Introductory C++ and the Monte Carlo method - c++

Question

So I'm just getting introduced to programming, and I have to say that this is the single most rewarding yet frustrating thing I've ever done. I'm taking on projects of increasing difficulty, the most recent of which involves the use of the Monte Carlo method and plenty of loops. The following is the code that is completed so far:
#include <iostream>
#include <cstdlib>
#include <cmath>
#include <string>
#include <ctime>
using namespace std;
int main ()
{
srand (time(0));
string operation;
cout << "Using the letters 'o', or 'q', please indicate if you would like to simulate once, or quit the program: " << endl;
cin >> operation;
string reservoir_name; // Creating variables for reservoir
double reservoir_capacity;
double outflow;
double inflow_min;
double inflow_max;
if (operation == "q")
{
cout << "Exiting program." << endl;
system ("pause");
return 0;
}
while (operation == "o") // Choose one or multiple simulations.
{
string reservoir_name; // Creating variables for reservoir function
double reservoir_capacity;
double inflow_min = 0;
double inflow_max = 0;
double inflow_range = inflow_min + inflow_max;
double inflow_difference = inflow_max - inflow_min;
double inflow_threshold = .9 * inflow_range/2; // Math for acceptable flow threshold.
cout << "What is the name of the reservoir?" << endl;
cin.ignore ();
getline (cin,reservoir_name); // Grab whole string for reservoir name.
cout << "What is the capacity of the reservoir in MAF (Millions of Acre Feet)?" << endl;
cin >> reservoir_capacity;
cout << "What is the minimum inflow?" << endl;
cin >> inflow_min;
cout << "What is the maximum inflow?" << endl;
cin >> inflow_max;
cout << "What is the required outflow?" << endl;
cin >> outflow;
inflow_range = inflow_min + inflow_max;
inflow_threshold = .9 * inflow_range/2;
cin.ignore ();
if (outflow > inflow_threshold) // Check for unacceptable outflow levels.
{
cout << "Warning! The outflow is over 90% of the average inflow. Simulation aborted. Returning to main menu." << endl;
}
else
{
const int number_simulations = 10;
double fill_level = 0;
int years = 1;
cout << "Running simulation." << endl;
for (int i = 1; i < number_simulations; i++) // Each year
{
for (years; fill_level < reservoir_capacity; years++ )
{
double r = rand() * 1.0 / RAND_MAX;
double x = inflow_min + inflow_range * r;// SHOULD be between minimum inflow and maximum inflow.
if (fill_level < 0)
{
fill_level = 0;
}
} // Simulate the change of water level.
cout << years << endl;
}
}
cout << "What would you like to do now?" << endl; // Saving for later. The menu re-prompt message and code.
cout << "Using the letters 'o', or 'q', please indicate if you would like to simulate once, or quit the program: " << endl;
cin >> operation;
}
system ("pause");
return 0;
}
So I suppose 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.
double r = rand() * 1.0 / RAND_MAX;
double x = inflow_min + inflow_range * r;// SHOULD be between minimum inflow and maximum inflow.
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 reservoir_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 fill_level simulation for loop.
When I try to run the program as you see it here, it will go all the way up until the "Running simulation" and then it won't proceed any further. Does someone more experienced than myself understand what I'm saying and know what is happening?

Answer 1

for (years; fill_level < reservoir_capacity; years++ )
{
double r = rand() * 1.0 / RAND_MAX;
double x = inflow_min + inflow_range * r;// SHOULD be between minimum inflow and maximum inflow.
if (fill_level < 0)
{
fill_level = 0;
}
} // Simulate the change of water level.
You never increase fill_level in this loop. It's an infinite loop.