c++ - What difference between rand() and random() functions?

Question

Once, my teacher taught me to use randomize() and random() function for generating pseudorandom numbers in C++ Builder. Now I prefer working in VS 2012, but when I tried to use these functions there it says that "identifier not found", even when I added #include <stdlib.h>. After some time of Googling I found that there are also rand() and srand() functions. What is the difference between them and which is it better to use?

Answer 1

randomize() and random() are not part of the standard library. Perhaps your teacher wrote functions with these names for use in your class, or maybe you really mean random() and srandom() which are part of POSIX and not available on Windows. rand() and srand() are part of the standard library and will be provided by any standard conforming implementation of C++.

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You should avoid rand() and srand() and use the new C++11 <random> library. <random> was added as part of the C++11 standard (and VS2012 does provide it).

Video explaining why: rand() Considered Harmful

• rand() is typically a low quality pRNG and not suitable for applications that need a reasonable level of unpredictability. <random> provides a variety of engines with different characteristics suitable for many different use cases.

• Converting the results of rand() into a number you can use directly usually relies on code that is difficult to read and easy to get wrong, whereas using <random> distributions is easy and produces readable code.

• The common methods of generating values in a given distribution using rand() further decrease the quality of the generated data. % generally biases the data and floating point division still produces non-uniform distributions. <random> distributions are higher quality as well as more readable.

• rand() relies on a hidden global resource. Among other issues this causes rand() to not be thread safe. Some implementations make thread safety guarantees, but this is not required standard. Engines provided by <random> encapsulate pRNG state as objects with value semantics, allowing flexible control over the state.

• srand() only permits a limited range of seeds. Engines in <random> can be initialized using seed sequences which permit the maximum possible seed data. seed_seq also implements a common pRNG warm-up.

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example of using <random>:

#include <iostream>
#include <random>

int main() {
  // create source of randomness, and initialize it with non-deterministic seed
  std::random_device r;
  std::seed_seq seed{r(), r(), r(), r(), r(), r(), r(), r()};
  std::mt19937 eng{seed};

  // a distribution that takes randomness and produces values in specified range
  std::uniform_int_distribution<> dist(1,6);

  for (int i=0; i<100; ++i) {
    std::cout << dist(eng) << '
';
  }
}