My result is not truly random; How can I fix this? - c++

Question

float genData(int low, int high);
int main(){
srand(time(0));
float num = genData(40, 100);
cout << fixed << left << setprecision(2) << num << endl;
return 0;
}
float genData(int low, int high) {
low *= 100;
high *= 100 + 1;
int rnd = rand() % (high - low) + low;
float newRand;
newRand = (float) rnd / 100;
return newRand;
}
I'm expecting a random number between 40 and 100 inclusively with two decimal places.
eg: 69.69, 42.00
What I get is the same number with different decimal values, slowly increasing every time I run the program.

Answer 1

Use the <random> header for that:
#include <iostream>
#include <random>
float getData(int const low, int const high) {
thread_local auto engine = std::mt19937{ std::random_device{}() };
auto dist = std::uniform_int_distribution<int>{ low * 100, high * 100 };
return dist(engine) / 100.0f;
}
int main() {
for (auto i = 0; i != 5; ++i) {
std::cout << getData(40, 100) << '\n';
}
}

Answer 2

Wrong range
int rnd = rand() % (high - low) + low; does not generate the right range.
float genData(int low, int high) {
low *= 100;
// high *= 100 + 1;
high = high*100 + 1;
expecting a random number between 40 and 100 inclusively with two decimal places. eg: 69.69, 42.00
That is [40.00 ... 100.00] or 10000-4000+1 different values
int rnd = rand() % (100*(high - low) + 1) + low*100;
float frnd = rnd/100.0f;
rand() weak here when RAND_MAX is small
With RAND_MAX == 32767, int rnd = rand() % (high - low) + low; is like [0... 32767] % 6001 + 40000. [0... 32767] % 6001 does not provide a very uniform distribution. Some values coming up 6 times and others 7-ish.

Answer 3

If you are using C++ 11 you can use better random number generators that will give you diversity in the generated numbers in addition to being a lot faster.
Quick Example:
#include <random> // The header for the generators.
#include <ctime> // To seed the generator.
// Generating random numbers with C++11's random requires an engine and a distribution.
mt19937_64 rng(seed);
// For instance, we will create a uniform distribution for integers in the (low, high) range:
uniform_int_distribution<int> unii(low, high);
// Show the random number
cout << unii(rng) << ' ';
You can follow this article for more explanation from here.