Question

I am trying to create a program that calculates different data about butterfly populations. My main issue is that I keep receiving an error for my potentialPopulation equation, where I square my ratioFactor value. The error is that the value I am squaring “may not have been initialized”. I know I need to set the ratioFactor to a value, but the value will be unknown until the inputs are entered. Also, I am a beginner, so if anyone sees any other errors I would really appreciate any help. Thank you

// This program calculates butterfly population estimates
//   Inputs  : males,   estimated number of male butterflies
//             females, estimated number of female butterflies
//   Outputs : total butterflies, sex ratio, variance
// Written by: Charlie
//   Modified: Oct 26, 2012 by Daniel Kellogg
//

import java.util.Scanner;
import java.text.DecimalFormat;
public class Hwk7 {
    public static void main (String[] args) {
            int males, females;

            int totalButterflies, sexRatio, ratioVariance, genderDifferences, matingPairs, growthFactor, ratioFactor, potentialPopulation, x;

            Scanner stdin = new Scanner(System.in);

            System.out.println("\nButterfly Estimator\n");
            System.out.print("Enter the estimated males population: ");
            males = stdin.nextInt();
            System.out.print("Enter the estimated females population: ");
            females = stdin.nextInt();

            totalButterflies  = males + females;
            sexRatio          = males / females;
            ratioVariance     = males % females;
            genderDifferences = males - females;
            matingPairs       = males * females;
            growthFactor      = (int)(Math.sqrt(matingPairs));

            if (sexRatio != 0){
                    ratioFactor       = growthFactor / sexRatio;

             if (sexRatio == 0){
                   ratioFactor = (int)(Math.sqrt(ratioVariance));
            }
            ratioFactor = x;
            potentialPopulation = x^2;

            System.out.println("\nTotal Butterflies: " + totalButterflies );
            System.out.println("Sex Ratio        : " + sexRatio );
            System.out.println("Variance         : " + ratioVariance );
            System.out.println("Gender Differences: " + genderDifferences );
            System.out.println("Possible Mating Pairs: " + matingPairs );
            DecimalFormat oneDigit = new DecimalFormat("#.000");
            System.out.println("Growth Factor: " + growthFactor + oneDigit.format(growthFactor));
            DecimalFormat twoDigit = new DecimalFormat("#.0");
            System.out.println("Ratio Factor: " + ratioFactor + twoDigit.format(ratioFactor));
            DecimalFormat threeDigit = new DecimalFormat("##0");
            System.out.println("Potential Population: " + potentialPopulation + threeDigit.format(potentialPopulation));
    }
}

Answer 1

The number one best piece of advice you can get about writing code is to make each function do exactly one thing. I’ve refactored your class below to break out your key functionality into their own, particular methods. Then, when you have a problem with each method you can solve the problem in that method.

Note too that in your sexRatio zero divisor case, you are getting the sqrt of a variable (ratioVariance) you never set, or ask for input on. You then immediately reset the ratioFactor to x – a mysterious variable that is also never set.

import java.util.Scanner;
import java.text.DecimalFormat;
public class Hwk7 {
    private Scanner stdin = new Scanner(System.in);//This needs to be used throughout your class

    //Do these defaults make sense?
    private int males = 0;
    private int females = 0;

    private int totalButterflies  = 0;
    private double sexRatio       = 0;
    private int ratioVariance     = 0;
    private int genderDifferences = 0;
    private int matingPairs       = 0;
    private double growthFactor   = 0;
    private int potentialPopulation = 0;


    public static double getInput(String message, int input) {
        System.out.print(message);
        input = stdin.nextInt();
    }

    public static void main (String[] args) {
        Hwk7 hw = new Hwk7();
        hw.run();
    }

    public void run() {
        System.out.println("\nButterfly Estimator\n");

        getInput("Enter the estimated males population: ", males);
        getInput("Enter the estimated females population: ", females);

        calculateResults();
        printResults();
    }

    public void calculateResults() {
        totalButterflies  = males + females;
        sexRatio          = males / females;
        ratioVariance     = males % females;
        genderDifferences = males - females;
        matingPairs       = males * females;
        growthFactor      = (int)(Math.sqrt(matingPairs));
        ratioFactor       = calculateRatioFactor(growthFactor, sexRatio);
        potentialPopulation = x^2;//where are you getting x from!?
    }

    //Note in your original implementation you calculate this and then immediately
    //change it to the value 'x'! This is clearly wrong.
    public static double calculateRatioFactor(int growthFactor, int sexRatio) {
        if (sexRatio == 0) {
            return Math.sqrt(RATIOVARIANCE);//Ratio variance is never set!
        } else {
            return growthFactor / sexRatio;
        }
    }

    public static void printResults(int males, int females) {
       System.out.println("\nTotal Butterflies: " + totalButterflies );
       System.out.println("Sex Ratio        : " + sexRatio );
       System.out.println("Variance         : " + ratioVariance );
       System.out.println("Gender Differences: " + genderDifferences );
       System.out.println("Possible Mating Pairs: " + matingPairs );
       DecimalFormat oneDigit = new DecimalFormat("#.000");
       System.out.println("Growth Factor: " + growthFactor + oneDigit.format(growthFactor));
       DecimalFormat twoDigit = new DecimalFormat("#.0");
       System.out.println("Ratio Factor: " + ratioFactor + twoDigit.format(ratioFactor));
       DecimalFormat threeDigit = new DecimalFormat("##0");
       System.out.println("Potential Population: " + potentialPopulation + threeDigit.format(potentialPopulation));
    }
}