I have an atomic integer array of size 10. I am using this array to organize numbers 1-10 sent in by threads. This 1-10 will eventually be able to change to be a range of numbers larger than 10 and the list is to contain the 10 greatest numbers in that range. I can see the numbers going into the loops and recognizing that they are greater than a number currently there. However, there is never more than 2 numbers in the array when it is printed out. I have tried to trace my code in debug mode, however, it looks as if it is working as intended to me. I feel like there may be a simple error to my logic? I am completely sure all values are entering in the function as I have triple checked this. I start at the end of the array which should contain the highest value and then swap downwards once the slot has been determined. I would appreciate the assistance. This is just a simple experiment I am doing in order to grasp the basics before I try to tackle a homework assignment.
Here an example of my code:
public class testing{
static AtomicIntegerArray maxList = new AtomicIntegerArray(10);
final static int n = 10;
static void setMax(int value)
{
for(int i = 9; i >= 0; i--)
{
if(value > maxList.get(i))
{
int temp = maxList.get(i);
maxList.set(i,value);
if(i == 0)
{
maxList.set(i, value);
}
else
{ for(int j = i-1; j > 0; j--)
{
maxList.set(j, temp);
temp = maxList.get(j-1);
}
}
break;
}
}
public static void main(String[] args)
{
for (int i = 0; i < n; i++)
{
setMax(i);
}
}
}
Here is an example of how it is being called:
Brooke, there is a small bug in your 'j' loop. You had saved the state of a variable (temp), however your logic in the j loop lost the state. This new logic preserves the state of the previous element in the list.
Try this:
for (int j = i - 1; j >= 0; j--) {
int t2 = maxList.get(j);
maxList.set(j, temp);
temp = t2;
}
So as part of a task, I was asked to create an array with randomized values within a range, and then to sort it from smallest to biggest (I used a bubble sort), and then to firstly, print the sum of all the elements in the array, then to list them from smallest to biggest.
My issue is that I keep getting the ArrayIndexOutOfBoundsException error, but cannot find where this problem lies.
You can see in the code that I've put in the randomArrays method, a for loop that creates the random values for the array size I declared in the main method, then, underneath the for loop, I've created an if statement that checks to see if an element's value is bigger than the element after it, if it is, it swaps the place of the elements, until they're all sorted into smallest to biggest, and the loop is terminated.
Any help is much appreciated, thank you :)
public class MyArray {
public static void main(String[] args) {
int[] elements = new int[50];
int min = 0;
int max = 50;
randomArrays(elements, max, min);
}
public static void randomArrays(int[] elements, int max, int min) {
int range = max - min; //defines the range of the integers
int temp;
boolean fixed = false;
while (fixed == false) {
fixed = true;
for (int i = 0; i < elements.length; i++) {
elements[i] = min + (int) (Math.random() * range);
while (i < elements.length) {
if (elements[i] > elements[i + 1]) {
//if 8 > 5
temp = elements[i + 1];
//store 5 in temp
elements[i + 1] = elements[i];
//put the 8 in the 5's place
elements[i] = temp;
fixed = false;
}
i++;
}
}
}
}
//System.out.println(elements[i]);
}
My issue is that I keep getting the ArrayIndexOutOfBoundsException
error, but cannot find where this problem lies.
Problem lies in the condition of the for loop. You get ArrayOutOfBounds exception when i=49 and then you try to access i+1 index which doesn't exists.
Change
for (int i = 0; i < elements.length; i++)
to
for (int i = 0; i < elements.length-1; i++)
As you can already see that your code is going out of the arrays limit.
if you look at your code, following is where this is happening
while (i < elements.length) {
Its this while loop part, so if you change it to correctly loop thru the right number of elements, your problem will be resolved..change your while loop code with this one
while (i < elements.length-1) {
I need to implement a function which does a k-way merge sort on an unsorted array or integers.
The function takes in two parameters, an integer K, which is the "way" of the sort and always a power of 2. The second parameter is the array of integers to be sorted, whose length is also a power of 2.
