array with non repeating numbers from a range in ascending order, java

Question

Im trying to generate an array with 1000 integers of non-repeating numbers in ascending order from 0 to 10,000

So far what I have is:

 public static void InitArray(int[] arr) { // InitArray method
     int i, a_num; // int declared
     Random my_rand_obj = new Random(); // random numbers

     for (i = 0; i <= arr.length-1; i++) // for loop
     {
        a_num = my_rand_obj.nextInt(10000); // acquiring random numbers from 0 - 10000
            arr[i] = a_num; // numbers being put into array (previoulsy declared of size 1000)
     }

  }
public static void ShowArray(int[] arr) { // ShowArray method
     int i; // int declared
     for (i = 0; i <= arr.length-1; i++) { // for loop
        System.out.print(arr[i] + " "); // show current array content
     }
     System.out.println(); // empty line
  }
public static void Sort(int[] arr) { // SortArray method
     int i, min, j; // int decalred
     for (i = 0; i < arr.length-1; i++) { // for loop
        min = i; // min is i
        for (j = i + 1; j < arr.length; j++) { // nested for loop
           if (arr[j] < arr[min]) { // if statement
              min = j; // j is the new minimum
           }
        }       
        int swap = arr[min]; // swap "method"
        arr[min] = arr[i];
        arr[i] = swap;
     }  
  }

Is there any way to check the numbers are not repeating? Is there a function besides the random generator that will let me generate numbers without repeating? Thanks for any help

You can generate numbers simply by iterating over a for loop using the index - it would meet your requirements as stated. — Scary Wombat
– Scary Wombat, Commented Mar 31, 2014 at 7:04
If you want to be sure that there are non-repeating elements, convert your array to a HashSet, sort it and check its size vs the array's size.. (there are several other "efficient" methods as well.). BTW you are re-inventing the wheel .. Set allows only unique values, why not use it instead of array? — TheLostMind
– TheLostMind, Commented Mar 31, 2014 at 7:05
How to generate random unique numbers? - This should be a decent read and probably you can then convert the Set/List to an array and sort it using Arrays.sort(). — Rahul
– Rahul, Commented Mar 31, 2014 at 7:08

john Smith · Accepted Answer · 2014-03-31 07:10:18Z

1

You can declare array of size 10,000 and init the array in away that each cell in the array will holds the value of it's index:

int [] arr= new  int[10000];
for (int i=0 i < arr.length; i++){
     arr[i] = i
}

Now you can shuffle the array using java Collections. and take the first 1000 items from the array and sort then using java sort.

answered Mar 31, 2014 at 7:10

john Smith

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2 Comments

Hungry Blue Dev Over a year ago

OP needs random nos. from 0-10,000 alright, but OP needs an array of size only 1,000.

john Smith Over a year ago

That is way he should take the first 1000 items as I mentions in the answer.

Martin Prikryl · Accepted Answer · 2014-03-31 07:19:02Z

1

This will do I believe..

HashSet hs = new HashSet();
for(int i=0;i< arr.length;i++)
    hs.add(arr[i]);
List<Integer> integers=new ArrayList<>(hs);
Collections.sort(integers);

edited Mar 31, 2014 at 7:19

Martin Prikryl

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answered Mar 31, 2014 at 7:15

Dileep

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2 Comments

Hungry Blue Dev Over a year ago

Not fair... I was just gonna write that! ;-)

Dileep Over a year ago

@ambigram_maker LOL ;-)

Hungry Blue Dev · Accepted Answer · 2014-03-31 09:08:03Z

A very simple solution is to generate the numbers cleverly. I have a solution. Though it may not have an even distribution, it's as simple as it can get. So, here goes:

public static int[] randomSortedArray (int minLimit, int maxLimit, int size) {
    int range    = (maxLimit - minLimit) / size;
    int[] array = new int[size];
    Random rand = new Random();
    for (int i = 0; i < array.length; i++ ) {
        array[i] = minLimit + rand.nextInt(range) + range * i;
    }
    return array;
}

So, in your case, call the method as:

int randomSortedArray = randomSortedArray(0, 10_000, 1_000);

It's very simple and doesn't require any sorting algorithm. It simply runs a single loop which makes it run in linear time (i.e. it is of time complexity = O(1)).

As a result, you get a randomly generated, "pre-sorted" int[] (int array) in unbelievable time!

Post a comment if you need an explanation of the algorithm (though it's fairly simple).

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