Develop Reference

After counting sort the result array has one more element (0) than the original one

So I have a problem, this method is supposed to sort an array of integers by using counting sort. The problem is that the resulting array has one extra element, zero. If the original array had a zero element (or several) it's fine, but if the original array didn't have any zero elements the result starts from zero anyway.
e.g. int input[] = { 2, 1, 4 }; result -> Sorted Array : [0, 1, 2, 4]
Why would this be happening?
public class CauntingSort {
public static int max(int[] A)
{
int maxValue = A[0];
for(int i = 0; i < A.length; i++)
if(maxValue < A[i])
maxValue = A[i];
return maxValue;
}
public static int[] createCountersArray(int[] A)
{
int maxValue = max(A) + 1;
int[] Result = new int[A.length + 1];
int[] Count = new int[maxValue];
for (int i = 0; i < A.length; i++) {
int x = Count[A[i]];
x++;
Count[A[i]] = x;
}
for (int i = 1; i < Count.length; i++) {
Count[i] = Count[i] + Count[i - 1];
}
for (int i = A.length -1; i >= 0; i--) {
int x = Count[A[i]];
Result[x] = A[i];
x--;
Count[A[i]] = x;
}
return Result;
}
}

You are using int[] Result = new int[A.length + 1]; which makes the array one position larger. But if you avoid it, you'll have an IndexOutOfBounds exception because you're supposed to do x-- before using x to access the array, so your code should change to something like:
public static int[] createCountersArray(int[] A)
{
int maxValue = max(A) + 1;
int[] Result = new int[A.length];
int[] Count = new int[maxValue];
for (int i = 0; i < A.length; i++) {
int x = Count[A[i]];
x++;
Count[A[i]] = x;
}
for (int i = 1; i < Count.length; i++) {
Count[i] = Count[i] + Count[i - 1];
}
for (int i = A.length -1; i >= 0; i--) {
int x = Count[A[i]];
x--;
Result[x] = A[i];
Count[A[i]] = x;
}
return Result;
}
Here you go: tio.run

int maxValue = max(A) + 1;
Returns the highest value of A + 1, so your new array with new int[maxValue] will be of size = 5;
The array Result is of the lenght A.lenght + 1, that is 4 + 1 = 5;
The first 0 is a predefinied value of int if it is a ? extends Object it would be null.

The leading 0 in your result is the initial value assigned to that element when the array is instantiated. That initial value is never modified because your loop that fills the result writes only to elements that correspond to a positive number of cumulative counts.
For example, consider sorting a one-element array. The Count for that element will be 1, so you will write the element's value at index 1 of the result array, leaving index 0 untouched.
Basically, then, this is an off-by-one error. You could fix it by changing
Result[x] = A[i];
to
Result[x - 1] = A[i];
HOWEVER, part of the problem here is that the buggy part of the routine is difficult to follow or analyze (for a human). No doubt it is comparatively efficient; nevertheless, fast, broken code is not better than slow, working code. Here's an alternative that is easier to reason about:
int nextResult = 0;
for (int i = 0; i < Count.length; i++) {
for (int j = 0; j < Count[i]; j++) {
Result[nextResult] = i;
nextResult++;
}
}
Of course, you'll also want to avoid declaring the Result array larger than array A.

Delete max element in array

Wrote some code to try to find the maximum element in an un-ordered array and then delete it from the array. My first loop has the logic to find the maximum element while the second loop should take in the variable from the first loop, look ahead one space and then insert into the max element.
My below code seems to work for my second idea .. but does not find the right max array element.
My array has the following values {6, 3, 5, 2, 7, 9, 4}. It is finding the max array element to be 7 .. it should be 9.
public void deleteMax() {
// set value for comparison starting from the beginning of the array
int arrayMax = arr[0];
for (int i = 0; i < nElems; i++) {
if (arr[i] > arrayMax) {
arrayMax = arr[i];
for (int k = i; k < nElems; k++) {
arr[k] = arr[k + 1];
nElems--;
break;
}
}
}
}

why not use jQuery $.grep & Math.max like:
var arr = [6, 3, 5, 2, 7, 9, 4];
var maxNum = Math.max.apply(null, arr);
var newArr = $.grep( arr, function( n ) {
return n != maxNum;
});
console.log(newArr);
Fiddle
EDIT:
Well didn't realize you're using Java as the question showed in JavaScript section...
in Java, You can find max number in array like
int maxNum = arr.get(0); // get the first number in array
for (int i = 1; i < arr.length; i++) {
if ( arr.get(i) > maxNum) {
maxNum = array.get(i);
}
}
arr.remove(maxNum);

Well,, you don't need any second loop.
Only one loop and two variables called, f.ex. MaxElementPosition and MaxElementValue, which are updated every time inside the loop if the number on this position is greater than the last MaxElementValue, update both value and position.
While the loop you do need the value for comparing. In the end you only need the position.

