# Random Number Generator Followup: Choosing the Generator Algorithm and the Distribution

This question is a follow-up from my previous code review question. This question regards the ability to choose a predefined random number generator algorithm and also choose a generator distribution.

For context, here is the full code in its current state:

#include <algorithm>
#include <boost/program_options.hpp>
#include <boost/algorithm/string/predicate.hpp>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <limits>
#include <random>
#include <vector>

enum returnID {
success = 0,
known_err = 1,
other_err = 2,
zero_err = 3,
conflict_err = 4,
overd_err = 5,
underd_err = 6,
exclude_err = 7,
round_prec = 8,
vect_nan = 9,
gen_err = 10,
success_help = -1
};

bool filter(const long double rand, const int precision, const std::vector<std::string> & fx, bool(*predicate)(const std::string&, const std::string&)) {
std::ostringstream oss;
oss << std::fixed << std::setprecision(precision) << rand;
const auto str_rand = oss.str();
return std::none_of(fx.begin(), fx.end(), [&](auto const & s) { return predicate(str_rand, s); });
}

struct program_args {
long long number;
long double lbound, ubound;
bool ceil, floor, round, trunc; // mutually exclusive
int precision;
std::vector<long double> excluded;
bool norepeat, stat_min, stat_max, stat_median,
stat_avg, bad_random, list, quiet, numbers_force;
std::vector<std::string> prefix, suffix, contains;
std::string delim = "\n", generator = "mt19937";
};

returnID parse_args(program_args & args, int argc, char const * const * argv) {
static auto const ld_prec = std::numeric_limits<long double>::max_digits10;

namespace po = boost::program_options;
po::options_description desc("Options");
desc.add_options()
("help,h", "produce this help message")
("number,n", po::value<long long>(&args.number)->default_value(1),
"count of numbers to be generated")
("lbound,l", po::value<long double>(&args.lbound)->default_value(0.0),
"minimum number (ldouble) to be generated")
("ubound,u", po::value<long double>(&args.ubound)->default_value(1.0),
"maximum number (ldouble) to be generated")
("ceil,c", po::bool_switch(&args.ceil)->default_value(false),
"apply ceiling function to numbers")
("floor,f", po::bool_switch(&args.floor)->default_value(false),
"apply floor function to numbers")
("round,r", po::bool_switch(&args.round)->default_value(false),
"apply round function to numbers")
("trunc,t", po::bool_switch(&args.trunc)->default_value(false),
"apply truncation to numbers")
("precision,p", po::value<int>(&args.precision)->default_value(ld_prec),
"output precision (not internal precision, cannot be > ldouble precision)")
("exclude,x", po::value<std::vector<long double> >(&args.excluded)->multitoken(),
"exclude numbers from being printed, best with --ceil, --floor, --round, or --trunc")
("norepeat", po::bool_switch(&args.norepeat)->default_value(false),
"exclude repeated numbers from being printed, best with --ceil, --floor, --round, or --trunc")
("stat-min", po::bool_switch(&args.stat_min)->default_value(false),
"print the lowest value generated")
("stat-max", po::bool_switch(&args.stat_max)->default_value(false),
"print the highest value generated")
("stat-median", po::bool_switch(&args.stat_median)->default_value(false),
"print the median of the values generated")
("stat-avg", po::bool_switch(&args.stat_avg)->default_value(false),
"print the average of the values generated")
("prefix", po::value<std::vector<std::string> >(&args.prefix)->multitoken(),
"only print when the number begins with string(s)")
("suffix", po::value<std::vector<std::string> >(&args.suffix)->multitoken(),
"only print when the number ends with string(s)")
("contains", po::value<std::vector<std::string> >(&args.contains)->multitoken(),
"only print when the number contains string(s)")
("list", po::bool_switch(&args.list)->default_value(false),
"print numbers in a list with positional numbers prefixed")
("delim", po::value<std::string>(&args.delim),
"change the delimiter")
("quiet,q", po::bool_switch(&args.quiet)->default_value(false),
"disable number output, useful when paired with stats")
("numbers-force", po::bool_switch(&args.numbers_force)->default_value(false),
"force the count of numbers output to be equal to the number specified")
("generator,g", po::value<std::string>(&args.generator),
"change algorithm for the random number generator:\n - minstd_rand0\n - minstd_rand"
"\n - mt19937 (default)\n - mt19937_64\n - ranlux24_base\n - ranlux48_base"
"\n - ranlux24\n - ranlux48\n - knuth_b\n - default_random_engine"
"\n - badrandom (std::rand)");

