QuestionAlternate.java

package Q17_03_Random_Set;

import CtCILibrary.AssortedMethods;

public class QuestionAlternate {

	/* Random number between lower and higher, inclusive */
	public static int rand(int lower, int higher) { 
		return lower + (int)(Math.random() * (higher - lower + 1));
	}
	
	/* pick M elements from original array, using only elements 0 through i (inclusive).*/
	public static int[] pickMRecursively(int[] original, int m, int i) {
		if (i + 1 < m) { // Not enough elements
			return null; 
		} else if (i + 1 == m) { // Base case -- copy first m elements into array
			int[] set = new int[m];
			for (int k = 0; k < m; k++) {
				set[k] = original[k];
			}
			return set;
		} else {
			int[] set = pickMRecursively(original, m, i - 1);
			int k = rand(0, i);
			if (k < m) {
				set[k] = original[i];
			}
			return set;
		}
	}	

	/* pick M elements from original array.*/
	public static int[] pickMIteratively(int[] original, int m) {
		int[] subset = new int[m];
		
		/* Fill in subset array with first part of original array */
		for (int i = 0; i < m ; i++) {
			subset[i] = original[i];
		}
		
		/* Go through rest of original array. */
		for (int i = m; i < original.length; i++) {
			int k = rand(0, i);
			if (k < m) {
				subset[k] = original[i];
			}
		}
		
		return subset;
	}
	
	public static void main(String[] args) {
		int[] cards = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
		System.out.println(AssortedMethods.arrayToString(cards));
		int[] set = pickMIteratively(cards, 4);
		System.out.println(AssortedMethods.arrayToString(set));
	}	

}