
>
>
> // floodfill_recursive_lambda.
> // Implementation in tricky C++
> // C can not do it
>
> #include <cstddef>
>
> int floodfill4(
> unsigned char *grey,
> int width, int height,
> int x, int y,
> unsigned char target, unsigned char dest)
> {
> if (width < 1 || height < 1)
> return 0;
> if (x < 0 || x >= width || y < 0 || y >= height)
> return 0;
>
> size_t w = width, h = height;
> if (grey[y*w+x] != target)
> return 0;
>
> auto core = [=] (auto& a_ref, size_t x, size_t y) {
> if (x >= w || y >= h)
> return;
> auto idx = y*w+x;
> if (grey[idx] == target) {
> grey[idx] = dest;
> a_ref(a_ref, x - 1, y);
> a_ref(a_ref, x + 1, y);
> a_ref(a_ref, x, y - 1);
> a_ref(a_ref, x, y + 1);
> }
> };
> core(core, x, y);
> return 1;
> }
>
> // end of floodfill_recursive_lambda.
>
>
> In the second variant in order to make it compile at all I had to uses
> very dirty trick with lambda passed as parameter to itself. I copied it
> from Stack Overflow, but don't pretend to understand why it works and
> why it needed in the first place.
For this part the answer is simple. The lambda only captures names that
are defined at its point of definition, and core is not defined until
the end of the declarator which include the initialisation -- the lambda
you are defining. The lambda stored in 'core', can't therefore refer to
the name core because core was not defined before the lambda. You can,
instead, do this (untested):
#include <functional>
int floodfill4(
unsigned char *grey,
int width, int height,
int x, int y,
unsigned char target, unsigned char dest)
{
if (width < 1 || height < 1)
return 0;
if (x < 0 || x >= width || y < 0 || y >= height)
return 0;
size_t w = width, h = height;
if (grey[y*w+x] != target)
return 0;
std::function<void(size_t, size_t)> core;
core = [=] (size_t x, size_t y) {
if (x >= w || y >= h)
return;
auto idx = y*w+x;
if (grey[idx] == target) {
grey[idx] = dest;
core(x - 1, y);
core(x + 1, y);
core(x, y - 1);
core(x, y + 1);
}
};
core(x, y);
return 1;
}
> But that's only part of the story.
> The other part is that the first variant is 1.2x faster.
Interesting, though this is not really what a lambda is for. What you
want here is a plain lexical nested function -- it's purpose being just
an auxiliary function that can refer to the outer scope so as to require
fewer parameters.
It would be interesting to see is gcc's nested function extension
produced something that was faster than a lambda.
Note: this is a very old problem and actually pre-dates the computing
era. Combinatory logic had to come up with the Y combinator to make
recursive "functions", and (slightly more recently) some Lisps have two
forms of 'let' to deal with this issue.
--
Ben.
Back to comp.lang.c++ | Previous | Next — Previous in thread | Next in thread | Find similar
thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-25 19:54 +0300
Re: thread about the pros and cons of lambdas, but more about cons Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-25 19:17 +0200
Re: thread about the pros and cons of lambdas, but more about cons Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-25 19:55 +0200
Re: thread about the pros and cons of lambdas, but more about cons Paavo Helde <eesnimi@osa.pri.ee> - 2024-09-25 22:53 +0300
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-25 23:30 +0300
Re: thread about the pros and cons of lambdas, but more about cons Paavo Helde <eesnimi@osa.pri.ee> - 2024-09-26 00:04 +0300
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-26 11:17 +0300
Re: thread about the pros and cons of lambdas, but more about cons Paavo Helde <eesnimi@osa.pri.ee> - 2024-09-26 11:25 +0300
Re: thread about the pros and cons of lambdas, but more about cons Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-26 10:28 +0200
Re: thread about the pros and cons of lambdas, but more about cons Paavo Helde <eesnimi@osa.pri.ee> - 2024-09-26 11:49 +0300
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-26 11:38 +0300
Re: thread about the pros and cons of lambdas, but more about cons Ben Bacarisse <ben@bsb.me.uk> - 2024-09-25 23:13 +0100
Re: thread about the pros and cons of lambdas, but more about cons Ben Bacarisse <ben@bsb.me.uk> - 2024-09-25 23:28 +0100
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-26 10:41 +0300
Re: thread about the pros and cons of lambdas, but more about cons David Brown <david.brown@hesbynett.no> - 2024-09-26 10:29 +0200
Re: thread about the pros and cons of lambdas, but more about cons Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-26 11:35 +0200
Re: thread about the pros and cons of lambdas, but more about cons David Brown <david.brown@hesbynett.no> - 2024-09-26 13:27 +0200
Re: thread about the pros and cons of lambdas, but more about cons Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-26 13:31 +0200
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-26 15:25 +0300
Re: thread about the pros and cons of lambdas, but more about cons Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-26 14:58 +0200
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-26 16:53 +0300
Re: thread about the pros and cons of lambdas, but more about cons Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-26 15:54 +0200
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-27 16:55 +0300
Re: thread about the pros and cons of lambdas, but more about cons Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-26 10:01 -0700
Re: thread about the pros and cons of lambdas, but more about cons Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-26 19:04 +0200
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-26 20:20 +0300
Re: thread about the pros and cons of lambdas, but more about cons Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-26 10:38 -0700
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-27 16:27 +0300
Re: thread about the pros and cons of lambdas, but more about cons Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-28 04:06 -0700
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-26 10:58 +0300
Re: thread about the pros and cons of lambdas, but more about cons Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-26 10:27 -0700
Re: thread about the pros and cons of lambdas, but more about cons Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-26 19:32 +0200
Re: thread about the pros and cons of lambdas, but more about cons Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-26 14:09 -0700
Re: thread about the pros and cons of lambdas, but more about cons Michael S <already5chosen@yahoo.com> - 2024-09-27 16:15 +0300
Re: thread about the pros and cons of lambdas, but more about cons Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-09-28 04:25 -0700
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