Question

i have a list 1 column and 100 rows each with a number
the number on each row may not be unique
i need to output the unique list of numbers sorted according to their rank , which is less if the number of its repetitions is more. least ranked number i.e 1 is on the top

now here is how i am planning to solve this problem

first i want to define an array of the structure would be like this

struct abc[100]
{
int number
int occurrence = 1;
}

now i want to go through the list for each row and check if the number exist in the array of structure or not. if it does not i want to store the number in a abc[row].number however if the number exists in the array of structure i want to increment the occurrence of that particular record.

at the end i would get a array of structure filled with each unique number and number of time it occurs together as records.

Is this style of programming a good way ? defining structures and moving through them is looking a tedious job to me can you suggest me a better way ? I’m a beginner programmer please feel free to give me any sort of advice

Answer 1

Here is an example that returns a Map of item frequencies. The method is generic and will therefore work for any type of item, not just Integer. I’m returning a SortedMap (TreeMap implementation) and hence the function runs with complexity O(n log n). However, I could have opted for a HashMap implementation, reducing the complexity to O(n) (as insert performance for HashMap is O(1)).

import java.util.*;

public class Main {
  public static void main(String[] args) {
    List<Integer> l = new LinkedList<Integer>();
    l.add(5);
 l.add(10);
 l.add(2);
 l.add(5);
 l.add(20);

 System.err.println(freq(l));
  }

  private static <T> SortedMap<T, Integer> freq(Collection<? extends T> c) {
 SortedMap<T, Integer> ret = new TreeMap<T, Integer>();

 for (T t : c) {
     Integer fq = ret.get(t);
     ret.put(t, fq == null ? 1 : fq + 1);
 }

 return ret;
  }    
}