Your inner loop is irrelevant, you only have a 1D array, so it makes no sense to do any inner iteration.
If you insist on performing no sorting on the array, then you could do something like this:
public void deleteMax() {
// set value for comparison starting from the beginning of the array
int arrayMax = arr[0];
int maxIndex = 0;
for (int i = 0; i < nElems; i++) {
if (arr[i] > arrayMax) {
arrayMax = arr[i];
maxIndex = i;
}
}
arr.remove(maxIndex);
}
Once the for loop finishes, we remove the item at maxIndex

#Alex is on the right track, but there is no remove function that can be called on an array in java. All that you need is the variable maxIndex that he gets, which of course will be the first occurence of this maximum, so if you need to remove every occurence of the maximum, that would be a different issue. So once you use #Alex code:
int arrayMax = arr[0];
int maxIndex = 0;
for (int i = 0; i < nElems; i++) {
if (arr[i] > arrayMax) {
arrayMax = arr[i];
maxIndex = i;
}
}
This would be much easier if instead of an array, you were to use an ArrayList, in which case the code would look like:
int arrayMax = arr.get(0);
int maxIndex = 0;
for (int i = 0; i < nElems; i++) {
if (arr[i] > arrayMax) {
arrayMax = arr[i];
maxIndex = i;
}
}
arr.remove(maxIndex);
Otherwise you would have to create a temp array to remove the value:
int[] temp = new int[arr.length-1];
for(int i = 0, index = 0; index < nElems; i++, index++)
{
if(index == maxIndex) index++;
temp[i] = arr[index];
}
arr = temp;

Inserting number into random array

I need some help inserting the number 8 into an array that gives me random values. The array must be in order. For example if I had an array of (1,5,10,15), I have to insert the number 8 between 5 and 10. I am having a problem on how I can figure our a way to find the index where 8 will be placed because the array is random, it can be anything. Here is my code so far :
public class TrickyInsert {
public static void main(String[] args) {
int[] mysteryArr = generateRandArr();
//print out starting state of mysteryArr:
System.out.print("start:\t");
for ( int a : mysteryArr ) {
System.out.print( a + ", ");
}
System.out.println();
//code starts below
// insert value '8' in the appropriate place in mysteryArr[]
int[] tmp = new int[mysteryArr.length + 1];
int b = mysteryArr.length;
for(int i = 0; i < mysteryArr.length; i++) {
tmp[i] = mysteryArr[i];
}
tmp[b] = 8;
for(int i =b ; i<mysteryArr.length; i++) {
tmp[i+1] = mysteryArr[i];
}
mysteryArr = tmp;
any tips? thanks!

Simply add the number then use Arrays.sort method,
int b = mysteryArr.length;
int[] tmp = new int[b + 1];
for(int i = 0; i < b; i++) {
tmp[i] = mysteryArr[i];
}
tmp[b] = 8;
mysteryArr = Arrays.sort(tmp);

In your example the random array is sorted. If this is the case, just insert 8 and sort again.

Simply copy the array over, add 8, and sort again.
int[] a = generateRandArr();
int[] b = Arrays.copyOf(a, a.length + 1);
b[a.length] = 8;
Arrays.sort(b);

int findPosition(int a, int[] inputArr)
{
for(int i = 0; i < inputArr.length; ++i)
if(inputArr[i] < a)
return i;
return -1;
}
int[] tmpArr = new int[mysteryArr.length + 1];
int a = 8; // or any other number
int x = findPosition(a, mysteryArr);
if(x == -1)
int i = 0;
for(; i < mysteryArr.length; ++i)
tmpArr[i] = mysteryArr[i];
tmpArr[i] = a;
else
for(int i = 0; i < mysteryArr.length + 1; ++i)
if(i < x)
tmpArr[i] = mysteryArr[i];
else if(i == x)
tmpArr = a;
else
tmpArr[i] = mysteryArr[i - 1];

I will suggest using binary search to find the appropriate index. Once you locate the index, you can use
System.arraycopy(Object src, int srcIndex, Obj dest, int destIndex, int length)
to copy the left half to your new array (with length one more than the existing one) and then the new element and finally the right half. This will stop the need to sort the whole array every time you insert an element.
Also, the following portion does not do anything.
for(int i =b ; i<mysteryArr.length; i++) {
tmp[i+1] = mysteryArr[i];
}
since int b = mysteryArr.length;, after setting int i =b ;, i<mysteryArr.length; will be false and hence the line inside this for loop will never execute.

