Infinite loop code

Question

I'm trying to code an Integrate class which will give me the integral of a function using the Trapezoidal rule. For some reason, the code I have never resolves to a value and stays stuck in the "sum +=" step:

public static double Trap(int exponent, int a, int b, int n) {
    double h = (double) (b-a)/n;

    double sum = 0;
    for(int i = a; i <= b; i+=h)
        sum += (Math.pow(i, exponent) + Math.pow(i+1, exponent))/2; //trouble!

    return h * sum;
}

public static void main(String[] args) {
    System.out.println(Trap(3,1,3,10)); //integral of x^3 from 1 to 3, 
    divided into 10 parts
}

Don't try to "bold" code. Use a comment instead.

Hovercraft Full Of Eels
– Hovercraft Full Of Eels

2013-03-26 01:45:36 +00:00
Commented Mar 26, 2013 at 1:45 — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Mar 26, 2013 at 1:45
I could imagine that h is always 0.

8192K
– 8192K

2013-03-26 01:48:22 +00:00
Commented Mar 26, 2013 at 1:48 — 8192K
– 8192K, Commented Mar 26, 2013 at 1:48
Got it. Changed everything to a double

Haque1
– Haque1

2013-03-26 01:52:34 +00:00
Commented Mar 26, 2013 at 1:52 — Haque1
– Haque1, Commented Mar 26, 2013 at 1:52

Chris Gerken · Accepted Answer · 2013-03-26 01:49:05Z

4

You declared i as an int, so when you add a double (1/3) to it, it rounds down and keeps the same value.

Change i to be a double.

In fact, you ought to make everything a double.

answered Mar 26, 2013 at 1:49

Chris Gerken

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1 Comment

ecbrodie Over a year ago

"you ought to make everything a double"...take that with a grain of salt though. There are many cases when you should be using int instead of double.

gerrytan · Accepted Answer · 2013-03-26 01:52:37Z

1

Your code has a problem with type casting, specifically on the loop increment expression i+=h.

In your case, if h is a double with value 0.2, and i is integer, 0.2 will be casted to 0.

To illustrate this, please try run following code example:

public static void main(String[] args) {

    int i = 1;
    i += 0.2;

    System.out.println(i); // will always output 1
}

answered Mar 26, 2013 at 1:52

gerrytan

41.2k12 gold badges88 silver badges99 bronze badges

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