This program works very well with integers, but not doubles. There are no errors, but the program returns -1. Sorry if this is a stupid question, but I am new to programming.
public class binarySearchProject
{
public static int binarySearch(double[] arr, double x, int high, int low)
{
int mid=(high+low)/2;
if(high==low || low==mid || high==mid)
{
return -1;
}
if(arr[mid]>x)
{
return binarySearch(arr, x, high, mid);
}
else if(arr[mid]<x)
{
return binarySearch(arr, x, mid, low);
}
else if(arr[mid]==x)
{
return mid;
}
return -1;
}
public static void main(String args[])
{
double i = 45.3;
double[] a = {-3, 10, 5, 24, 45.3, 10.5};
int size = a.length;
System.out.println(binarySearch(a, i, size, 0));
}
}
You should change the conditions:
if (arr[mid] > x) should be if (arr[mid] < x)
else if (arr[mid] < x) should be else if (arr[mid] > x)
Also note that in order to make this work, the array must be sorted (That's the whole point of binary search), you can use Arrays#sort:
Arrays.sort(a);
I recommend you rename your class so it begins with an upper case (Following Java Naming Conventions).
As #tobias_k pointed out:
For binary search to work, you need to sort the array first.
See Wikipedia for details.
Related
public class binsearch {
public static void main(String args[])
{
int arr[]={2,45,-21,56,23};
int target=45;
int answer=binarysearch(arr, target);
System.out.println(answer);
}
static int binarysearch(int arr[],int target)
{
int start=0;
int end=arr.length-1;
int mid=start+end/2;
while(start<=end)
{
if(target<arr[mid])
{
mid=end-1;
}
else if(target>arr[mid])
{
mid=mid+1;
}
else
{
return mid;
}
}
return -1;
}
}
I have tried running this code multiple times but it just doesnt run. I dont think there is any problem with the logic for binary search in this code. Please do help.Thank you.
Your code runs. It has an infinite loop so it never terminates.
In order for a binary search to work. The array must be sorted in ascending order. So just sort the array before you do the binary search. Below code uses class java.util.Arrays to sort the array but you can sort it anyway you like. Just make sure that the array is sorted before you do the binary search.
Also, the calculation of mid needs to be inside the while loop because it always changes since its value is determined by the values of both start and end and those values are changed inside the while loop.
Note that I changed the name of the class so as to adhere to Java naming conventions. The conventions make it easier for other people to read and understand your code.
import java.util.Arrays;
public class BinSearch {
static int binarysearch(int arr[], int target) {
int start = 0;
int end = arr.length - 1;
while (start <= end) {
int mid = start + ((end - start) / 2);
if (target < arr[mid]) {
end = mid - 1;
}
else if (target > arr[mid]) {
start = mid + 1;
}
else {
return mid;
}
}
return -1;
}
public static void main(String[] args) {
int arr[] = {2, 45, -21, 56, 23};
Arrays.sort(arr);
int target = 45;
int answer = binarysearch(arr, target);
System.out.println(answer);
}
}
The answer is 3 because, after the sort, 45 is the second last element in the [sorted] array because it is the second largest number in the array.
If you want to search without sorting the array then a binary search is not appropriate.
So it's pretty simple to find the max of an array using a for loop or a while loop, but I wanted to try it out with recursion. For some reason, the substring doesn't work - it says "cannot find symbol". Why is this? My strategy is continue subdividing and comparing the two sides until there is only one left which should be the max....am I doing it right? Thanks
public static int max(int[] array) {
if (array.length == 1) {
return array[0];
} else {
int mid = (array.length) / 2;
int leftmax = max(array.substring(0, mid));
int rightmax = max(array.substring(mid, array.length));
if (leftmax > rightmax) {
return leftmax;
} else {
return rightmax;
}
}
}
You are going to want to use Arrays.copyOfRange. Substring isn't going to work on an array.
int[] firstHalf = Arrays.copyOfRange(original, 0, original.length/2);
int[] secondHalf = Arrays.copyOfRange(original, original.length/2, original.length);
I can't comment on your algorithm.
Since array is of type int[] not String, you cannot use substring(). Instead, keep track of which indexes you are searching through. Copying the array each iteration is a waste of both space and time.
int max( int[] array ){ return max( array, 0, array.length - 1 ); }
int max( int[] array, int low, int high )
{
if (low == high) {
return array[low];
}
else {
int mid = (high + low) / 2;
int leftmax = max(array, low, mid );
int rightmax = max(array, mid, high );
if (leftmax > rightmax) {
return leftmax;
}
else {
return rightmax;
}
}
}
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));
}
}
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).
import java.util.Arrays;
public class BinarySearch
{
public static void main(String[] args)
{
int[] whitelist = In.readInts(args[0]);
Arrays.sort(whitelist);
StdOut.println(whitelist);
while(!StdIn.isEmpty())
{
int key = StdIn.readInt();
if (rank(key, whitelist) < 0)
StdOut.println(key);
}
}
public static int rank(int key, int[] a)
{
return rank(key, a, 0, a.length-1);
}
public static int rank(int key, int[] a, int low, int high)
{
if (low > high) return -1;
int mid = low + (high - low)/2;
if (key < a[mid]) return rank(key, a, low, mid - 1);
else if (key > a[mid]) return rank(key, a, mid + 1, high);
else return mid;
}
}
I don't really know your StdIn, In and StdOut classes (these are not in the standard Java distribution).
