generating random #'s w/out duplicates

Anon

Hi There!

I am a senior in high school currently trying to learn java on my own by working though an intro book (I am into math and so I figured I might like programming). I have just finished learning about loops, and at the back of that chapter there was a "challenge" question (without an answer!) that I have been wondering about. It says to write a While loop to generate random numbers between 1 and 100 and that STOPS looping after it generates the same number twice. I know how to generate random numbers between 1 and 100, that's no problem, but i have no idea how to code the condition of not generating the same random number twice. The use of a boolean array has been suggested to me but i'm not quite sure how that works. Any help you could provide would be greatly appreciated. Thank you so much.

sarahk

Don't know about the "boolean" bit but I'd have an array of the numbers you have generated.

So
I'd generate a number
check to see if it's in the array
yes - exit, all don
no - keep going.

Any more than that and we'd be cheating you out of the challenge. By the way, I bet there's a dozen ways to solve it! Some good, some awful and the challenge is to find the way that is either faster, lighter on memory, easier to maintain - all of which are valid reasons to code in a particular style. Alot of bosses will forgo speed & efficiency to have maintainable code because they know that by the time the code needs rework you'll be off on the next project (or job) and the new person has to be able to read it easily.

Anon

Ok, I don't really like the boolean array idea, because it can take up a lot of space. In your problem, you're asked to generate numbers from 1 to 100, so it shouldn't be too bad...

So, if you generate the number 34, then set array[33] as true. If you generate 1, then set array[0] as true. And you keep generating. Each time you generate a random number, you check with the boolean array. If that spot is true, then the number has already been generated so exit. Something like this:

// create boolean array of 100 elements
while (num = randomNumber()) {
if (array[num-1] == true) {
break;
} else {
array[num-1] = true;
}
}

So you use the boolean array to keep track of which ones you've generated.

HTH,

Diego

Anon

Thanks so much for taking the time to respond to my question...your idea helps so much...I get it now! Thank you.

Anon

right...but how do you have an array of the numbers you've generated when that number keeps on growing (and arrays have to be declared a definite size)?...perhaps I am misunderstanding. I think I understand a similar way to solve the problem now. Thanks so much for your help!