Java-Sorting a 2-D array quickly

Question

I have a 2D string array consisting of values like as

{ "Home","0.1256784"  
 "Contact","-0.56789"  
 "Refer","1.36589"  
 "Next","3.678456" }

I have to sort the array based upon the second element(double value) and obtain a result such as like

{"Contact","-0.56789"  
 "Home","0.1256784"  
 "Refer","1.36589"  
 "Next","3.678456" }

I have used some bubble sort code to sort it and it works, but and i have to know how can i make the sorting more efficient than my one in faster manner.I tried some code posted previously for the questions related to mine but i can't get the task done.
My Code:

String tt="",tk="";
for(int i=1;i<myarray.length;i++)
 {  
for(int j=1;j<myarray.length-1;j++)
       {
       if(Double.parseDouble(myarray[i][1])<Double.parseDouble(myarray[j][1]))
            {
                    tk=myarray[i][1];  
                    tt=myarray[i][0];  
                    myarray[i][1]=myarray[j][1];  
                    myarray[i][0]=myarray[j][0];  
                    myarray[j][1]=myarray;  
                    myarray[j][0]=myarray;  


            }
          }
        }

try merge sort it runs in O(n log n) as opposed to O(n^2) like bubble sort: en.wikipedia.org/wiki/Merge_sort — Aly
– Aly, Commented Feb 17, 2011 at 9:13

maerics · Accepted Answer · 2011-02-17 09:29:00Z

1

public class Sort2D {
  public static void main(String args[]) {
    String ss[][] = { 
      {"Home", "0.1256784"},
      {"Contact", "-0.56789"},
      {"Refer", "1.36589"},
      {"Next", "3.678456"}
    };
    Arrays.sort(ss, new Comparator<String[]>() {
      public int compare(String[] s1, String[] s2) {
        double d1 = Double.parseDouble(s1[1]);
        double d2 = Double.parseDouble(s2[1]);
        return Double.compare(d1, d2);
      }
    });
    for (String[] s : ss) {
      System.out.println(s[0] + ": " + s[1]);
    }
  }
}

edited Feb 17, 2011 at 9:29

answered Feb 17, 2011 at 9:18

maerics

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5 Comments

Aly Over a year ago

why are you overriding equals? other than that solution looks good +1

Naveen Over a year ago

Thanks for th e reply.I tried the above code and when i run the code some NullPointerException is thrown.It occurs in the line 'Arrays.sort(ss, new Comparator<String[]>()'

maerics Over a year ago

@Aly - good catch, I removed the "equals" override; I added it because it appears in the Comparator interface but it is not needed for this simple example because Object provides a default implementation.

maerics Over a year ago

@Naveen - this code compiles and runs on its own, you'll have to fit it into your own code structure somehow; I only demonstrated the part you asked about.

Naveen Over a year ago

I got it worked.The error i made was that i started the array from ss[1][1] instaed of ss[0][1].So it reported NullpointerException on reading ss[0][1].Thanks a lot maerics..

Augusto · Accepted Answer · 2011-02-17 09:13:42Z

1

If it's a 2d array you can use Array.sort(String[], Comparator<String[]> comparator) and pass a custom comparator, which compares the 2nd element of the sub array.

answered Feb 17, 2011 at 9:13

Augusto

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Comments

Vinay Lodha · Accepted Answer · 2011-02-17 09:15:30Z

1

You can use Arrays.sortsort(Object[] a, Comparator c) and let java take care of it. You may find this link useful

answered Feb 17, 2011 at 9:15

Vinay Lodha

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Comments

Andreas Dolk · Accepted Answer · 2011-02-17 09:33:35Z

0

Alternative approach: you could copy the data to a TreeMap (in case all double values are unique) and let the map to the sorting:

Map<Double, String> map = new TreeMap<Double, String>();
for (String[] row:myArray) {
   map.put(Double.parseDouble(row[1]), row[1]);

The entrysets iterator now returns the values in ascending sort order.

answered Feb 17, 2011 at 9:33

Andreas Dolk

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