Unique Random Number Generator in Java

[Gunslinger08]

Does anyone know how to generate unique random numbers in Java? I need to generate a list of 10 random number below 50, and with no repeats. I feel like there must be something more elegant than checking each new number against the array, but I can't find it.

 

[kamper]

I don't know of any non-duplicating random number generators, but if I was checking and throwing away duplicates, I'd use a Map instead of an array. That way you get O(1) checking time instead of O(n).

 

[MartyMcFly3]

Nevermind it'd help if i read the whole thing.

 

[n0cmonkey]

There's gotta be a way to access /dev/random through JAVA.

 

[ntdz]

There is no such thing as a true random number generator

 

[Sunner]

Originally posted by: joshsquall
Does anyone know how to generate unique random numbers in Java? I need to generate a list of 10 random number below 50, and with no repeats. I feel like there must be something more elegant than checking each new number against the array, but I can't find it.

Well, the numbers having be unique kinda defies the random part...

 

[IHateMyJob2004]

Originally posted by: joshsquall
Does anyone know how to generate unique random numbers in Java? I need to generate a list of 10 random number below 50, and with no repeats. I feel like there must be something more elegant than checking each new number against the array, but I can't find it.

If you are only doing 50 numbers, just use an array. the time to compare is insignificant.

Oh ... Map. Then use Map.contains(arg)! That would be nice indeed!

 

[mundane]

Just use a Set, and keep adding until set.size() == 10

 

[Avalon]

Hey guys, saw this thread, and my problem was a bit similar, so I didn't want to clutter the forum with another thread.

I'm trying to figure out how to generate random numbers within the range of -100 to 100. As for the size of the array, it is left up to user input. Could be 10, could be 1000. Here is the actual generating part of my code.

for (i = 0; i < nums; i++)
{


array = (int)(numgen.nextDouble() * MAX);
System.out.println(array);
i++;
array = (int)(numgen.nextDouble() * MIN);
System.out.println(array);
}

Where nums is the number of wanted elements for the array, MAX is preset to 100, and MIN is preset to -100. Currently, this will just generate a positive then negative number. I'd like to have it randomly pick a positive/negative number, not alternate. Duplicate numbers are fine. Any tips?

 

[Soccer55]

Originally posted by: Avalon
Where nums is the number of wanted elements for the array, MAX is preset to 100, and MIN is preset to -100. Currently, this will just generate a positive then negative number. I'd like to have it randomly pick a positive/negative number, not alternate. Duplicate numbers are fine. Any tips?

Here's an idea for the positive and negative thing. Why don't you just randomly generate a number and let that decide whether your number will be + or -. It's been a while since I've done any java, so I don't remember the exact syntax or commands, so hopefully you'll get the idea from what I'm writing.

For example:

for (i = 0; i < nums; i++)
{
sign = Math.random();

if( sign >= .5)
{
array = (int)(numgen.nextDouble() * MAX);
System.out.println(array);
i++;
}

else
{
array = (int)(numgen.nextDouble() * MIN);
System.out.println(array);
i++;
}

}

So if the variable sign is < .5, you'll generate a negative number and if it's >= .5, you'll generate a positive number.

-Tom

 

[Armitage]

Originally posted by: joshsquall
Does anyone know how to generate unique random numbers in Java? I need to generate a list of 10 random number below 50, and with no repeats. I feel like there must be something more elegant than checking each new number against the array, but I can't find it.

Shuffle an array of the first 50 integers and take the first 10?
Not particularly elegant either

How about this:
1. Build a list of the first 50 integers - call it x
2. generate a random integer j between 1 and the size of x
3. copy x[j] to your list of random numbers
4. delete x[j] from x decreasing the size of x by one
5. GOTO 2.

 

[Avalon]

Did I mention I love you?

Thanks a bunch, works exactly as needed

 

[Armitage]

Originally posted by: Avalon
Hey guys, saw this thread, and my problem was a bit similar, so I didn't want to clutter the forum with another thread.

I'm trying to figure out how to generate random numbers within the range of -100 to 100. As for the size of the array, it is left up to user input. Could be 10, could be 1000. Here is the actual generating part of my code.

for (i = 0; i < nums; i++)
{


array = (int)(numgen.nextDouble() * MAX);
System.out.println(array);
i++;
array = (int)(numgen.nextDouble() * MIN);
System.out.println(array);
}

Where nums is the number of wanted elements for the array, MAX is preset to 100, and MIN is preset to -100. Currently, this will just generate a positive then negative number. I'd like to have it randomly pick a positive/negative number, not alternate. Duplicate numbers are fine. Any tips?

