Sorting numbers using Compare method

[Angela Truce]

Hello,

I have implemented a Doubly Linked List to sort elements (integer or character) into ascending order. My problem is that it reads 34 as 3, 56 as 5, 78 as 7 and so on. It also behaves the same for letters like AB is A and TY is T...etc.
I have converted the object elements into strings so that i can use the compare method like this:

String data = (String)current.getElement();
int comp = data.compareTo((String)next.getElement());

where the current is the DLNode current node and next is the DLNode next node after current node.

Can anyone figure out why it's doing this?

Thanks in advance.

[Keith Lynn]

Do you do any manipulation of the String in the class? It appears that you are removing the last character.

[Angela Truce]

Hi,
thanks for the reply. No there is no string manipulation, only string the objects such as (String)current.getElement().

[Keith Lynn]

Can you post the code?

[Stan James]

Is this the kind of thing you get?

1 
10
11
2
21
22
3
31
32

If so, that is the proper sort order for strings, but not for numbers. You have to right justify strings that are numbers to make them sort in numeric order. I used zeros to pad - spaces also work but they're a bit harder to see.

01
02
03
10
20
30

So your choices are to right justify strings or convert strings to some numeric type like Integer. One challenge with right justification is making sure your numbers are wide enough for the longest possible value. For positive ints that might be 10 digits. See what this line does:

value = ("0000000000" + value).substring(value.length());

[Edwin Dalorzo]

You could also write your own Comparator to order the Collection.

For instance:

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
 
public class Main {

static List<String> numbers = new ArrayList<String>();

public static void main(String args[]){
 
numbers.add("9");
numbers.add("8");
numbers.add("7");
numbers.add("6");
numbers.add("5");
numbers.add("4");
numbers.add("3");
numbers.add("2");
numbers.add("1");
numbers.add("0");

Collections.sort(numbers,new NumberComparator());
System.out.println(numbers);
}

public static class NumberComparator implements Comparator<String>{
 
public int compare(String arg0, String arg1) {

Integer n1 = Integer.valueOf(arg0.toString());
Integer n2 = Integer.valueOf(arg1.toString());

if(n1.intValue() > n2.intValue())
return 1;

if (n1.intValue() < n2.intValue())
return 2;

return 0;
}

}
 
}

The output is: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Regards,
Edwin Dalorzo

[Angela Truce]

thanks everyone, i padded out my numbers and they are in order now.
thanks!

[Stan James]

The custom comparator is a pretty good idea. I just got tired of typing before I got to it. (Honest!) It has more overhead doing two conversions on every compare, but it's reusable in other future projects is a more "correct" solution.