About std::rand, std::time, std::srand and std::experimental::randint

std::srand takes std::time(must include <ctime> and uses current time as seed for random generator) as parameter and seeds pseudo-random number generator used by std::rand() that by its instead returns a pseudo-random integral value between 0 and RAND_MAX, to know its value just print it, example:

#include <iostream>
#include <ctime>

int main(){
  std::srand(std::time(nullptr));

  int random_number = std::rand();
  int number_then_60 = std::rand() % 60;
  
  std::cout << "The maximum number is: " << RAND_MAX << '\n';
  std::cout << "Random number up to RAND_MAX: " << random_number << '\n';
  std::cout << "Random number up to 60: " << number_then_60 << '\n';

  return 0;
}

Random numbers are generated at runtime, that is, each time it is executed or each time you call a function with generated value from std::rand() the numbers will be different unless you remove the line: std::srand(std::time(nullptr)); or, in the case of the function, if using static.

If you also want numbers without signs, just make a casting, example: std::srand(static_cast<unsigned>(time(nullptr))), a little useless, but just to inform.

Nice to know that we can limit the size of the random number as in the example we used: std::rand() % 60, that is, it prints at most up to 60, but if we wanted a range between: 40 and 60, for example?

Of course we could create a condition and add or subtract according to the result value dynamically.

But that’s quite a bit of work, so comes the std::experimental::randint task. To use it we need to include the <experimental/random> header, example:

#include <iostream>
#include <ctime>
#include <experimental/random>

int main(){
  std::srand(std::time(nullptr));

  int value = std::experimental::randint(40,60);
  std::cout << "Random number between 40 and 60: " << value  << '\n';

  return 0;
}

If you think the name gets too long, try summarizing, eg: namespace rd = std::experimental; and implement: rd::randint(5,10);.

Alternatively you can use std::srand with ZERO time: std::srand(time(0)); or even without scope resolution: srand(time(0)); and rand() % num;, for example:

#include <iostream>
#include <ctime>

int main(){
  srand(time(0));
  int value = rand() % 10;
  std::cout << "Random number up to 10: " << value  << '\n';
  return 0;
}

Simple, right?!

cpp cppdaily