Java reverse array method

I'm trying to create a method that takes in an array and then returns that array in reverse. The code I wrote returns the array in reverse, but, the first two values are now 0. Anyone know what I did wrong?
public static int[] reverse(int[] x)
{
int []d = new int[x.length];
for (int i = 0; i < x.length/2; i++) // for loop, that checks each array slot
{
d[i] = x[i];
x[i] = x[x.length-1-i]; // creates a new array that is in reverse order of the original
x[x.length-1-i] = d[i];
}
return d; // returns the new reversed array
}

You are assigning values from an uninitialized array d to x - that's where the zeroes (default value for an int in Java) are coming from.
IIUC, you're mixing two reversing strategies.
If you're creating a new array, you needn't run over half of the original array, but over all of it:
public static int[] reverse(int[] x) {
int[] d = new int[x.length];
for (int i = 0; i < x.length; i++) {
d[i] = x[x.length - 1 -i];
}
return d;
}
Alternatively, if you want to reverse the array in place, you don't need a temp array, only a single variable (at most - there are also ways to switch two ints without an additional variable, but that's a different question):
public static int[] reverseInPlace(int[] x) {
int tmp;
for (int i = 0; i < x.length / 2; i++) {
tmp = x[i];
x[i] = x[x.length - 1 - i];
x[x.length - 1 - i] = tmp;
}
return x; // for completeness, not really necessary.
}

Here is a short way to do it.
public static int[] reverse(int[] x)
{
int[] d = new int[x.length]; //create new array
for (int i=x.length-1; i >= 0; i--) // revered loop
{
d[(x.length-i-1)]=x[i]; //setting values
}
return d; // returns the new reversed array
}

Its simple mistake; you are coping reversed data in x; and returning d. If you will return x, you will get complete revered data.
d[i] = x[i]; // you are copying first element to some temp value
x[i] = x[x.length-1-i]; // copied last element to first; and respective...
x[x.length-1-i] = d[i]; // copied temp element to first element; and temp elements are nothing but array d
So ultimately you have created revered array inside x and not in d. If you will return x you got your answer. And d which is just half baked; so you get default value of 0 for remainign half array. :)

int[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9};
System.out.println("The original Array: ");
for (int i = 0; i < array.length; i++) {
System.out.print(array[i] + " ");
}
System.out.println();
System.out.println("The Reverse Array is: ");
for (int i = array.length - 1; i >= 0; i--) {
System.out.print(array[i] + " ");
}

How do I shift array elements up one position in java?

I am working on a java assignment where I need to delete an integer element in an array and shift the below elements up on space to keep them in order. The array is currently random integers in descending order. I am not allowed to use array.copy because I will need to collect array usage information as part of the assignment. I have tried a ton of different ways of doing this but cannot seem to get it working.
public static void deletionArray(int anArray[], int positionToDelete) {
for (int j = anArray[positionToDelete] - 1; j < anArray.length; j++) {
System.out.println("j is " + j);
anArray[j] = anArray[j + 1];
}
displayArray(anArray);
}

You're iterating until anArray.length (exclusive), but inside the loop, you're accessing anArray[j + 1], which will thus be equal to anArray[anArray.length] at the last iteration, which will cause an ArrayIndexOutOfBoundsException.
Iterate until anArray.length - 1 (exclusive), and decide what should be stored in the last element of the array instead of its previous value.
You're also starting at anArray[positionToDelete] - 1, instead of starting at positionToDelete.

You have two bugs there.
Since this is an assignment, I won't give a complete answer - just a hint. Your loop definition is wrong. Think about this: what happens on the first and on the last iteration of the loop? Imagine a 5-element array (numbered 0 to 4, as per Java rules), and work out the values of variables over iterations of the loop when you're erasing element number, say, 2.

Use System.arraycopy faster than a loop:
public static void deletionArray( int anArray[], int positionToDelete) {
System.arraycopy(anArray, positionToDelete + 1, anArray,
positionToDelete, anArray.length - positionToDelete - 1);
//anArray[length-1]=0; //you may clear the last element
}

public static int[] deletionArray(int anArray[], int positionToDelete) {
if (anArray.length == 0) {
throw new IlligalArgumentException("Error");
}
int[] ret = new int[anArray.length - 1];
int j = 0;
for (int i = 0; i < anArray.length; ++i) {
if (i != positionToDelete) {
ret[j] = anArray[i];
++j;
}
}
return ret;
}
Why do we reserve a new array?
Because if don't, we would use C\C++-style array: an array and a "used length" of it.
public static int deletionArray(int anArray[], int positionToDelete, int n) {
if (n == 0) {
throw new IlligalArgumentException("Error");
}
for (int i = positionToDelete; i < n - 1; ++i) {
anArray[i] = anArray[i + 1];
}
return n - 1; // the new length
}

How's this ? Please note the comment, I don't think you can delete an element in an array, just replace it with something else, this may be useful : Removing an element from an Array (Java)
Updated with 'JB Nizet' comment :
public class Driver {
public static void main (String args []){
int intArray[] = { 1,3,5,6,7,8};
int updatedArray[] = deletionArray(intArray , 3);
for (int j = 0; j < updatedArray.length; j++) {
System.out.println(updatedArray[j]);
}
}
public static int[] deletionArray(int anArray[], int positionToDelete) {
boolean isTrue = false;
for (int j = positionToDelete; j < anArray.length - 1; j++) {
if(j == positionToDelete || isTrue){
isTrue = true;
anArray[j] = anArray[j + 1];
}
}
anArray[anArray.length-1] = 0; //decide what value this should be or create a new array with just the array elements of length -> anArray.length-2
return anArray;
}
}