Integer division issue.

[Jack816]

On line 87 I'm having an issue with integer division. I keep getting 1 as the result. I tried using static_cast, but that wasn't able to resolve my issue.

#include <iostream>
#include <fstream>
#include <cstdlib>
#include <iomanip>
using namespace std;

const double DEATH_RATE = 0.1;

const double CONVERSION_RATE = 0.00001;

const double GROWTH_RATE = 0.4;

const double WOLF_ABILITY = 0.0005;

const int CAPACITY = 30000;

const int YEARS = 101;

const int TABLE_INCREMENT = 10;

void validate(int[], int[], int, char*[]);
void populate(int[], int[]);
void computeMinimum(int[], int&);
void computeMaximum(int[], int&);
void writeOut(ofstream&, int, int, int, int, int[], int[]);


int main(int argc, char *argv[])
{
    int wolfMax = 0;
    int wolfMin = 0;
    int mooseMax = 0;
    int mooseMin = 0;

    int wolves[101];
    int moose[101];
    moose[0] = 10000;
    wolves[0] = 100;
    ofstream report;
    report.open("report.txt");
    cout << argv[1];

    validate(moose, wolves, argc, argv);
    populate(wolves, moose);
    computeMinimum(wolves, wolfMin);
    computeMaximum(wolves, wolfMax);
    computeMinimum(moose, mooseMin);
    computeMaximum(moose, mooseMax);
    writeOut(report, wolfMin, wolfMax, mooseMin, mooseMax, wolves, moose);

    return 0;
}

void computeMaximum(int data[], int& q)
{
    int comp = data[0];
    for(int k = 1; k < YEARS; k++)
    {
        if(comp < data[k])
        {
            comp = data[k];
            q = k;
        }
    }
}

void computeMinimum(int data[], int& p)
{
    int comp = data[0];
    for(int j = 1; j < YEARS; j++)
    {
        if(comp > data[j])
        {
            comp = data[j];
            p = j;
        }
    }
}

void populate(int wolves[], int moose[])
{


    for(int i = 1; i < YEARS; i++)
    {
        wolves[i] = ((1 - DEATH_RATE + CONVERSION_RATE * moose[i - 1]) * wolves[i -1]);
        moose[i] = ((1 + GROWTH_RATE - (GROWTH_RATE * moose[i - 1])) / (CAPACITY - WOLF_ABILITY * wolves[i - 1]) * moose[i - 1]); // This is where I'm having an issue.
    }

}

void validate(int moose[], int wolves[], int argc, char *argv[])
{
    if(argc == 3)
    {
        wolves[0] = atoi(argv[1]);
        moose[0] = atoi(argv[2]);
    }

}

void writeOut(ofstream& report, int wolfMin, int wolfMax, int mooseMin, int mooseMax, int wolves[], int moose[])
{
    report << "----------" << "Year" << "----------" << "Wolf" << "----------" << "Moose" << endl;
    for(int k = 0; k < YEARS; k += TABLE_INCREMENT)
    {
        report << setw(15) << k << setw(15) << wolves[k] << setw(15) <<moose[k] << endl;
    }
    report << "The minimum population for wolves was " << wolves[wolfMin] << " in year " << wolfMin << endl;
    report << "The maximum population for wolves was " << wolves[wolfMax] << " in year " << wolfMax << endl;
    report << "The minimum population for moose was " << moose[mooseMin] << " in year " << mooseMin << endl;
    report << "The minimum population for wolves was " << moose[mooseMax] << " in year " << mooseMax << endl;
}

[TheEurekaMan]

How did you use static_cast on line 87? Did you encapsulate the numerator or the whole quotient? To better help you, paste (or even better edit), the code above with the static_cast command.

[Jack816]

I believe this is what I tried.

((1 + GROWTH_RATE - static_cast<double>(GROWTH_RATE * moose[i - 1])) / (CAPACITY - WOLF_ABILITY * wolves[i - 1]) * moose[i - 1])

[TheEurekaMan]

I tried to do this:

moose[i] = static_cast<int>((1 + GROWTH_RATE - (GROWTH_RATE * moose[i - 1]))) / (CAPACITY - WOLF_ABILITY * wolves[i - 1]) * moose[i - 1]);

EDIT: For some reason unbeknownst to me, I'm not getting any output when I run your program. Brb while I troubleshoot that.

[Jack816]

I tried putting static_cast<int> in and I'm only getting zeroes now.

[Handy Andy]

Hello Jack816,

I do not think the problem is where to put static_cast<int>, but in the formula for moose it self.

I changed the function populate to this for testing:

void populate(int wolves[], int moose[])
{
	for (int i = 1; i < 5 /*YEARS*/; i++)
	{
		wolves[i] = ((1 - DEATH_RATE + CONVERSION_RATE * moose[i - 1]) * wolves[i - 1]);
		moose[i] = ((1 + GROWTH_RATE - (GROWTH_RATE * moose[i - 1])) / (CAPACITY - WOLF_ABILITY * wolves[i - 1]) * moose[i - 1]); // This is where I'm having an issue.
		cout << "\n formula =  " << ((1 + GROWTH_RATE - (GROWTH_RATE * moose[i - 1])) / (CAPACITY - WOLF_ABILITY * wolves[i - 1]) * moose[i - 1]);
		cout << "\n moose " << i << " = " << moose[i] << endl;
	}
}

I changed the for loop to < 5 just to keep it short. When I run this I notice when i is > 1 the output that I get is zero.

Also to avoid any problems you should initialize the array when you first start

int wolves[101]{ 0 };  // Will initialize each element to 0.
int moose[101]{ 0 };  // Will initialize each element to 0. 

Still have some thinking to do on the formula on line 87 as to what might work better.

Hope that helps,
Andy

[lastchance]

Jack816,

Could you possibly post the mathematical formulae that you are trying to encode in lines 86 and 87?

As a predator-prey population model there are some anomalies. Are you sure that it is bracketed correctly?

A biologically plausible version of lines 86 and 87 might be

        wolves[i] = wolves[i-1] *(1 - DEATH_RATE + CONVERSION_RATE * moose[i - 1] );
        moose[i] = moose[i-1] * ( 1 + GROWTH_RATE - GROWTH_RATE * moose[i-1]/CAPACITY  - WOLF_ABILITY * wolves[i-1] );

This produces a believable amount of moose (meese?).

(Line 112 has a minor typo in)