The function is to return an array containing the sorted elements. So far, I know how to implement a regular merge sort. How would I modify this code so that it implements a K-way merge sort? (Note: this function doesn't return the sorted array, I need help with that as well. It also doesn't take in K, since its a regular merge sort)
Below code:
public class MergeSort {
public static void main(String[] args) {
}
public static void mergeSort(int[] inputArray) {
int size = inputArray.length;
if (size < 2)
return;
int mid = size / 2;
int leftSize = mid;
int rightSize = size - mid;
int[] left = new int[leftSize];
int[] right = new int[rightSize];
for (int i = 0; i < mid; i++) {
left[i] = inputArray[i];
}
for (int i = mid; i < size; i++) {
right[i - mid] = inputArray[i];
}
mergeSort(left);
mergeSort(right);
merge(left, right, inputArray);
}
public static void merge(int[] left, int[] right, int[] arr) {
int leftSize = left.length;
int rightSize = right.length;
int i = 0, j = 0, k = 0;
while (i < leftSize && j < rightSize) {
if (left[i] <= right[j]) {
arr[k] = left[i];
i++;
k++;
} else {
arr[k] = right[j];
k++;
j++;
}
}
while (i < leftSize) {
arr[k] = left[i];
k++;
i++;
}
while (j < leftSize) {
arr[k] = right[j];
k++;
j++;
}
}
}
Regular merge sort is two-way sorting. You compare elements from the first and the second halves of array and copy smallest to output array.
For k-way sorting you divide input array into K parts. K indexes point to the first elements of every part. To effectively choose the smallest of them, use priority queue (based on binary heap) and pop the smallest element from the heap top at every step. When you pop element belonging to the m-th part, push the next element from the same part (if it still exists)
Let you have array length 16 and k = 4.
The first recursion level calls 4 mergesorts for arrays copied from indexes 0..3, 4..7, 8..11, 12..15.
The second recursion level gets length 4 array and calls 4 mergesorts for 1-element arrays.
The third recursion level gets length 1 array and immediately returns (such array is sorted).
Now at the second recursion level you merge 4 one-element arrays into one sorted array.
Now at the first recursion level you merge 4 four-element arrays into one sorted array length 16
So the original code is
// An (unsorted) integer list class with a method to add an
// integer to the list and a toString method that returns the contents
// of the list with indices.
//
// ****************************************************************
public class IntList {
private int[] list;
private int numElements = 0;
//-------------------------------------------------------------
// Constructor -- creates an integer list of a given size.
//-------------------------------------------------------------
public IntList(int size) {
list = new int[size];
}
//------------------------------------------------------------
// Adds an integer to the list. If the list is full,
// prints a message and does nothing.
//------------------------------------------------------------
public void add(int value) {
if (numElements == list.length) {
System.out.println("Can't add, list is full");
} else {
list[numElements] = value;
numElements++;
}
}
//-------------------------------------------------------------
// Returns a string containing the elements of the list with their
// indices.
//-------------------------------------------------------------
public String toString() {
String returnString = "";
for (int i = 0; i < numElements; i++) {
returnString += i + ": " + list[i] + "\n";
}
return returnString;
}
}
and
// ***************************************************************
// ListTest.java
//
// A simple test program that creates an IntList, puts some
// ints in it, and prints the list.
//
// ***************************************************************
import java.util.Scanner ;
public class ListTest {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
IntList myList = new IntList(10);
int count = 0;
int num;
while (count < 10) {
System.out.println("Please enter a number, enter 0 to quit:");
num = scan.nextInt();
if (num != 0) {
myList.add(num);
count++;
} else {
break;
}
}
System.out.println(myList);
}
}
I need to change the add method to sort from lowest to highest. This is what I tried doing.
// An (unsorted) integer list class with a method to add an
// integer to the list and a toString method that returns the contents
// of the list with indices.
//
// ****************************************************************
public class IntList {
private int[] list;
private int numElements = 0;
//-------------------------------------------------------------
// Constructor -- creates an integer list of a given size.
//-------------------------------------------------------------
public IntList(int size) {
list = new int[size];
}
//------------------------------------------------------------
// Adds an integer to the list. If the list is full,
// prints a message and does nothing.
//------------------------------------------------------------
public void add(int value) {
if (numElements == list.length) {
System.out.println("Can't add, list is full");
} else {
list[numElements] = value;
numElements++;
for (int i = 0; i < list.length; i++) {
if (list[i] > value) {
for (int j = list.length - 1; j > i; j--) {
list[j] = list[j - 1];
list[i] = value;
break;
}
}
}
for (in i = 0; i < list.length; i++) {
}
}
}
//-------------------------------------------------------------
// Returns a string containing the elements of the list with their
// indices.
//-------------------------------------------------------------
public String toString() {
String returnString = "";
for (int i = 0; i < numElements; i++) {
returnString += i + ": " + list[i] + "\n";
}
return returnString;
}
}
The outcome is very wrong. Any one able to steer me in the right direction? I can sort of see why what I have doesn't work, but I can't see enough to fix it.
