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 ?

Answer 1

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.

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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

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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; }
//...