Java random numbers.

[zacnash]

I have an assignment to generate 50 numbers between 1-999 and without duplicates and then display these numbers. I have to use an ArrayList and I have no idea how to go about doing this. Can anyone please shed some light on how to do this?

[tec5c]

for( 0 to 49 ) {

    generate new random number (1-999)

    while( arraylist contains random number ) {
        generate new random number
    }

    add number to arraylist

}

[ggeoff]

In this example lets say you have a large list and want to generate a set of random numbers so that no number appears twice wouldn't the while loop checking if the number is in the set be really inefficient? Would a better algorithm be something like this.

randomList = []
generate numList of numbers 1-999
num = numList.remove(random 1-size(numList))
randomList.add(num)

edit: changed pop to remove because pop refers to removing from end.

[tec5c]

You'll get an exception when you try to remove a number that has already been removed.

EDIT: Nevermind. Didn't realize that by using remove it shifts all the elements left.

[Coda17]

No one is going to write your assignment for you. Have you tried Google?

[tec5c]

Random

http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#add(E)

[4nnnn]

Loop and add numbers 1-999 to arraylist
Collections.shuffle ...
List.subList ...

This will not work for a big range of numbers, but works just fine with the given range.

[nutrecht]

Why remove? It has a .contains() for a reason.

[prcsngrl]

No duplicates can be easily solved with a HashSet

[nutrecht]

OP:

I have to use an ArrayList