po::variables_map vm;
po::store(po::parse_command_line(argc, argv, desc), vm);
po::notify(vm);

if(vm.count("help")) {
std::cout << desc << '\n';
return returnID::success_help;
}
if(args.number <= 0) {
std::cerr << "error: the argument for option '--number' is invalid (n must be >= 1)\n";
return returnID::zero_err;
}
if(args.ceil + args.floor + args.round + args.trunc > 1) {
std::cerr << "error: --ceil, --floor, --round, and --trunc are mutually exclusive\n";
return returnID::conflict_err;
}
if(args.ceil || args.floor || args.round || args.trunc) {
args.precision = 0;
}
if(args.precision > ld_prec) {
std::cerr << "error: --precision cannot be greater than the precision for <long double> ("
<< ld_prec << ")\n";
return returnID::overd_err;
}
if(args.precision <= -1) {
std::cerr << "error: --precision cannot be less than zero\n";
return returnID::underd_err;
}
if(vm.count("exclude") && vm["exclude"].empty()) {
std::cerr << "error: --exclude was specified without arguments (arguments are separated by spaces)\n";
return returnID::exclude_err;
}

std::vector<std::vector<std::string> > filters = {{args.prefix, args.suffix, args.contains}};
for(auto i : filters) {
for(auto j : i) {
if(j.find_first_not_of("0123456789.") != std::string::npos || std::count(std::begin(j), std::end(j), '.') > 1) {
std::cerr << "error: --prefix, --suffix, and --contains can only be numbers\n";
return returnID::vect_nan;
}
}
}

const std::vector<std::string> options {{"minstd_rand0", "minstd_rand", "mt19937",
"mt19937_64", "ranlux24_base", "ranlux48_base", "ranlux24",
"ranlux48", "knuth_b", "default_random_engine", "badrandom"}};
if(std::find(options.begin(), options.end(), args.generator) == options.end()) {
std::cerr << "error: --generator must be: minstd_rand0, minstd_rand, "
"mt19937, mt19937_64, ranlux24_base, ranlux48_base, "
"ranlux24, ranlux48, knuth_b, default_random_engine, badrandom\n";
return returnID::gen_err;
}

return returnID::success;
}

std::function<long double()> random_generator(const program_args & args) {
if(args.generator == "minstd_rand0") {
std::minstd_rand0 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "minstd_rand") {
std::minstd_rand generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "mt19937") {
std::mt19937 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "mt19937_64") {
std::mt19937_64 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "ranlux24_base") {
std::ranlux24_base generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "ranlux48_base") {
std::ranlux48_base generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "ranlux24") {
std::ranlux24 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "ranlux48") {
std::ranlux48 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "knuth_b") {
std::knuth_b generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "default_random_engine") {
std::default_random_engine generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "badrandom") {
std::srand(std::time(nullptr));
const auto min = args.lbound;
const auto scale = (args.ubound - args.lbound) / RAND_MAX;
return [min, scale]{ return min + (std::rand() * scale);};
}
}

int main(int ac, char* av[]) {
try {
program_args args;

switch(auto result = parse_args(args, ac, av)) {
case returnID::success: break;
case returnID::success_help: return 0;
default: return result;
}

std::vector<long double> generated;

std::cout.precision(args.precision);

const auto random = random_generator(args);
long long list_cnt = 0;

for(long long i = 1; i <= args.number;) {
if(!args.numbers_force) ++i;
if(args.list) ++list_cnt;
long double rand = random();

if(args.ceil) rand = std::ceil(rand);
else if(args.floor) rand = std::floor(rand);
else if(args.round) rand = std::round(rand);
else if(args.trunc) rand = std::trunc(rand);

if(!args.excluded.empty() && std::find(args.excluded.begin(), args.excluded.end(), rand) != args.excluded.end())
continue;
else if(args.norepeat && std::find(generated.begin(), generated.end(), rand) != generated.end())
continue;
else if(!args.prefix.empty() && filter(rand, args.precision, args.prefix, boost::starts_with))
continue;
else if(!args.suffix.empty() && filter(rand, args.precision, args.suffix, boost::ends_with))
continue;
else if(!args.contains.empty() && filter(rand, args.precision, args.contains, boost::contains))
continue;

