Same random numbers being created everytime the loop goes around. - c++

Question

I am a beginner at C++ and for one of my project involves loop inside loops and creating random numbers. Here is what I have so far:
`
using namespace std;
int main()
{
srand((unsigned int)time(0));
{
cout << "Name of reservoir: ";
string reservior_name;
cin >> reservior_name;
cout << "Capacity in MAF: ";
double capacity;
cin >> capacity;
cout << "Maximum inflow in MAF: ";
int max;
cin>> max;
cout << "minimum inflow in MAF: ";
int min;
cin >> min;
if(min>max)
{cout<<endl<<"Error: The minimum inflow is higher than the maximum inflow."<<endl
<< "Please re-enter your minimum inflow: ";
cin>>min;
}
double inflow_range= max-min;
cout <<"required outflow in MAF: ";
double required;
cin >> required;
if (required > 0.9 * (min + max)/2)
{
cout<<endl<< "Warning: required ouflow is over 90% of the average inflow."<<endl
<< "Returning to main menu ";
}
else
{ const int simulations = 10;
int water_level = 0;
int years = 1;
cout << "Running simulation..." << endl;
for (int i = 1; i <= simulations; i++)
{
int x = (rand()% (max-min + 1)) + min;
while (water_level < capacity)
{
//double r = rand() * 1.0 / RAND_MAX;
//double x = min + inflow_range * r;
//int x = (rand()% (max-min + 1)) + min;
if (water_level + x > required)
{
water_level = water_level + x - required;
}
else
{
water_level= 0;
}
years++;
}
cout <<"Simulation "<< i <<" took " << years <<" years to finish"<< endl;
}
}
}
system ("pause");
return 0;
}
`
So my main question is I'm running into a wall concerning setting up the for loops underneath "Running simulation" where I need to set up the first for loop to run the internal for loop 10 times, with each of those 10 iterations of the internal for loop coming up with random numbers for the range of acceptable results from the query for a random value. I've been told that the idea is to use the Monte Carlo method, i.e. I put in here both the Monte Carlo method and the normal random number generating method. Here it is:
for (int i = 1; i <= simulations; i++)
{
int x = (rand()% (max-min + 1)) + min;
while (water_level < capacity)
{
//double r = rand() * 1.0 / RAND_MAX;
//double x = min + inflow_range * r;
//int x = (rand()% (max-min + 1)) + min;
so the program will create a random value for the inflow. The idea is that the internal for loop will continue to run until the fill_level of the reservoir, which starts at 0, hits the capacity. The process of simulating how many years (each iteration of the internal for loop representing a year) is to be repeated 10 times by the parent for loop of the water_level simulation for loop.
The problem is that the random number that is supposed to created are the same number. THey are different every time I run it, but they are the same every time the loops repeat to make a new simulation. I have tried to figure out what the problem is for hours and still stuck. Any help is very appreciated.

Answer 1

The x is random in your code, the problem is the algorithm and calculation after that. See your code live.
You've forgotten to reset simulation parameter at each iteration, put these inside simulation loop:
--------------------------------------------+
|
for (int i = 1; i <= simulations; i++) |
{ |
int water_level = 0; <--+
int years = 1; <--+
int x = (rand() % (max - min + 1)) + min;
See the code after this edition: live code. The output is
Simulation 1 took 68 years to finish
Simulation 2 took 101 years to finish
Simulation 3 took 8 years to finish

Answer 2

With the code as shown, each iteration (simulation) gets a single value of x for all the years that are simulated. Your commented out code generates a new value of x for each year. Which is the method you want? I'm inclined to think that the inflow varies from year to year, so you should generate a new value of x for each year.
It also looks like you should reset years and water_level for each simulation.
cout << "Running simulation..." << endl;
for (int i = 1; i <= simulations; i++)
{
int water_level = 0;
int years = 1;
while (water_level < capacity)
{
int x = (rand() % (max - min + 1)) + min;
if (water_level + x > required)
water_level += x - required;
else
water_level = 0;
years++;
}
cout <<"Simulation "<< i <<" took " << years <<" years to finish"<< endl;
}
And for debugging, I'd want to print the control parameters (min, max, capacity, required), and then print the key values (year, x, water_level) on each iteration of the inner while loop until I was satisfied it was working correctly.