awdreg - 6 months ago 22
Java Question

# Java - A value resets to 0 unexpectedly

I'm making a derivative calculator that asks the user for the degree of their polynomial and then the coefficients for each term, partly because I'm an inexperienced programmer and can't parse input like

`3x^4/4 + sin(x)`
.

Here's my class.

``````package beta;

import java.math.RoundingMode;
import java.text.DecimalFormat;
import java.util.ArrayList;
import java.util.List;

import javax.swing.JOptionPane;

public class DerivativeCalculator
{
public DerivativeCalculator(String d, String v)
{
int degree = Integer.parseInt(d);

double value = Double.parseDouble(v);

coeffList = new ArrayList<Double>();
for (int i = 0; i <= degree; i++)
{
String console = JOptionPane.showInputDialog("Enter the coefficient of the "
+ "x^" + i + " term.");
Double coeff = Double.parseDouble(console);

}

}
public double calc()
{
double dx = 0.0001;

double x1 = value;
double y1 = 0;
for (int d = degree; d >= 0; d--)
{
y1 += coeffList.get(d) * Math.pow(x1, d);
}

double x2 = x1 + dx;
double y2 = 0;
for (int d = degree; d >= 0; d--)
{
y2 += coeffList.get(d) * Math.pow(x2, d);
}

double slope = (y2 - y1)/ (x2 - x1);

DecimalFormat round = new DecimalFormat("##.##");
round.setRoundingMode(RoundingMode.DOWN);

return Double.valueOf(round.format(slope));
}

public String getEquation()
{
String equation = "";
for (int d = degree; d >= 0; d--)
{
equation = equation + String.valueOf(coeffList.get(d)) + "x^" + String.valueOf(d) + " + ";
}
return equation;
}

public String getValue()
{
return String.valueOf(value);
}

private int degree;
private double value;
private List<Double> coeffList;

}
``````

Now here's my test class.

``````package beta;

import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.JApplet;
import javax.swing.JOptionPane;

public class DerivativeCalculatorTest extends JApplet
{
public void paint(Graphics g)
{
Graphics2D g2 = (Graphics2D)g;

String d = JOptionPane.showInputDialog("Enter the degree of your polynomial: ");
String v = JOptionPane.showInputDialog("Enter the x value "
+ "at which you want to take the derivative");

DerivativeCalculator myDerivCalc = new DerivativeCalculator(d, v);
g2.drawString(String.valueOf(myDerivCalc.calc()), 10, 100);
g2.drawString(myDerivCalc.getEquation(), 10, 40);
g2.drawString(myDerivCalc.getValue(), 10, 70);
}

}
``````

Running this creates an applet window that displays

``````5.0x^0+
0.0
0.0
``````

which is not the correct derivative.

I debugged my program and everything runs as expected until the view switches to my test class and it executes
`g2.drawString(String.valueOf(myDerivCalc.calc()), 10, 100);`

After it executes this line,
`degree`
(the degree of the polynomial) resets to 0 even though the user entered 5. This then messes up all the for loops in my class.

Why does this happen? Any suggestions on fixing this? Thanks

You redefine `degree` and `value` as local variables inside your constructor. They shadow your class variables with the same name.

Do not re-declare them.

``````  int degree = <something>;
``````  degree = <something>;