Why not just generate the random numbers between 0 to 200 and then subtract 100?

 

[notfred]

Is it THAT hard to read the documentation? math.random() is not some super-secret advanced function.

Link

 

[Armitage]

Originally posted by: notfred
Is it THAT hard to read the documentation? math.random() is not some super-secret advanced function.

Link

Ok, so it returns a random double between 0 and 1 ... how does that help with the OPs problem? Generating a set of "random" integers in some range without duplicates.

 

[Avalon]

Originally posted by: notfred
Is it THAT hard to read the documentation? math.random() is not some super-secret advanced function.

Link

Is it THAT hard to use a more helpful tone? I had no knowledge of such a documentation. This is my first time using java.

 

[notfred]

Originally posted by: Avalon

Originally posted by: notfred
Is it THAT hard to read the documentation? math.random() is not some super-secret advanced function.

Link

Is it THAT hard to use a more helpful tone? I had no knowledge of such a documentation. This is my first time using java.

Well, Sun's java documentation pages show up in the first 3 results for nearly any java related search on google, they're not hard to find (go search for "java random" on google).

Regardless, you know about them now.

 

[ahurtt]

Originally posted by: Armitage

Originally posted by: notfred
Is it THAT hard to read the documentation? math.random() is not some super-secret advanced function.

Link

Ok, so it returns a random double between 0 and 1 ... how does that help with the OPs problem? Generating a set of "random" integers in some range without duplicates.

All you have to do is take the resulting random double and multiply it by your upper limit (50 in this case) and assign the result to an int, no?
Add that resulting value to a set (as somebody else mentioned, since these data structures automatically discard or overwrite duplicates) and repeat until the size of the set is 10.

 

[Armitage]

Originally posted by: ahurtt

Originally posted by: Armitage

Originally posted by: notfred
Is it THAT hard to read the documentation? math.random() is not some super-secret advanced function.

Link

Ok, so it returns a random double between 0 and 1 ... how does that help with the OPs problem? Generating a set of "random" integers in some range without duplicates.

All you have to do is take the resulting random double and multiply it by your upper limit (50 in this case) and assign the result to an int, no?
Add that resulting value to a set (as somebody else mentioned, since these data structures automatically discard or overwrite duplicates) and repeat until the size of the set is 10.

Yep, that should work - though depending on the number of unique random numbers you need relative to the range you can pick from, it may not be very efficient. Imagine an extreme case - picking 99 unique random numbers between 1 and 100. After you get 50 of them, you'll be discarding every other pick. You will need roughly 100 picks to get the last number. And internal to the set it will have to do a log(n) operation for every pick which could get expensive for large n. Of course both issues are trivial for the OP's 10 out of 50 problem statement.

Anyway, my point was that notfred's post was the documentation on random() which doesn't seem relavent to the OP's question.

 

[ahurtt]

Originally posted by: Armitage

Originally posted by: ahurtt

Originally posted by: Armitage

Originally posted by: notfred
Is it THAT hard to read the documentation? math.random() is not some super-secret advanced function.

Link

Ok, so it returns a random double between 0 and 1 ... how does that help with the OPs problem? Generating a set of "random" integers in some range without duplicates.

All you have to do is take the resulting random double and multiply it by your upper limit (50 in this case) and assign the result to an int, no?
Add that resulting value to a set (as somebody else mentioned, since these data structures automatically discard or overwrite duplicates) and repeat until the size of the set is 10.

Yep, that should work - though depending on the number of unique random numbers you need relative to the range you can pick from, it may not be very efficient. Imagine an extreme case - picking 99 unique random numbers between 1 and 100. After you get 50 of them, you'll be discarding every other pick. You will need roughly 100 picks to get the last number. And internal to the set it will have to do a log(n) operation for every pick which could get expensive for large n. Of course both issues are trivial for the OP's 10 out of 50 problem statement.

Anyway, my point was that notfred's post was the documentation on random() which doesn't seem relavent to the OP's question.

In the extreme case you mentioned, it would be more efficient to start with a collection of all 100 numbers, choose a random one, and discard it. Nobody would ever do something like pick 99 unique random numbers between 1 and 100.

Here's the code, I did your homework for you OP:

Set randoms = new HashSet();
while (randoms.size() < 10) {
int result = (int) (Math.random() * 49);
randoms.add(new Integer(result));
}


When done, the set randoms will contain 10 random numbers ranging from 0 to 49.

