This is entirely implementation specific, but it appears that in the C++ environment you’re working in, RAND_MAX is equal to INT_MAX.

Because of this, RAND_MAX + 1 exhibits undefined (overflow) behavior, and becomes INT_MIN. While your initial statement was dividing (random # between 0 and INT_MAX)/(INT_MAX) and generating a value 0 <= r < 1, now it's dividing (random # between 0 and INT_MAX)/(INT_MIN), generating a value -1 < r <= 0
In order to generate a random number 1 <= r < 2, you would want
r = ((double) rand() / (RAND_MAX)) + 1
rand() / double(RAND_MAX) generates a floating-point random number between 0 (inclusive) and 1 (inclusive), but it's not a good way for the following reasons (because RAND_MAX is usually 32767):
The number of different random numbers that can be generated is too small: 32768. If you need more different random numbers, you need a different way (a code example is given below)
The generated numbers are too coarse-grained: you can get 1/32768, 2/32768, 3/32768, but never anything in between.
Limited states of random number generator engine: after generating RAND_MAX random numbers, implementations usually start to repeat the same sequence of random numbers.
Due to the above limitations of rand(), a better choice for generation of random numbers between 0 (inclusive) and 1 (exclusive) would be the following snippet (similar to the example at http://en.cppreference.com/w/cpp/numeric/random/uniform_real_distribution ):
#include

#include

#include

int main()

{

std::mt19937_64 rng;

// initialize the random number generator with time-dependent seed

uint64_t timeSeed = std::chrono::high_resolution_clock::now().time_since_epoch().count();

std::seed_seq ss{uint32_t(timeSeed & 0xffffffff), uint32_t(timeSeed>>32)};

rng.seed(ss);

// initialize a uniform distribution between 0 and 1

std::uniform_real_distribution

// ready to generate random numbers

const int nSimulations = 10;

for (int i = 0; i < nSimulations; i++)
{
double currentRandomNumber = unif(rng);
std::cout << currentRandomNumber << std::endl;
}
return 0;
}
This is easy to modify to generate random numbers between 1 (inclusive) and 2 (exclusive) by replacing unif(0, 1) with unif(1, 2).

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