marielle marielle - 7 months ago 10
Java Question

Take always a different random element in range

I need to select two different integers that belong to a certain range

[min, max]
.

Example:
rds = 3
,
min = 0
,
max = 9
. If the first random element is
r1 = 6
, then
r2
must be different from
6
and
r3
must be different from
r1
and
r2
.

I thought of building an array containing all the numbers belonging to
[min, max]
:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
.

I select a random number between
0
and
9
with return
ThreadLocalRandom.current().nextInt(0, 10)
and then I swap
r1
and the last element of the array.
To find
r2
I apply the random function from
0
to the penultimate position in the array, and so on.

This is the code:

public void takeRandomXposition(int rds, int[] positions) {
int k = positions.length;
int min = 0;
int max = k - 1;
for(int i = 0; i < rds; i++) {
int r = random(positions[min], positions[max - i]);
swapPositions(positions, r, max - i);
}
}

public int random(int min, int max) {
return ThreadLocalRandom.current().nextInt(min, max + 1);
}

public void swapPositions(int[] positions, int index, int last) {
int indexA = positions[index];
int lastA = positions[last];
positions[index] = lastA;
positions[last] = indexA;
}


This method works for the first steps, but not anymore.
The problem is that the elements in the array are moved and mixed, but I try the random number in a range.
I don't know how to explain, I give an example in which the method doesn't work.

positions: [ 0 1 2 3 4 5 6 7 8 9 ]
rds = 3
k = 10
min = 0
max = 9

i = 0
search r in range [0, 9]
r = 5
positions: [ 0 1 2 3 4 9 6 7 8 5 ]

i = 1
search r in range [0, 8]
r = 3
positions: [ 0 1 2 8 4 9 6 7 3 5 ]

i = 2
search r in range [0, 7]
r = 5
positions: [ 0 1 2 8 4 7 6 9 3 5 ]


In addition there is also a problem when
r = 0
:

positions: [ 0 1 2 3 4 5 6 7 8 9 ]
rds = 3
k = 10
min = 0
max = 9

i = 0
search r in range [0, 9]
r = 0
positions: [ 9 1 2 3 4 5 6 7 8 0 ]

i = 1
search r in range [9, 8]
Exception in thread "main" java.lang.IllegalArgumentException: bound must be greater than origin


So, I don't want to find a random number in a range
[min, max]
but a random number in the contents of an array.

Thanks.

Answer

Store your generated random numbers in a list and keep generating until you find one which is not in the list.

LinkedList<Integer> list = new LinkedList<>();
final int min = 0;
final int max = 10;

while(list.size() < max) // Condition to not keep iterating forever
{
    do 
    {
        int random = ThreadLocalRandom.current().nextInt(min, max + 1);

    } while (list.contains(random));

    list.add(random);
}

You can also use an approach with arrays, as you were trying to do

int min = 0;
int max = 10;
int array[] = new int[max + 1];

int filledArray = 0;

while(filledArray < max + 1) // Condition to not keep iterating forever
{
    do 
    {
        int random = ThreadLocalRandom.current().nextInt(min, max + 1);

    } while (array[random] == 1);

    array[random] = 1;
    filledArray++;
}

EDIT: Im not completly sure if this is what you are looking for. What I understood is that you want to find random numbers in a given range without getting twice the same random number

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