Trapezoidal Rule implementation in Java - need help

[Alexandros Stefanidis]

Hi!
I have found this code for implementing the trapezoidal rule of numerical integration:

package trap;

/**
 *
 * @author HP
 */
// File:     Trap.java
// Purpose:  Calculate area using trapezoidal rule
//
// Input:    a:  left endpoint of interval
//           b:  right endpoint of interval
//           n:  number of trapezoids
// Output:   estimate of area between the x-axis, x = a, x = b, and
//           the graph of f(x) using n trapezoids
//
// Compile:  javac Trap.java
// Usage:    java Trap
//
// Notes:    f(x) is hardwired as a private static member.

import static java.lang.Math.cos;
import java.util.Scanner;

public class Trap {

   public static void main(String args[]) {
      double   area;      // Store result in area
      double   a, b;      // Left and right endpoints
      int      n;         // Number of trapezoids
      double   h;         // Trapezoid base width

      Scanner sc = new Scanner(System.in);

      System.out.println("Enter a, b, and n");
      a = sc.nextDouble();
      b = sc.nextDouble();
      n = sc.nextInt();

      h = (b-a)/n;
      area = trap(a, b, n, h);

      System.out.println("With n = " + n + " trapezoids, our estimate");
      System.out.print("of the area from " + a + " to " + b);
      System.out.println(" = " + area);
      
   }  // main

   // Method:       trap
   // Purpose:      Estimate area using the trapezoidal rule
   // Input args:   a: left endpoint
   //               b: right endpoint
   //               n: number of trapezoids
   //               h: stepsize = length of base of trapezoids
   // Return val:   Trapezoidal rule estimate of area between x-axis,
   //               x = a, x = b, and graph of f(x)
   // Note:         Need *static* so trap can be called from main without 
   //               instantiating an object
   static double trap(double a, double b, int n, double h) {
       double area;   // Store result in area 
       double x;
       int i;
   
       area = (f(a) + f(b))/2.0;
       for (i = 1; i <= n-1; i++) {
           x = a + i*h;
           area = area + f(x);
       }
       area = area*h;
   
       return area;
   } //  trap  


   // Method:     f
   // Purpose:    Compute value of function being integrated
   // Input arg:  x
   // Note:       Need *static* so f can be called from trap without 
   //             instantiating an object
   static double f(double x) {
      double return_val;  
   
      return_val = (8+4*cos(x));
      return return_val;
   }  // f

}  // class Trap

The code above was copied from the web page: http://www.cs.usfca.edu/~peter/cs220/code/trap/Trap.java

All I have changed in the code is the function to be integrated -> (8+4cos(x))
and added the

import static java.lang.Math.cos;

line to handle the cos(x) function.

When run I give the values of 0 (for a), 1.57 (for b) and it throws an exception at this point.

Here is the exception:

Enter a, b, and n
0
1.5
Exception in thread "main" java.util.InputMismatchException
	at java.util.Scanner.throwFor(Scanner.java:864)
	at java.util.Scanner.next(Scanner.java:1485)
	at java.util.Scanner.nextDouble(Scanner.java:2413)
	at trap.Trap.main(Trap.java:41)
C:\Users\HP\AppData\Local\NetBeans\Cache\8.1\executor-snippets\run.xml:53: Java returned: 1
BUILD FAILED (total time: 5 seconds)

What can I do for this?
Thanks

[Piet Souris]

hi Alexandros,

to my surprise I got exactly the same error. It has to with the Locale that the Scanner is using. In my Windows settings, I have a dot as decimal separator, but my Locale is 'nl-NL' and that seems to have the comma as separator.

So, use 3,4 instead of 3.4 and you should be fine (hopefully).

double x = Double.parseDouble("3.4");
System.out.println(3.0 / 4); 

have absolutely no problems with a dot.

Unfortunately, I saw no clear way to make the Scanner use my Windows settings. I hope a Scanner specialist can explain how to handle this in a satisfying way.

[Piet Souris]

By the way:
in java 8 you can turn the function into a parameter Function<Double, Double>, so you're not stuck to the current cos-function. For instance:

static double trap(double a, double b, int n, double h, Function<Double, Double> f) {
    double area;   // Store result in area 
    double x;
    int i;

    area = (f.apply(a) + f.apply(b)) / 2.0;
    for (i = 1; i <= n - 1; i++) {
        x = a + i * h;
        area = area + f.apply(x);
    }
    area = area * h;
    return area;
}

And you would call it like this:

area = trap(a, b, n, h, x -> 8 * Math.cos(x));
area = trap(a, b, n, h, x -> Math.exp(-.5 * x * x) / Math.sqrt(2 * Math.PI));

[Alexandros Stefanidis]

Hi Piet,

by changing the seperator from a dot to a comma works
now just fine!
Thanks

[Campbell Ritchie]

If I had written that code, I would make sure to hide it away where nobody could see it. I would be embarrassed to expose it to public gaze.
The exception is caused by your entering the arguments in the wrong order. The message telling you to enter three numbers is confusing at best; you are supposed to enter double double int in that order.

Moving to one of our Java® fora.

Yes, there is a way to get keyboard input reliably via a Scanner object. Start by searching my posts for those containing both Scanner and utility class.