How can i go about adding the elements of a sorted array which contain a specific string prefix using binary search and those elements as the order they appear in the array to a arraylist..
It is not hard to code but i am having difficulty with binary search. To use the string prefix the String class provides startswith. I just need help to start the binary search
public static <T extends Comparable<T>> ArrayList prefixMatch(T[] list,
String prefix) {
}
Use the binarySearch method from the API.
String[] objString = {"a","b","c"};
System.out.println(Arrays.binarySearch(objString,"c"));
Or if you want to create your own Binary search implementation. Here it is.
/* BinarySearch.java */
public class BinarySearch {
public static final int NOT_FOUND = -1;
public static int search(int[] arr, int searchValue) {
int left = 0;
int right = arr.length - 1;
return binarySearch(arr, searchValue, left, right);
}
private static int binarySearch(int[] arr, int searchValue, int left, int right) {
if (right < left) {
return NOT_FOUND;
}
/*
int mid = mid = (left + right) / 2;
There is a bug in the above line;
Joshua Bloch suggests the following replacement:
*/
int mid = (left + right) >>> 1;
if (searchValue > arr[mid]) {
return binarySearch(arr, searchValue, mid + 1, right);
} else if (searchValue < arr[mid]) {
return binarySearch(arr, searchValue, left, mid - 1);
} else {
return mid;
}
}
}
public class BinarySearchTest {
public static void main(String[] args) {
int[] arr = {1, 5, 2, 7, 9, 5};
Arrays.sort(arr);
System.out.println(BinarySearch.search(arr, 2));
}
}
binarySearch(T[] a, int fromIndex, int toIndex, T key, Comparator<? super T> c) method. You could create a comparator and pass it to compare only the prefixes.I had a similar requirement - given the array of Strings find indexes of each string which starts with a new letter, i.e. for Africa Angela Beach Bamboo Zorro, I needed algorithm which would return [ 0, 2, 4 ]
I found that there's a modification of well known Binary Search algorithm which uses 'deferred equality test' and this has the side effect of finding exactly needed indexes - ones which start the range.
You can read about it in this Wikipedia article (also there's a very simple example based on which I implemented my own).
