function returns all the same random number

Question

I have a function in c++, which returns a random float number between two numbers. Here is the code:

float randFloatNumBetween(float lower, float upper) {
    srand((unsigned)time(NULL));
    return lower + ((float) rand() / RAND_MAX) * (upper - lower); 
}

When I call this function with parameters where lower is 0.0 and upper 10.0, it returns numbers like:

How can I make my function returns different numbers before and after the comma? Numbers like:

Mark Ransom · Accepted Answer · 2014-06-18T18:40:25.010

Obviously you need to change the seed value each time you call srand. Based on the comments you've left this doesn't appear to be your problem, but it needs to be pointed out.

Some random number generators (I know Microsoft's rand to be one) have a correlation between the initial seed and the first number generated. Since the time doesn't change very fast, the numbers you get from seeding with it tend to be correlated as well.

The best way around this is to use a better random number generator such as the C++11 ones in the <random> header: http://en.cppreference.com/w/cpp/numeric/random or their predecessors in the boost library.

Otherwise just make sure you generate a couple of throw-away random numbers after you seed it.

score 1 · Answer 2 · edited May 23 '17 at 12:21

1

You are calling srand in each iteration but srand should be called only once. Place srand((unsigned)time(NULL)); outside of this function, in the file scope.

edited May 23 '17 at 12:21

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answered Jun 18 '14 at 18:18

haccks

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But now I dont understand this, why does it everytime i call the program still return the same number in current time? Shouldn't it also change the first 2 digits when I run for a second time program? – Xerath Jun 18 '14 at 18:24
No. It will not. Read the accepted answer in the link provided. – haccks Jun 18 '14 at 18:28

score 1 · Answer 3 · answered Jun 18 '14 at 18:41

If your compiler supports C++11 features you can do the following:

#include <chrono>
#include <random>
#include <iostream>
#include <type_traits>

template<typename T>
typename std::enable_if<std::is_floating_point<T>::value, T>::type randNumBetween(T lower, T upper, std::default_random_engine &gnr) {
  if(lower > upper) std::swap(lower, upper);
  std::uniform_real_distribution<T> distribution(lower, upper);
  return distribution(gnr);
}

template<typename T>
typename std::enable_if<std::is_integral<T>::value, T>::type randNumBetween(T lower, T upper, std::default_random_engine &gnr) {
  if(lower > upper) std::swap(lower, upper);
  std::uniform_int_distribution<T> distribution(lower, upper);
  return distribution(gnr);
}

int main() {
  auto seed = std::chrono::system_clock::now().time_since_epoch().count();
  std::default_random_engine gnr(seed);
  for(auto i(0); i < 10; ++i) std::cout << randNumBetween(10, 20, gnr) << std::endl;
  for(auto i(0); i < 10; ++i) std::cout << randNumBetween(10.0, 20.0, gnr) << std::endl;
  return 0;
}

DEMO

In this nice video lecture Stephan T. Lavavej explains why use of rand() is considered harmful and unsafe.

function returns all the same random number

3 Answers3

DEMO