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+// FILE: pondlife.cxx

+// A simple simulation program to model the fish and weeds in a pond

+

+#include <iostream>    // Provides cin, cout

+#include <iomanip>     // Provides setw

+#include <cstdlib>     // Provides EXIT_SUCCESS, rand, size_t

+#include <vector>      // Provides the vector template class

+#include "organism.h"  // Provides Herbivore, Plant classes

+using namespace std;

+using namespace main_savitch_14;

+

+// PROGRAM CONSTANTS

+const size_t MANY_WEEDS      = 2000; // Number of weeds in the pond

+const double WEED_SIZE       =   15; // Initial size of each weed, in ounces

+const double WEED_RATE       =  2.5; // Growth rate of weeds, in ounces/week

+const size_t INIT_FISH       =  300; // Initial number of fish in the pond

+const double FISH_SIZE       =   50; // Fish size, in ounces

+const double FRACTION        =  0.5; // A fish must eat FRACTION times its size

+                                     // during one week or it will die.

+const int    AVERAGE_NIBBLES =   30; // Average number of plants nibbled by

+                                     // a fish over one week.

+const double BIRTH_RATE      = 0.05; // At the end of each week, some fish have

+                                     // babies. The total number of new fish

+                                     // born is the current number of fish times

+                                     // the BIRTH_RATE (rounded down to an

+                                     // integer).

+

+// Samples weed and fish objects to copy into the vectors of the main program:

+const plant SAMPLE_WEED(WEED_SIZE, WEED_RATE);

+const herbivore SAMPLE_FISH(FISH_SIZE, 0, FISH_SIZE * FRACTION);

+

+// PROTOTYPES for the functions used in the program:

+void pond_week(vector<herbivore>& fish, vector<plant>& weeds);

+// Precondition: weeds.size( ) > 0.

+// Postcondition: On average, each fish has nibbled on AVERAGE_NIBBLES plants,

+// and then simulate_week has been activated for each fish and each weed. Any

+// fish that died are removed from the fish bag, and then

+// BIRTH_RATE * fish.size( ) new fish have been added to the fish bag.

+

+double total_mass(const vector<plant>& collection);

+// Postcondition: The return value is the total mass of all the plants in the

+// collection.

+

+    

+int main( )

+{

+    vector<plant> weeds(MANY_WEEDS, SAMPLE_WEED);    // A vector of weeds

+    vector<herbivore> fish(INIT_FISH, SAMPLE_FISH);  // A vector of weeds

+    int many_weeks;   // Number of weeks to simulate

+    int i;            // Loop control variable

+

+    // Get number of weeks, and format the output.

+    cout << "How many weeks shall I simulate? ";

+    cin >> many_weeks;

+    cout.setf(ios::fixed);

+    cout << "Week    Number    Plant Mass" << endl;

+    cout << "       of Fish   (in ounces)" << endl;

+

+    // Simulate the weeks.

+    for (i = 1; i <= many_weeks; i++)

+    {

+        pond_week(fish, weeds);

+        cout << setw(4) << i;

+        cout << setw(10) << fish.size( );

+        cout << setw(14) << setprecision(0) << total_mass(weeds);

+        cout << endl;

+    }

+

+    return EXIT_SUCCESS;

+}

+

+double total_mass(const vector<plant>& collection)

+{

+    double answer;

+    vector<plant>::const_iterator p;

+    answer = 0;

+    for (p = collection.begin( ); p != collection.end( ); ++p)

+        answer += p->get_size( );

+

+    return answer;

+}

+

+void pond_week(vector<herbivore>& fish, vector<plant>& weeds)

+{

+    // Variables for an index and an iterator for the weeds:

+    vector<plant>::iterator      wi;

+    vector<plant>::size_type     weed_index;

+

+    // Variables for an index, an iterator, and counters for the fish:

+    vector<herbivore>::iterator  fi;

+    vector<herbivore>::size_type fish_index;

+    vector<herbivore>::size_type new_fish_population;

+

+    size_t many_iterations;  // How many random nibbles to simulate

+    size_t i;                // Loop counter

+

+    // Have randomly selected fish nibble on randomly selected plants.

+    many_iterations = AVERAGE_NIBBLES * fish.size( );

+    for (i = 0; i < many_iterations; ++i)

+    {

+        fish_index = rand( ) % fish.size( );  // Index of a random fish

+        weed_index = rand( ) % weeds.size( ); // Index of a random weed

+        fish[fish_index].nibble(weeds[weed_index]);

+    }

+

+    // Simulate the weeks for the weeds.

+    for (wi = weeds.begin( ); wi != weeds.end( ); ++wi)

+        wi->simulate_week( );

+

+    // Simulate the weeks for the fish, and count how many died.

+    fi = fish.begin( );

+    while (fi != fish.end( ))

+    {

+        fi->simulate_week( );

+        if (!fi->is_alive( ))

+            fish.erase(fi);

+	else

+	    ++fi;

+    }

+

+    // Calculate the new number of fish, and reset the fish vector to this size. 

+    // If this adds new fish to the vector, then those new fish will be equal to

+    // the SAMPLE_FISH that is used for all our fish:

+    new_fish_population = (1 + BIRTH_RATE) * fish.size( );

+    fish.resize(new_fish_population, SAMPLE_FISH);

+}