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Re: This code is effective for modest-sized numbers, but can you tell what it does?

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Graeme C. <graec58@gmail.com> wrote:
> using namespace std;

I'd stop using that. The prefixes increase code readability.
(Yes, they do.)

> typedef long long Int;

It's clearer if you use the newer alternative:

    using Int = long long;

>             string s(argv[1]);
>             istringstream iss(s);
>             iss >> N;           // we'll try this one

It's easier to just say N = std::atoll(argv[1]);
(Ostensibly we don't care about exact validity of the argument).

>     vector<vector<Facs> > vf;
>     vf.push_back(vector<Facs>());

Could just write:

    std::vector<std::vector<Facs>> vf(1);

>     Int lo = 2;
>     Int hi = 4;
>     Int gen = 1;
>     while (N > lo) {          
...
>         lo=hi;
>         hi*=2;
>         gen++;
>     }

Isn't that just the same thing as:

    Int gen = 1;
    for(Int lo = 2, hi = 4; lo < N; lo *= 2, hi *= 2, ++gen)

>         cout << "generation " << gen << endl;
>         cout.flush();

Outputting std::endl already flushes the stream. There's no need to flush
it again.

>         for (int i=0; i<vf[gen-1].size(); i++) { // from previous gen

std::vector::size() returns a value of type std::size_t. You shouldn't be
using an int with it.

>             const Int& a = vf[gen-1][i].a;
>             const Int& b = vf[gen-1][i].b;

As others have commented there's no need to use references here. In fact,
it's likely going to produce less efficient code (I'm not sure the compiler
is allowed to optimize the references away).

Handing elementary types by value is more efficient than handling them by
reference.

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This code is effective for modest-sized numbers, but can you tell what it does? "Graeme C." <graec58@gmail.com> - 2022-04-22 04:41 -0700
  Re: This code is effective for modest-sized numbers, but can you tell what it does? Christian Gollwitzer <auriocus@gmx.de> - 2022-04-22 17:19 +0200
    Re: This code is effective for modest-sized numbers, but can you tell what it does? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-04-22 17:42 +0200
      Re: This code is effective for modest-sized numbers, but can you tell what it does? red floyd <no.spam.here@its.invalid> - 2022-04-22 09:16 -0700
  Re: This code is effective for modest-sized numbers, but can you tell what it does? Frederick Gotham <cauldwell.thomas@gmail.com> - 2022-04-22 09:22 -0700
    Re: This code is effective for modest-sized numbers, but can you tell what it does? "Graeme C." <graec58@gmail.com> - 2022-04-22 10:24 -0700
      Re: This code is effective for modest-sized numbers, but can you tell what it does? Christian Gollwitzer <auriocus@gmx.de> - 2022-04-22 20:33 +0200
        Re: This code is effective for modest-sized numbers, but can you tell what it does? "Graeme C." <graec58@gmail.com> - 2022-04-22 17:57 -0700
  Re: This code is effective for modest-sized numbers, but can you tell what it does? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-22 19:18 -0700
    Re: This code is effective for modest-sized numbers, but can you tell what it does? "Graeme C." <graec58@gmail.com> - 2022-04-22 19:40 -0700
      Re: This code is effective for modest-sized numbers, but can you tell what it does? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-22 20:00 -0700
        Re: This code is effective for modest-sized numbers, but can you tell what it does? "Graeme C." <graec58@gmail.com> - 2022-04-23 04:21 -0700
  Re: This code is effective for modest-sized numbers, but can you tell what it does? Juha Nieminen <nospam@thanks.invalid> - 2022-04-25 05:48 +0000
    Re: This code is effective for modest-sized numbers, but can you tell what it does? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-25 05:27 -0700
      Re: This code is effective for modest-sized numbers, but can you tell what it does? Öö Tiib <ootiib@hot.ee> - 2022-04-25 05:40 -0700
        Re: This code is effective for modest-sized numbers, but can you tell what it does? Vir Campestris <vir.campestris@invalid.invalid> - 2022-04-25 16:48 +0100
          Re: This code is effective for modest-sized numbers, but can you tell what it does? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-30 03:51 -0700
            Re: This code is effective for modest-sized numbers, but can you tell what it does? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-30 08:28 -0700
              Re: This code is effective for modest-sized numbers, but can you tell what it does? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-30 10:30 -0700
              Re: This code is effective for modest-sized numbers, but can you tell what it does? Manfred <noname@add.invalid> - 2022-04-30 19:20 +0200
                Re: This code is effective for modest-sized numbers, but can you tell what it does? Manfred <noname@add.invalid> - 2022-04-30 23:20 +0200
                Re: This code is effective for modest-sized numbers, but can you tell what it does? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-30 10:40 -0700
              Re: This code is effective for modest-sized numbers, but can you tell what it does? Manfred <noname@add.invalid> - 2022-04-30 19:12 +0200
        Re: This code is effective for modest-sized numbers, but can you tell what it does? Juha Nieminen <nospam@thanks.invalid> - 2022-04-26 05:20 +0000
          Re: This code is effective for modest-sized numbers, but can you tell what it does? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-29 16:12 -0700
      Re: This code is effective for modest-sized numbers, but can you tell what it does? Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-25 15:44 +0300
        Re: This code is effective for modest-sized numbers, but can you tell what it does? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-05-10 07:18 -0700
          Re: This code is effective for modest-sized numbers, but can you tell what it does? scott@slp53.sl.home (Scott Lurndal) - 2022-05-10 14:34 +0000
            Re: This code is effective for modest-sized numbers, but can you tell what it does? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-05-24 05:43 -0700
          Re: This code is effective for modest-sized numbers, but can you tell what it does? Paavo Helde <eesnimi@osa.pri.ee> - 2022-05-10 20:34 +0300
            Re: This code is effective for modest-sized numbers, but can you tell what it does? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-06-04 07:57 -0700

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