But the reason for your output is this line:
StdOut.println(whitelist);
This will probably simply call whilelist.toString(), and the toString() method for arrays outputs something like you showed ([I stands for int[], and 1484a8a is something like a memory address, yes.)
If you want to print the contents of the array, use
StdOut.println(Arrays.toString(whitelist))
instead, or use a loop.
I'm trying to implement QuickSort algorithm program in Java, but I'm getting incorrect answer.
public class QuickSort {
public static void main(String[] args){
int arr[]={12,34,22,64,34,33,23,64,33};
int i=0;
int j=arr.length;
while(i<j){
i=quickSort(arr,i,i+1,j-1);
}
for(i=0;i<arr.length;i++)
System.out.print(arr[i]+" ");
}
public static int quickSort(int arr[],int pivot,int i,int j){
if(i>j) {
swap(arr,pivot,j);
return i;
}
while(i<arr.length&&arr[i]<=arr[pivot]) {
i++;
}
while(j>=1&&arr[j]>=arr[pivot]) {
j--;
}
if(i<j)
swap(arr,i,j);
return quickSort(arr,pivot,i,j);
}
public static void swap(int[] arr,int i,int j) {
int temp;
temp=arr[i];
arr[i]=arr[j];
arr[j]=temp;
}
}
The above program giving me the output as: 12 23 22 33 34 33 64 34 64
Could anyone please tell me how can I get my desire result?
The problem is that this is not really how quicksort works. Quicksort is a recursive algorithm that should only be called once from outside of itself. The idea is that at each iteration, you partition the array into two halves - the left half contains all elements less than the pivot, and the right half contains all elements greater than / equal to the pivot. Then you quicksort the two halves, and finally put the pivot in the middle.
If the side that you are quicksorting is less than 3 elements long, you can just swap the two elements or leave them, and that part of the array is done.
But it doesn't look like your code is doing that at all - you are calling Quicksort 6 times from your client, and within the quicksort function you are making at most one swap. So this is not a case where someone is going to be able to look at your code and debug it by telling you to move a swap or something. You need to revisit your logic.
Check out the Wikipedia diagram for a visual example of what is supposed to happen in a single iteration:
http://en.wikipedia.org/wiki/File:Partition_example.svg
There are open source implementations of quicksort in Apache Harmony and Apache Mahout, probably amongst many others. You can read them.
public static int partition(int[] a, int p, int r){
int i=p,j=r,pivot=a[r];
while(i<j){
while(i<r && a[i] <= pivot){
i++;
}
while(j>p && a[j]>pivot){
j--;
}
if(i<j){
swap(a, i, j);
}
}
return j;
}
public static void quickSort(int[] a, int p, int r){
if(p<r){
int q=partition(a, p, r);
if(p==q){
quickSort(a, p+1, r);
}else if(q==r){
quickSort(a, p, r-1);
}else {
quickSort(a, p, q);
quickSort(a, q+1, r);
}
}
}
public static void swap(int[] a, int p1, int p2){
int temp=a[p1];
a[p1]=a[p2];
a[p2]=temp;
}
here is a quicksort algorithm
package drawFramePackage;
import java.awt.geom.AffineTransform;
import java.util.ArrayList;
import java.util.ListIterator;
import java.util.Random;
public class QuicksortAlgorithm {
ArrayList<AffineTransform> affs;
ListIterator<AffineTransform> li;
Integer count, count2;
/**
* #param args
*/
public static void main(String[] args) {
new QuicksortAlgorithm();
}
public QuicksortAlgorithm(){
count = new Integer(0);
count2 = new Integer(1);
affs = new ArrayList<AffineTransform>();
for (int i = 0; i <= 128; i++){
affs.add(new AffineTransform(1, 0, 0, 1, new Random().nextInt(1024), 0));
}
affs = arrangeNumbers(affs);
printNumbers();
}
public ArrayList<AffineTransform> arrangeNumbers(ArrayList<AffineTransform> list){
while (list.size() > 1 && count != list.size() - 1){
if (list.get(count2).getTranslateX() > list.get(count).getTranslateX()){
list.add(count, list.get(count2));
list.remove(count2 + 1);
}
if (count2 == list.size() - 1){
count++;
count2 = count + 1;
}
else{
count2++;
}
}
return list;
}
public void printNumbers(){
li = affs.listIterator();
while (li.hasNext()){
System.out.println(li.next());
}
}
}
also available with description at nathan's computer knowledge with a description
[code]
[/code]
``
Your loop is not working properly. Refer the code which is solve your problem about Quick Sort
static void quickSort (int[] numbers, int low, int high)
{
int i=low;
int j=high;
int temp;
int middle=numbers[(low+high)/2];
while (i<j) {
while (numbers[i]<middle) {
i++;
}
while (numbers[j]>middle) {
j--;
}
if (i<=j) {
temp=numbers[i];
numbers[i]=numbers[j];
numbers[j]=temp;
i++;
j--;
}
}
if (low<j) {
quickSort(numbers, low, j);
}
if (i<high) {
quickSort(numbers, i, high);
}
}
Refer Quick sort.
Please find comprehensive working code for quick sort algorithm implemented in Java here,
http://tech.bragboy.com/2010/01/quick-sort-in-java.html