generated.push_back(rand);

if(!args.quiet) {
if(args.list && args.numbers_force) std::cout << i << ".\t";
if(args.list) std::cout << list_cnt << ".\t";
std::cout << std::fixed << rand << args.delim;
if(args.numbers_force) ++i;
}
}

if(args.delim != "\n" && !args.quiet) std::cout << '\n';

if((args.stat_min || args.stat_max || args.stat_median || args.stat_avg) && !args.quiet)
std::cout << '\n';

if(args.stat_min || args.stat_max) {
auto minmax = std::minmax_element(generated.begin(), generated.end());
if(args.stat_min) std::cout << "min: " << *minmax.first << '\n';
if(args.stat_max) std::cout << "max: " << *minmax.second << '\n';
}

if(args.stat_median) {
auto midpoint = generated.begin() + generated.size() / 2;
std::nth_element(generated.begin(), midpoint, generated.end());
auto median = *midpoint;
if(generated.size() % 2 == 0)
median = (median + *std::max_element(generated.begin(), midpoint)) / 2;
std::cout << "median: " << median << '\n';
}
if(args.stat_avg) {
long double sum = std::accumulate(generated.begin(), generated.end(), 0.0);
std::cout << "avg: " << sum / generated.size() << '\n';
}

return returnID::success;

} catch(std::exception & e) {
std::cerr << "error: " << e.what() << '\n';
return returnID::known_err;

} catch(...) {
std::cerr << "error: exception of unknown type!\n";
return returnID::other_err;
}
}


Here is the portion of the code that this question is about:

std::function<long double()> random_generator(const program_args & args) {
if(args.generator == "minstd_rand0") {
std::minstd_rand0 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "minstd_rand") {
std::minstd_rand generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "mt19937") {
std::mt19937 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "mt19937_64") {
std::mt19937_64 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "ranlux24_base") {
std::ranlux24_base generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "ranlux48_base") {
std::ranlux48_base generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "ranlux24") {
std::ranlux24 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "ranlux48") {
std::ranlux48 generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "knuth_b") {
std::knuth_b generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "default_random_engine") {
std::default_random_engine generator{(std::random_device())()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };

} else if(args.generator == "badrandom") {
std::srand(std::time(nullptr));
const auto min = args.lbound;
const auto scale = (args.ubound - args.lbound) / RAND_MAX;
return [min, scale]{ return min + (std::rand() * scale);};
}
}


Which is then called using:

const auto random = random_generator(args);
long double rand = random();


This std::function was mainly created by Toby Speight through his response to my previous question.

While Toby's original code was very good, when I added a lot more predefined generators, it seemed to be repetitive.

My question is this: can this std::function be simplified, and can it also work with the ability to choose a distribution as well?

If I were to simply create a separate if-else for each possibility of generator-and-distribution it would get out of hand quickly.

• Mix inheritance and templates. This is the perfect problem for it, in my opinion Aug 25, 2017 at 1:08

## Mixing fire with ice

On the one hand, we have inheritance, which is a great tool in cases where change of behavior during runtime is required (switch is a tool for that too, but its kind of static). On the other hand, we have templates, which are great when things are known beforehand. So, lets mix the two:

class random_generator
{
public:
virtual long double operator()() = 0;
};

#include <utility>

template <typename StdGenerator,
typename Distribution = std::uniform_real_distribution<long double>>
class std_random_generator: public random_generator
{
StdGenerator gen;
Distribution distr;
public:
template <typename ... Args>
std_random_generator(Distribution&& distr_,
std::forward<Args>(gen_args)...)
{}

virtual long double operator() override;
};

class cstd_random
{
const long double lbound;
const long double ubound;
public:
cstd_random(const long double lbound_,
const long double ubound_);

virtual long double operator()() override;

private:
//insert guard for initialization from multiple threads
};


Then you can just define a factory function:

std::unique_ptr<random_generator> create_random_generator(const random_generator_args& args)
{
//big if else block...
}


It is possible to make it a static function of random_generator, but I think it should be fine.

Then, duplication should go away:

if(args.generator == "knuth_b") {
return std::make_unique<std_random_generator<std::knuth_b>>({args.lbound, args.ubound}, std::random_device()());
} //etc
else if (args.generator == "bad_random")
{
return std::make_unique<cstd_random>(args.lbound, args.ubound);
}


## Code review

(std::random_device())();


Using std::random_device{}() is generally better, in my opinion, to dodge all of the vexing parse issues.

