Tutorial

Sequential search is the simplest approach. Given a collection you try every element in the collection until you have found the element or until you reach the end of the collection.

Sequential Search is extremely easy to implement.

Create the Java project "de.vogella.algorithms.search.sequential" and a package with the same name.

Create the following program.

				
package de.vogella.algorithms.search.sequential;

public class SequentialSearch {
	public static boolean contains(int[] a, int b){
		for (int i : a) {
			if (i==b){
				return true;
			}
		}
		return false; 
	}
}

			

You can use the following JUnit test to validate your sort method. To learn about JUnit please see JUnit Tutorial .

				
package de.vogella.algorithms.search.sequential;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

import org.junit.Test;

public class SequentialSearchTest {

	@Test
	public void testContains() {
		int[]a = {1, 2, 3, 4, 5, 19, 17, 7};
		assertTrue(SequentialSearch.contains(a, 17));
		assertTrue(SequentialSearch.contains(a, 1));
		assertTrue(SequentialSearch.contains(a, 2));
		assertTrue(SequentialSearch.contains(a, 3));
		assertTrue(SequentialSearch.contains(a, 4));
		assertFalse(SequentialSearch.contains(a, 10));
	}

}

			

See Complexity Analysis for an introduction to the topic.

Sequential search has a average and worst-case runtime of O(N).

Binary search requires that the collection is already sorted. For example by Quicksort or Mergesort. Binary search checks the element in the middle of the collection. If the search element smaller or greater then the found element then a sub-array is defined which is then search again. If the searched element is smaller then the found element then the sub-array is from the start of the array until the found element. If the searched element is larger then the found element then the sub-array is from the found element until the end of the array. Once the searched element is found or the collection is empty then the search is over.

Create the Java project "de.vogella.algorithms.search.binary" and a package with the same name.

Create the following program.

				
package de.vogella.algorithms.search.binary;

public class BinarySearch {
	public static boolean contains(int[] a, int b) {
		if (a.length == 0) {
			return false;
		}
		int low = 0;
		int high = a.length-1;

		while(low <= high ) {
			int middle = (low+high) /2; 
			if (b> a[middle] ){
				low = middle +1;
			} else if (b< a[middle]){
				high = middle -1;
			} else { // The element has been found
				return true; 
			}
		}
		return false;
	}
}

			

You can use the following JUnit test to validate your sort method. To learn about JUnit please see JUnit Tutorial .

				
package de.vogella.algorithms.search.binary;


import org.junit.Test;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;

public class BinarySearchTest {

	@Test
	public void testContains() {
		int[]a = {1, 2, 3, 4, 5, 7, 17,  19 };
//		assertTrue(BinarySearch.contains(a, 17));
		assertTrue(BinarySearch.contains(a, 1));
		assertTrue(BinarySearch.contains(a, 2));
		assertTrue(BinarySearch.contains(a, 3));
		assertTrue(BinarySearch.contains(a, 4));
		assertFalse(BinarySearch.contains(a, 10));
	}

}

			

See Complexity Analysis for an introduction to the topic.

Binary search cuts the search space in each iteration into half and has therefore O(lg N) runtime behavior.

http://www.vogella.de/code/codejava.html Source Code of Examples