 

[Armitage]

Originally posted by: ahurtt

Originally posted by: Armitage

Originally posted by: ahurtt

Originally posted by: Armitage

Originally posted by: notfred
Is it THAT hard to read the documentation? math.random() is not some super-secret advanced function.

Link

Ok, so it returns a random double between 0 and 1 ... how does that help with the OPs problem? Generating a set of "random" integers in some range without duplicates.

All you have to do is take the resulting random double and multiply it by your upper limit (50 in this case) and assign the result to an int, no?
Add that resulting value to a set (as somebody else mentioned, since these data structures automatically discard or overwrite duplicates) and repeat until the size of the set is 10.

Yep, that should work - though depending on the number of unique random numbers you need relative to the range you can pick from, it may not be very efficient. Imagine an extreme case - picking 99 unique random numbers between 1 and 100. After you get 50 of them, you'll be discarding every other pick. You will need roughly 100 picks to get the last number. And internal to the set it will have to do a log(n) operation for every pick which could get expensive for large n. Of course both issues are trivial for the OP's 10 out of 50 problem statement.

Anyway, my point was that notfred's post was the documentation on random() which doesn't seem relavent to the OP's question.

In the extreme case you mentioned, it would be more efficient to start with a collection of all 100 numbers, choose a random one, and discard it.

Which is the solution I suggested earlier.

Nobody would ever do something like pick 99 unique random numbers between 1 and 100.

maybe not 99 of 100 but it's essentially shuffling a list - which I've certainly used before. In any case, my point is simply to understand where your algorithm may start to implode.

Here's the code, I did your homework for you OP:

Set randoms = new HashSet();
while (randoms.size() < 10) {
int result = (int) (Math.random() * 49);
randoms.add(new Integer(result));
}


When done, the set randoms will contain 10 random numbers ranging from 0 to 49.

 

[ahurtt]

Originally posted by: Armitage

Originally posted by: ahurtt

Originally posted by: Armitage

Originally posted by: ahurtt

Originally posted by: Armitage

Originally posted by: notfred
Is it THAT hard to read the documentation? math.random() is not some super-secret advanced function.

Link

Ok, so it returns a random double between 0 and 1 ... how does that help with the OPs problem? Generating a set of "random" integers in some range without duplicates.

All you have to do is take the resulting random double and multiply it by your upper limit (50 in this case) and assign the result to an int, no?
Add that resulting value to a set (as somebody else mentioned, since these data structures automatically discard or overwrite duplicates) and repeat until the size of the set is 10.

Yep, that should work - though depending on the number of unique random numbers you need relative to the range you can pick from, it may not be very efficient. Imagine an extreme case - picking 99 unique random numbers between 1 and 100. After you get 50 of them, you'll be discarding every other pick. You will need roughly 100 picks to get the last number. And internal to the set it will have to do a log(n) operation for every pick which could get expensive for large n. Of course both issues are trivial for the OP's 10 out of 50 problem statement.

Anyway, my point was that notfred's post was the documentation on random() which doesn't seem relavent to the OP's question.

In the extreme case you mentioned, it would be more efficient to start with a collection of all 100 numbers, choose a random one, and discard it.

Which is the solution I suggested earlier.

Nobody would ever do something like pick 99 unique random numbers between 1 and 100.

maybe not 99 of 100 but it's essentially shuffling a list - which I've certainly used before. In any case, my point is simply to understand where your algorithm may start to implode.

Here's the code, I did your homework for you OP:

Set randoms = new HashSet();
while (randoms.size() < 10) {
int result = (int) (Math.random() * 49);
randoms.add(new Integer(result));
}


When done, the set randoms will contain 10 random numbers ranging from 0 to 49.

Obviously, as you should be able to tell from my previous post, I wouldn't use this algorithm to solve the problem you brought up. But that isn't the problem the OP wanted to solve.

 

[leggomyeggroll]

java does not provide unique random number generator. it's pseudo random. try running the program a second time and you'll have the same results.

ntdz, there are many implementations of true random number generators. one way is the use of quantum mechanics which is thought of to revolve around true randomness

 

[ArmchairAthlete]

http://java.sun.com/j2se/1.3/docs/api/java/rmi/server/UID.html
http://java.sun.com/j2se/1.5.0/docs/api/java/util/UUID.html

.hashcode for an int instead of string.

10 below 50? well you'll have to check if you already generated it then because the odds are fairly high.