So I realize I was not very descriptive here the first time. With the exception of the add method modifications the code was not my doing. My assignment is to only touch the add method to sort the array to print out smallest to largest. This is a beginners class and we do little to no practice my only tools for this are some basic understandings of loops and arrays.
I tried redoing it again and came up with this:
if(list[numElements-1] > value){
for(int i=0; i<numElements; i++){
if(list[i]>value){
for(int j = numElements; j>i; j-- ){
list[j] = list[j-1];
}
list[i] = value;
break;
}
}
numElements++;
}
else
{
list[numElements] = value;
numElements++;
}
my input was:8,6,5,4,3,7,1,2,9,10
the output was: 1,10,1,9,10,1,1,2,9,10
this thing is kicking my butt. I understand I want to check the input number to the array and move all numbers higher than it up one space and enter it behind those so it is sorted on entry, but doing so is proving difficult for me. I apologize if my code on here is hard to follow formatting is a little odd on here for me and time only allows for me to do my best. I think break is not breaking the for loop with i like i thought it would. Maybe that is the problem.
The biggest bug I see is using list.length in your for loop,
for(int i = 0; i <list.length; i++)
you have numElements. Also, I think it's i that needs to stop one before like,
for(int i = 0; i < numElements - 1; i++)
and then
for (int j = numElements; j > i; j--)
There are two lines that have to be moved out of the inner loop:
for (int i = 0; i < list.length; i++) {
if (list[i] > value) {
for (int j = list.length - 1; j > i; j--) {
list[j] = list[j - 1];
// list[i] = value;
// break;
}
list[i] = value;
break:
}
}
In particular, the inner break means that the loop that is supposed to move all larger elements away to make room for the new value only runs once.
You might want to include Java.util.Arrays which has its own sort function:
http://www.tutorialspoint.com/java/util/arrays_sort_int.htm
Then you can do:
public void add(int value) {
if (numElements == list.length) {
System.out.println("Can't add, list is full");
}
else {
list[numElements] = value;
numElements++;
Arrays.sort(list);
//Or: Java.util.Arrays.sort(list);
}
}
As Eliott remarked, you are getting confused between list.length (the capacity of your list) and numElements (the current size of your list). Also, though, you do not need to completely sort the list on each addition if you simply make sure to insert each new element into the correct position in the first place. You can rely on the rest of the list already to be sorted. Here's a simple and fast way to do that:
public void add(int value) {
if (numElements == list.length) {
System.out.println("Can't add, list is full");
} else {
int insertionPoint = Arrays.binarySearch(list, 0, numElements);
if (insertionPoint < 0) {
insertionPoint = -(insertionPoint + 1);
}
System.arrayCopy(list, insertionPoint, list, insertionPoint + 1,
numElements - insertionPoint);
list[insertionPoint] = value;
numElements += 1;
}
}
That will perform better (though you may not care for this assignment), and it is much easier to see what's going on, at least for me.
Here are some hints.
First, numElements indicates how many elements are currently in the list. It's best if you change it only after you have finished adding your item, like the original method did. Otherwise it may confuse you into thinking you have more elements than you really do at the moment.
There is really no need for a nested loop to do proper adding. The logic you should be following is this:
I know everything already in the list is sorted.
If my number is bigger then the biggest number (which is the one indexed by numElements-1, because the list is sorted) then I can just add my number to the next available cell in the array (indexed by numElements) and then update numElements and I'm done.
If not, I need to start from the last element in the array (careful, don't look at the length of the array. The last element is indexed by numElements-1!), going down, and move each number one cell to the right. When I hit a cell that's lower than my number, I stop.
Moving all the high numbers one cell to the right caused one cell to become "empty". This is where I'm going to put my number. Update numElements, and done.
Suppose you want to add the number 7 to this array:
┌─┬──┬──┬─┬─┐
│3│14│92│-│-│
└─┴──┴──┴─┴─┘
⬆︎ Last element
You move everything starting from the last element (92) to the right. You stop at the 3 because it's not bigger than 7.
┌─┬─┬──┬──┬─┐
│3│-│14│92│-│
└─┴─┴──┴──┴─┘
(The second element will probably still contain 14, but you're going to change that in the next step so it doesn't matter. I just put a - there to indicate it's now free for you to enter your number)
┌─┬─┬──┬──┬─┐
│3│7│14│92│-│
└─┴─┴──┴──┴─┘
⬆︎ Updated last element
This requires just one loop, without nesting. Be careful and remember that the array starts from 0, so you have to make sure not to get an ArrayIndexOutOfBoundsException if your number happens to be lower than the lowest one.
