CODEWITHSUNDEEP
javagenericsdouble
ruh roh
ruh roh

Reputation:

Java doesn't seem to be comparing Doubles right

I created a Linked List, with insert, search and remove functions. I also created a iterator for it. Now, suppose I do this:

myList<Integer> test = new myList();
test.insert(30);
test.insert(20);
test.insert(10);
myList.iterator it = test.search(20);
if(it.hasNext())
    System.out.println(it.next());

And voila, it works (it prints the value of the element at the node, in this case 20). Now, if I do this:

myList<Double> test = new myList();
test.insert(30.1);
test.insert(20.1);
test.insert(10.1);
myList.iterator it = test.search(20.1);
if(it.hasNext())
    System.out.println(it.next());

It doesn't, because the iterator is pointing to null. Here is the implementation of the search function:

public iterator search(T data)
{
    no<T> temp = first;
    while( (temp != null) && (temp.data != data) )
        temp = temp.next;
    return (new iterator(temp));
}

Here's how I know there's something fishy with the comparisons: If I change part of the above code like this:

while( (temp != null) && (temp.data != data) )
     System.out.println(temp.data + " " + data);
     temp = temp.next;

I can see it printing the numbers in the list. It prints, at one point, "20.1 20.1" (for example). So how can I fix this? The function appears to be right, but it just seems as if Java isn't comparing the numbers correctly.

EDIT: wth, BigDecimal gave me the same kind of problem too.

EDIT 2: equals() worked, didn't realize something else was amiss. Sorry.

Upvotes: 1

Views: 3682

Answers (2)

Ari Ronen
Ari Ronen

Reputation: 2232

Note that using .equals() to compare doubles may lead to errors. Double.equals() uses this as its equality test:

 d1.doubleValue() == d2.doubleValue()

Doubles and floats are approximations of numbers stored in a fixed space in memory.

In order to properly compare floating point numbers, you need to be aware that due to the nature of floats, there will be some error.

see: http://www.google.com/search?q=floating+point+equality

A quick and easy way to compare doubles is to use Math.abs(a-b)<ACCEPTABLE_ERROR where ACCEPTABLE_ERROR could be .000000000001 depending on what exactly your doing. (Note this doesn't handle edge cases such as NaN and INFINITY)

Upvotes: 11

cletus
cletus

Reputation: 625037

You don't want the != operator for this. It comapres references. You want the .equals() method:

public iterator search(T data)
{
    no<T> temp = first;
    while (!data.equals(temp.data)) {
        temp = temp.next;
    }
    return (new iterator(temp));
}

Also, watch out for auto-boxing. You may find that test.search(20.1) boxes 20.1 to a Float not a Double, which will probably break your comparison. Compare the results with test.search(20.1d). If I recall correctly, the expression:

new Float(20.1).equals(new Double(20.1))

is false.

Upvotes: 2

Related Questions