Sunday, January 15, 2023

Digest for comp.lang.c++@googlegroups.com - 10 updates in 3 topics

quanzhou <anhonghe@gmail.com>: Jan 15 11:37AM -0800

auto [a, b, ...] = x;
auto [a, ..., b] = y;
auto [...,a, b] = z;
 
helps to get the last member variable of the right side type.
Bonita Montero <Bonita.Montero@gmail.com>: Jan 15 04:06PM +0100

Now it's perfect:
 
#include <iostream>
#include <unordered_set>
#include <charconv>
#include <random>
#include <concepts>
 
using namespace std;
 
int main( int argc, char** argv )
{
try
{
if( argc < 4 )
return
cout << argv[0] << " n from to" << endl,
EXIT_FAILURE;
auto parse = []( char const *str, char const *err )
{
size_t value;
if( from_chars_result fcr = from_chars( str, str + strlen( str ),
value ); (bool)fcr.ec || *fcr.ptr )
throw invalid_argument( err );
return value;
};
size_t n = parse( argv[1], "wrong number of values" );
if( !n )
return EXIT_SUCCESS;
size_t
from = parse( argv[2], "wrong from-value" ),
to = parse( argv[3], "wrong to-value" );
if( from > to )
swap(from, to);
if( n - 1 > to - from )
return
cout << "n is too small" << endl,
EXIT_FAILURE;
unordered_set<size_t> values;
mt19937_64 mt;
uniform_int_distribution<size_t> uid( from, to );
if( to - from != (size_t)-1 && to - from + 1 < n / 2 )
{
values.reserve( n );
while( values.size() < n )
{
size_t value;
do
value = uid( mt );
while( values.contains( value ) );
values.emplace( value );
}
}
else
{
if( to - from == (size_t)-1 )
throw bad_alloc();
values.reserve( to - from + 1 );
size_t i = from - 1;
do
values.emplace( ++i );
while( i != to );
while( values.size() > n )
for( ; ; )
{
auto itRemove = values.find( uid( mt ) );
if( itRemove == values.end() )
continue;
values.erase( itRemove );
break;
}
}
for( size_t value : values )
cout << value << endl;
}
catch( exception const &exc )
{
return
cout << exc.what() << endl,
EXIT_FAILURE;
}
}
 
If less than half of the range of numbers is selected (n < (to - from
+ 1) / 2) the unordered map is filled up to n elements and if an element
is found that's already there another value is chosen. If more than half
of the range is selected (n >= (to - from + 1) / 2) the set is filled up
to contain the whole range and the elements are incrementally removed at
a random value.
Ralf Goertz <me@myprovider.invalid>: Jan 15 04:47PM +0100

Am Sun, 15 Jan 2023 16:06:00 +0100
 
> Now it's perfect:

[snip]
 
> more than half of the range is selected (n >= (to - from + 1) / 2)
> the set is filled up to contain the whole range and the elements are
> incrementally removed at a random value.
 
What's wrong with a clean two line solution:
 
 
#include <algorithm>
#include <iostream>
#include <random>
#include <charconv>
#include <vector>
#include <cstring>
 
using namespace std;
 
int main( int argc, char** argv )
{
try
{
if( argc < 4 )
return
cout << argv[0] << " n from to" << endl,
EXIT_FAILURE;
auto parse = []( char const *str, char const *err )
{
size_t value;
if( from_chars_result fcr = from_chars( str, str + strlen( str ),
value ); (bool)fcr.ec || *fcr.ptr )
throw invalid_argument( err );
return value;
};
size_t n = parse( argv[1], "wrong number of values" );
if( !n )
return EXIT_SUCCESS;
size_t
from = parse( argv[2], "wrong from-value" ),
to = parse( argv[3], "wrong to-value" );
if( from > to )
swap(from, to);
if( n - 1 > to - from )
return
cout << "n is too small" << endl,
EXIT_FAILURE;
mt19937_64 mt;
vector<int> values( to - from );
// ------
//only two lines to find random subset of fixed size:
fill(values.begin(), values.begin() + n, 1);
shuffle(values.begin(), values.end(), mt);
// ------
for( size_t i=0; i < values.size(); ++i )
if( values[i]) cout << i+from << endl;
}
catch( exception const &exc )
{
return
cout << exc.what() << endl,
EXIT_FAILURE;
}
}
Bonita Montero <Bonita.Montero@gmail.com>: Jan 15 05:13PM +0100

Am 15.01.2023 um 16:47 schrieb Ralf Goertz:
>> the set is filled up to contain the whole range and the elements are
>> incrementally removed at a random value.
 
> What's wrong with a clean two line solution:
 
You might have less numbers than to - from + 1 (you omitted the 1 on
allocation of the vector).
Bart <bc@freeuk.com>: Jan 15 04:27PM

On 15/01/2023 15:06, Bonita Montero wrote:
> Now it's perfect:
 
You said that last time too:
 
On 08/01/2023 05:01, Bonita Montero wrote:
> Now it's perfect:
 
I guess it's more perfect?
Bonita Montero <Bonita.Montero@gmail.com>: Jan 15 05:42PM +0100

Am 15.01.2023 um 17:27 schrieb Bart:
 
> On 08/01/2023 05:01, Bonita Montero wrote:
> > Now it's perfect:
 
> I guess it's more perfect?
 
The code I've shown yet is also perfect in terms of performance,
but not just functionally.
Malcolm McLean <malcolm.arthur.mclean@gmail.com>: Jan 15 08:57AM -0800

On Sunday, 15 January 2023 at 15:48:01 UTC, Ralf Goertz wrote:
> EXIT_FAILURE;
> }
> }
 
Where N is the range and M the number of elements to choose, it's O(N)
in space and time. So unsuitable if N is very much larger than M.
gazelle@shell.xmission.com (Kenny McCormack): Jan 15 05:11PM

In article <tq14n7$2cgbf$1@dont-email.me>,
>Now it's perfect:
 
I just need 11,780 votes. Give me a break here.
 
--
If Jeb is Charlie Brown kicking a football-pulled-away, Mitt is a '50s
housewife with a black eye who insists to her friends the roast wasn't
dry.
"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>: Jan 14 04:14PM -0800

On 5/12/2022 12:16 PM, Chris M. Thomasson wrote:
> Using my experimental vector field to generate a fractal formation. Here
> is generation two:
 
> https://fractalforums.org/gallery/1612-120522191048.png
 
An experiment... The music and geometry are mine, EXCEPT, the model of
the girl:
 
A test of the Thomasson Fractal System. Other models are welcome.
https://youtu.be/n13GHyYEfLA Using some of my experimental MIDI music...
Btw, the model for the girl can be found here: https://skfb.ly/6zGMG
"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>: Jan 14 07:35PM -0800

On 1/4/2023 3:23 PM, Alf P. Steinbach wrote:
 
>> https://skfb.ly/oCqn7
 
> You should put this to music by Nirvana, In Bloom.
 
> - Alf
 
https://youtu.be/usADINi17cI
 
;^) ?
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