What are the bounds of std::minstd_rand? - c++

Question

From header <random>, there is an engine called std::minstd_rand.
Are the bounds for this engines random numbers inclusive or exclusive? It's got min and max functions, but I specifically want to know whether the random simulation can equal min or max, as opposed to just be bounded by them.
Documentation uses the word "between", which is of course not unambigous.

Answer 1

t's got min and max functions, but I specifically want to know whether the random simulation can equal min or max.
Yes.
From https://en.cppreference.com/w/cpp/numeric/random/linear_congruential_engine#Characteristics (emphasis mine):
min [static]
gets the smallest possible value in the output range
(public static member function)
max [static]
gets the largest possible value in the output range
(public static member function)

Answer 2

I think that we can be a little more precise.
As you can see std::minstd_rand is really std::linear_congruential_engine<std::uint_fast32_t, 48271, 0, 2147483647> hence the emphasis of my co-answerer.
To simply check that indeed the generator can produce the value returned by min() max() we can use this simpler generator.
std::linear_congruential_engine<std::uint_fast32_t, 3, 0, 5>
If we test it
#include <random>
#include <iostream>
int main()
{
std::linear_congruential_engine<std::uint_fast32_t, 3, 0, 5> gen;
std::cout << (std::uint_fast32_t) std::linear_congruential_engine<std::uint_fast32_t, 3, 0, 5>::min() << std::endl;
std::cout << (std::uint_fast32_t) std::linear_congruential_engine<std::uint_fast32_t, 3, 0, 5>::max() << std::endl<< std::endl;
gen.seed(1);
for (int i = 0; i < 5; i++)
{
std::cout << gen() << std::endl;
}
std::cout << std::endl;
return 0;
}
we get
1
4
3
4
2
1
3
As you can see the generator did hit the min and max. So
min [static] gets the smallest possible value in the output range (public static member function)
max [static] gets the largest possible value in the output range (public static member function)
The generator follows this rule https://en.wikipedia.org/wiki/Linear_congruential_generator .
So basically min() returns 1 and max() returns m-1. No smarter than this.
If I take another generator std::linear_congruential_engine<std::uint_fast32_t, 3, 0, 6>
and test it
#include <random>
#include <iostream>
int main()
{
std::linear_congruential_engine<std::uint_fast32_t, 3, 0, 6> gen;
std::cout << (std::uint_fast32_t) std::linear_congruential_engine<std::uint_fast32_t, 3, 0, 6>::min() << std::endl;
std::cout << (std::uint_fast32_t) std::linear_congruential_engine<std::uint_fast32_t, 3, 0, 6>::max() << std::endl << std::endl;
for (int s = 0; s < 6; s++)
{
gen.seed(1);
for (int i = 0; i < 5; i++)
{
std::cout << gen() << " ";
}
std::cout << std::endl;
}
return 0;
}
the output is
1
5
3 3 3 3 3
3 3 3 3 3
3 3 3 3 3
3 3 3 3 3
3 3 3 3 3
3 3 3 3 3
Whatever the seed the output is always 3. so much for hitting the min and max
To finally answer the question
Are the bounds for this engines random numbers inclusive or exclusive?
I don't know. It depends. And I don't know if somebody has ever proved that for whatever seed, you eventually always hit min and max for the std::minstd_rand generator… But that is more of a mathexchange question...