awdreg - 1 year ago 79

Java Question

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);

coeffList.add(coeff);

}

}

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`

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

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Answer Source

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.

Instead of

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

you need

```
degree = <something>;
value = <something>;
```

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