Created
April 16, 2014 12:22
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Java implementation of an algorithm for sampling without replacement by Donald Knuth
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import static java.util.Arrays.asList; | |
import java.util.Arrays; | |
import java.util.Iterator; | |
import java.util.List; | |
import java.util.Random; | |
import com.google.common.collect.AbstractIterator; | |
public class Sample { | |
public static <T> Iterator<T> sOfN(final Integer sampleSize, final Integer populationSize, final Iterator<T> populationItems) { | |
return new AbstractIterator<T>() { | |
Integer n = sampleSize; | |
Integer N = populationSize; | |
Integer t = 0; | |
Integer m = 0; | |
Random random = new Random(); | |
@Override | |
protected T computeNext() { | |
while (m<n) { | |
T next = populationItems.next(); | |
double u = random.nextDouble(); | |
if ((N-t)*u >= n-m) { | |
t++; | |
} else { | |
t++; m++; | |
return next; | |
} | |
} | |
return endOfData(); | |
} | |
}; | |
} | |
public static void main(String[] args) { | |
int[] result = new int[10]; | |
List<Integer> digits = asList(0,1,2,3,4,5,6,7,8,9); | |
for (int i=0; i<100000; i++) { | |
Iterator<Integer> sample = Sample.sOfN(3, 10, digits.iterator()); | |
while(sample.hasNext()) { | |
result[sample.next()]++; | |
} | |
} | |
System.out.println(Arrays.toString(result)); | |
} | |
} |
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