## Very high coupling

There are no clear concepts here, except for the end user. I see two main distinct modules here: random number generation, and argument parsing. I found that most of the members in program_arguments are not used in random generator initialization. May be it would make sense for parser to return it, but random generator initialization needs far less of them. It doesn't look like it is passing a lot of arguments, and they can be passed by const reference anyway.

## No automated tests

Tests are important (I've written part many times now and out of ideas what to write). They're a big investment with net income in the longer run.

## No build scripts

May be older people are able to deal with build errors, but I hate them. Also, it turns out some CMake versions support only a boost version lower than specified. So, as a result I just didn't check what I've written, even though I've written a complete program. There are also package managers, like Conan, but I think having CMake build script first would be better.

## Function pointers

I believe this is the first time I've seen a raw function pointer on this site in 1.5 years. I don't read every post, but I read majority of them. I think that accepting any callable should be fine.

• I believe this is the example that proves that C++ is the pinnacle of all of programming languages. Very powerful, yet simple abstraction. Aug 25, 2017 at 2:54
• Good suggestion of std::random_device{}() - the parenthesised form came from my earlier review, where I passed it as a function argument, and was vexed to find it parsed as a typename unless I helped the compiler. The braced form is much better, and I'm shamefaced to have overlooked that solution! Perhaps I should be concerned that so many parens didn't bother me - have I being doing too much Lisp? Aug 25, 2017 at 6:12
• @TobySpeight, thanks, adjusted the content of the paragraph. Though I am not sure what Standard has to say about that. Probably vexing parse is correct. Aug 25, 2017 at 6:14

Below is the solution that I came to (posted a day late):

I decided to take Incomputable's idea of templates and run with it, in my own way:

template<typename GEN> std::function<long double()> r_gen(const program_args & args) {
GEN generator{std::random_device{}()};
std::uniform_real_distribution<long double> dis{args.lbound, args.ubound};
return [dis, generator]() mutable -> auto { return dis(generator); };
}

long double random(const program_args & args) {
if(args.generator == "minstd_rand0") return r_gen<std::minstd_rand0>(args)();
else if(args.generator == "minstd_rand") return r_gen<std::minstd_rand>(args)();
else if(args.generator == "mt19937_64") return r_gen<std::mt19937_64>(args)();
else if(args.generator == "ranlux24_base") return r_gen<std::ranlux24_base>(args)();
else if(args.generator == "ranlux48_base") return r_gen<std::ranlux48_base>(args)();
else if(args.generator == "ranlux24") return r_gen<std::ranlux24>(args)();
else if(args.generator == "ranlux48") return r_gen<std::ranlux48>(args)();
else if(args.generator == "knuth_b") return r_gen<std::knuth_b>(args)();
else if(args.generator == "default_random_engine") return r_gen<std::default_random_engine>(args)();
else if(args.generator == "badrandom")
return args.lbound + (std::rand() / (RAND_MAX / (args.ubound - args.lbound)));
else return r_gen<std::mt19937>(args)();
}


Which can then be called by:

long double rand = random(args);


Notice that the case for args.generator == "badrandom" does not include std::srand(). If it contained std::srand(), it would be called constantly, and thus would return the same number over and over (that's why it's called badrandom after all). Instead, I use:

if(args.generator == "badrandom") std::srand(std::time(nullptr));


before the main for loop.

You may also notice that my template (r_gen<>()) does not include a typename for the distribution, even though it is possible to do so via:

template<typename GEN, typename DIST> // etc.


When going through the distributions, I noticed different distributions work differently (not really surprising). For example, std::uniform_int_distribution returns int, std::bernoulli_distribution returns bool, etc. Some of them have varying arguments, some just using a single number for the bound, or only using arguments to define a seed, etc.

For these reasons, I decided not to include a choice of distribution, at least not yet. A solution would be to pass lbound and ubound as optional arguments... we'll see.

While I will accept this as the answer, because it is what I ended up using, I want to say that Incomputable's answer is what inspired the code, and some of their suggestions I have also implemented (such as std::random_device{}()). I love the idea of using tests, and it's something I've had on my list to look into.

Here's the final code. Let me know if I've done something horribly wrong.