Solved

Best way to pick a random subset from a collection

Posted on 2013-12-07
6
393 Views
Last Modified: 2013-12-07
I have a large ArrayList and I would like to generate a random subset list of four, without repeats.  I was wondering what the best way to do this would be in Java

Thanks in advance!
0
Comment
Question by:kpbarem
6 Comments
 
LVL 16

Expert Comment

by:krakatoa
ID: 39703617
Out of interest - why does it need to be 'random', is there some sort of security angle involved?
0
 
LVL 84

Expert Comment

by:ozo
ID: 39703636
0
 

Author Comment

by:kpbarem
ID: 39703647
ah well the application I am developing is using the google places API, which will return different results based on different criteria.  However, my application only displays a subset of the places generated.  That subset is supposed to change every time the application is loaded.  It may be difficult to explain without going into a lot of details.
0
Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

 
LVL 84

Expert Comment

by:ozo
ID: 39703663
0
 
LVL 37

Accepted Solution

by:
TommySzalapski earned 500 total points
ID: 39703683
If you don't want to mess up the initial array, I would generate a list of indexes and do the first four iterations of the standard shuffle algorithm. Then the first four positions of the index array would be your items.

This is the standard shuffle algorithm that I use (where there are N items in the list)
int temp;
for (int i = 0; i < N; i++)
{
   index = RandBetween(i, N-1);
   //swap them
   temp = array[i];
   array[i] = array[index];
   array[index] = temp;
   swaper(array, i, index);
}

int RandBetween(int Min, int Max)
{
   return Min + (int)(Math.random() * ((Max - Min) + 1))
}

Open in new window


Forgive the c++ syntax. I think this is mostly the same as the way you would do it in Java.

Note that you don't need to go all the way to N. You can just go to 4.
You can see that you might swap item 0 with item 0 etc. This is fine. This algorithm gives equal chance of every possible combination.
0
 

Author Closing Comment

by:kpbarem
ID: 39703973
I implemented something similar to this.  Thanks
0

Featured Post

Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Introduction This article is the last of three articles that explain why and how the Experts Exchange QA Team does test automation for our web site. This article covers our test design approach and then goes through a simple test case example, how …
Computer science students often experience many of the same frustrations when going through their engineering courses. This article presents seven tips I found useful when completing a bachelors and masters degree in computing which I believe may he…
Viewers will learn about if statements in Java and their use The if statement: The condition required to create an if statement: Variations of if statements: An example using if statements:
This theoretical tutorial explains exceptions, reasons for exceptions, different categories of exception and exception hierarchy.

910 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

23 Experts available now in Live!

Get 1:1 Help Now