2D Array values frequency

Question

If I have a 2D array that is arranged as follows :

  String X[][] = new String [][] {{"127.0.0.9", "60", "75000","UDP", "Good"},
                              {"127.0.0.8", "75", "75000","TCP", "Bad"},
                      {"127.0.0.9", "75", "70000","UDP", "Good"},
                      {"127.0.0.1", "", "70000","UDP", "Good"},
                      {"127.0.0.1", "75", "75000","TCP", "Bad"}
                                   };

I want to know the frequency of each value .. so I27.0.0.9 gets 2. How can I do a general solution for this ? In Java or any algorithm for any language ?

polygenelubricants · Accepted Answer · 2010-05-15 14:44:12Z

It looks like you need a custom data type to encapsulate each row rather than using a String[][], but to answer your question more directly, you can use a Map<String,Integer> for each column. A HashMap<String,Integer> can be expected to do this in optimal time.

Here's a snippet to demonstrate the idea:

import java.util.*;

public class Frequency {
    static void increment(Map<String,Integer> map, String key) {
        Integer count = map.get(key);
        map.put(key, (count == null ? 0 : count) + 1);
    }
    public static void main(String[] args) {
        String table[][] = new String[][] {
            {"127.0.0.9", "60", "75000","UDP", "Good"},
            {"127.0.0.8", "75", "75000","TCP", "Bad"},
            {"127.0.0.9", "75", "70000","UDP", "Good"},
            {"127.0.0.1", "", "70000","UDP", "Good"},
            {"127.0.0.1", "75", "75000","TCP", "Bad"}
        };
        final int M = table.length;
        final int N = table[0].length;
        List<Map<String,Integer>> maps = new ArrayList<Map<String,Integer>>();
        for (int i = 0; i < N; i++) {
            maps.add(new HashMap<String,Integer>());
        }
        for (String[] row : table) {
            for (int i = 0; i < N; i++) {               
                increment(maps.get(i), row[i]);
            }
        }
        for (Map<String,Integer> map : maps) {
            System.out.println(map);
        }
        System.out.println(maps.get(0).get("127.0.0.9"));
    }
}

This produces the following output: each line is a frequency map for each column:

{127.0.0.9=2, 127.0.0.8=1, 127.0.0.1=2}
{=1, 60=1, 75=3}
{75000=3, 70000=2}
{UDP=3, TCP=2}
{Good=3, Bad=2}
2

Note: if you don't care about mixing values from all columns together, then you only need one Map, instead of a List<Map> one for each column. This would make the design even worse, though. You really should encapsulate each row into a custom type, instead of a having everything mixed as String[][].

For example some of those columns really looks like they should be an enum.

enum Protocol { UDP, TCP; }
enum Condition { Good, Bad; }
//...

vodkhang · Accepted Answer · 2010-05-15 14:36:43Z

1

You can use Map for Java as above question, use Dictionary for C# The general algorithm is that you have a table/ array of key - value. And you can give the datastructure a key, it will find for you the correct value for that key

answered May 15, 2010 at 14:36

vodkhang

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Comments

Kris · Accepted Answer · 2010-05-15 14:46:21Z

1

Don't store everything as Strings. This slows down processing and is not elegant. Why store a port number (which is an int) as a string? or a boolean value "good" "bad" as a string? Create a custom class that have fields with types corresponding to their semantic.

answered May 15, 2010 at 14:46

Kris

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Comments

Winter · Accepted Answer · 2010-05-15 15:57:25Z

// I think this is an example of what Vodkhang is describing

public static void main(String[] args){
  String X[][] = new String [][] {{"127.0.0.9", "60", "75000","UDP", "Good"},
                          {"127.0.0.8", "75", "75000","TCP", "Bad"},
                  {"127.0.0.9", "75", "70000","UDP", "Good"},
                  {"127.0.0.1", "", "70000","UDP", "Good"},
                  {"127.0.0.1", "75", "75000","TCP", "Bad"}
                               };

   final int M = X.length;
   final int N = X[0].length;

   HashMap<Object, ArrayList> map = new HashMap();

   for(int i = 0; i < M; i++){
       for(int j = 0; j < M; j++){
           String s = X[i][j];
           if( map.containsKey(s) ){
               // add the string to the existing array list
               ArrayList al = map.get(s);
               al.add(s);
               map.put(s,al);
           } else {
               // make a new node
               ArrayList al = new ArrayList();
               al.add(s);
               map.put(s, al);
           }
       }
   }

   // now loop through each pair in the map
   // for each arraylist print out the size
   Set<Object> set = map.keySet();
   for(Object s: set){
       ArrayList al = map.get(s);
       System.out.println(s.toString() + " " + al.size() );
   }

}

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