One problem I spotted is that: you are trying to insert the newly added number into the array. However your loop:
for (int i = 0; i < list.length; i++) {
if (list[i] > value) {
for (int j = list.length - 1; j > i; j--) {
list[j] = list[j - 1];
list[i] = value;
break;
}
}
}
is always looped through the total length of the array, which is always 10 in your test, rather than the actual length of the array, i.e. how many numbers are actually in the array.
For example, when you add the first element, it still loops through all 10 elements of the array, although the last 9 slots does not have value and are automatically assigned zero.
This caused your if statement always returns true:
if (list[i] > value)
if you have to write the sort algorithm yourself, use one of the commonly used sorting algorithm, which can be found in Wikipedia.
If any one was curious I finally worked it out. Thank you to everyone who replied. This is what i ended up with.
public void add(int value)
{
if(numElements == 0){
list[numElements] = value;
numElements++;
}
else{
list[numElements] = value;
for(int check = 0; check < numElements; check++){
if(list[check] > value){
for(int swap = numElements; swap> check; swap--){
list[swap] = list[swap-1];
}
list[check] = value;
break;
}
}
numElements++;
}
}
so my original is the same but we have to make another class
A Sorted Integer List
File IntList.java contains code for an integer list class. Save it to your project and study it; notice that the only things you can do are create a list of a fixed size and add an element to a list. If the list is already full, a message will be printed. File ListTest.java contains code for a class that creates an IntList, puts some values in it, and prints it. Save this to your folder and compile and run it to see how it works.
Now write a class SortedIntList that extends IntList. SortedIntList should be just like IntList except that its elements should always be in sorted order from smallest to largest. This means that when an element is inserted into a SortedIntList it should be put into its sorted place, not just at the end of the array. To do this you’ll need to do two things when you add a new element:
Walk down the array until you find the place where the new element should go. Since the list is already sorted you can just keep looking at elements until you find one that is at least as big as the one to be inserted.
Move down every element that will go after the new element, that is, everything from the one you stop on to the end. This creates a slot in which you can put the new element. Be careful about the order in which you move them or you’ll overwrite your data!
Now you can insert the new element in the location you originally stopped on.
All of this will go into your add method, which will override the add method for the IntList class. (Be sure to also check to see if you need to expand the array, just as in the IntList add() method.) What other methods, if any, do you need to override?
To test your class, modify ListTest.java so that after it creates and prints the IntList, it creates and prints a SortedIntList containing the same elements (inserted in the same order). When the list is printed, they should come out in sorted order.
I've been working on selection sort and bubble sort using recursion. I've finally come up with two methods, and they worked perfectly fine. But as I took a final look at those, they look like just one one method which is selectionSortRecursive. Could you tell me the difference (or are they the same)?
public static void selectionSortRecursive(Comparable[] list, int n)
{
Comparable temp;
if ( n > 1 ){
for ( int i = 0; i < n - 1; i++ )
{
if(list[i].compareTo(list[i + 1]) > 0){
temp = list[i];
list[i] = list[i + 1];
list[i + 1] = temp;
}
}
selectionSortRecursive(list, n - 1);
}
}
public static void bubbleSortRecursive( Comparable[] list, int n)
{
Comparable tmp;
if (n >1) {
for (int i = 0; i < n - 1; i++)
{
if(list[i+1].compareTo(list[i]) < 0)
{
tmp = list[i];
list[i] = list[i+1];
list[i+1] = tmp;
}
}
bubbleSortRecursive( list, n - 1);
}
}
The only line which is different is
if(list[i].compareTo(list[i + 1]) > 0){
and
if(list[i+1].compareTo(list[i]) < 0)
and provided compareTo is implemented correctly this will do the same thing.
BTW the if(n > 1) check is redundant. And I would move tmp to the most inner scope you can.
Both sorts are bubble sorts. A bubble sort "bubbles" values to the top/right position.
A selection sort selects the lowest/highest value repeatedly, swap in the selected with the position it needs to place it. i.e. the swap would be outside the loop to find the lowest/highest.
It's both bubble sort, one bubbles the element from the top to the bottom, the other one does it the way round. Selection sort is different: it searches the smallest elements of all remaining (unsorted) elements and places it in the next slot, it does not change any other elements. Bubble sort, instead, always compares tow elements and swaps them is the first one is bigger (or smaller) than the second one - which is what you are doing.