Java Error 'error: cannot find symbol'

Question

I am in the process of learning Java. Below is the program that I've been trying to compile, but cannot figure out why 'x' in line 38 provides the following error: 'cannot find symbol'. Any help would be greatly appreciated.

import java.util.Scanner;

class metropolis_HW2_7 {
    static int count = 0;

    public static void main(String[] args) {
        double a = 0.;
        double b = Math.PI;
        Scanner sc = new Scanner(System.in);
        while (true) {
            System.out.println(" Number of bins?");
            int nbin = sc.nextInt();
            if (nbin < 1)
                System.exit(0);
            double[] bin = new double[nbin];
            System.out.println(" Number of histories to run?");
            int N = sc.nextInt();
            double dx = (b - a) / nbin;
            for (int i = 0; i < N; i++) {
                if (count == 0) {
                    double squiggle1 = Math.PI * Math.random();
                    double squiggle2 = Math.PI * Math.random();
                    double y_1 = 2 * squiggle1 + Math.sin(squiggle1);
                    double y_2 = 2 * squiggle2 + Math.sin(squiggle2);
                    if (y_2 < y_1) {
                        squiggle1 = squiggle2;
                        double x = squiggle2;
                    } else {
                        squiggle1 = squiggle1;
                        double x = squiggle2 / squiggle1;
                    }
                    count++;
                } else {
                    double squiggle1;
                    double x = Sample(squiggle1);
                }
                int binNumber = (int) ((x - a) / dx);
                bin[binNumber] += 1.;
            }
            double x = a - dx / 2.;
            for (int i = 0; i < nbin; i++) {
                x += dx;
                bin[i] /= N * dx;
                System.out.printf(" Bin %1$5d Sample for x = %2$7.5f is %3$7.5f vs %4$7.5f Ratio (%5$f) \n", i, x, bin[i], PDF(x), bin[i] / PDF(x));
            }
        }
    }

    static double Sample(double squiggle1) {
        double squiggle2 = Math.PI * Math.random();
        double y_1 = 2 * squiggle1 + Math.sin(squiggle1);
        double y_2 = 2 * squiggle2 + Math.sin(squiggle2);
        if (y_2 < y_1) {
            squiggle1 = squiggle2;
            return squiggle2;
        } else {
            squiggle1 = squiggle1;
            return squiggle2 / squiggle1;
        }
        count++;
    }

    static double PDF(double x) {
        return (2 * x + Math.sin(x)) / (Math.pow(Math.PI, 2) + 2);
    }
}

Crossposted at coderanch.com/t/645832/java/java/…

maneesh
– maneesh

2015-02-09 06:17:16 +00:00
Commented Feb 9, 2015 at 6:17 — maneesh
– maneesh, Commented Feb 9, 2015 at 6:17

Stack Exchange Broke The Law · Accepted Answer · 2015-02-09 05:09:01Z

2

Variables only exist inside the scope (between { and }) they're declared in. You have three different variables called x, and none of them exist when the line int binNumber=(int)((x-a)/dx); is executed.

Declare a variable outside the if statements, and then assign it inside it, something like this: (I've removed most of your code to make this example clearer; obvious you still need it)

double x;
if (count==0) {
    if (y_2<y_1) {
        x=squiggle2;
    } else {
        x=squiggle2/squiggle1;
    }
} else {
    x=Sample(squiggle1);
}
int binNumber=(int)((x-a)/dx);

answered Feb 9, 2015 at 5:09

Stack Exchange Broke The Law

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Comments

manikandan · Accepted Answer · 2015-02-09 05:22:14Z

Declare double x variable globally.You are declared in else part thats why it could not find the variable.

Scope variable Example:

    int a = 80;  // Create a global variable "a"

void setup() {
  size(640, 360);
  background(0);
  stroke(255);
  noLoop();
}

void draw() {
  // Draw a line using the global variable "a"
  line(a, 0, a, height);

  // Create a new variable "a" local to the for() statement 
  for (int a = 120; a < 200; a += 2) {
    line(a, 0, a, height);
  }

  // Create a new variable "a" local to the draw() function
  int a = 300;
  // Draw a line using the new local variable "a"
  line(a, 0, a, height);  

  // Make a call to the custom function drawAnotherLine()
  drawAnotherLine();

  // Make a call to the custom function setYetAnotherLine()
  drawYetAnotherLine();
}

void drawAnotherLine() {
  // Create a new variable "a" local to this method
  int a = 320;
  // Draw a line using the local variable "a"
  line(a, 0, a, height);
}

void drawYetAnotherLine() {
  // Because no new local variable "a" is set, 
  // this line draws using the original global
  // variable "a", which is set to the value 80.
  line(a+2, 0, a+2, height);
}

Kon · Accepted Answer · 2015-02-09 05:07:43Z

0

The variable x is not declared in the scope in which it is used at that line. You are defining and assigning the double x inside two different if-blocks. Try declaring the variable in a broader scope (say, before the if-block, then assign it locally. Then it will be accessible in all 3 places.

Here's a simple example to explain what I mean:

void method()
{
    if (2 > 1)
        double x = 2;
    else
        double x = 3;

     System.out.println(x); //ERROR, because x is out of scope
}

So change it to

void method()
{
    double x = 0;
    if (2 > 1)
        x = 2;
    else
        x = 3;

     System.out.println(x); //No error; x is referenced in the same scope in which it is declared
}

answered Feb 9, 2015 at 5:07

Kon

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