Solved

Best way to pick a random subset from a collection

Posted on 2013-12-07
6
396 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
Free Tool: Port Scanner

Check which ports are open to the outside world. Helps make sure that your firewall rules are working as intended.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

 
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

Free Tool: Subnet Calculator

The subnet calculator helps you design networks by taking an IP address and network mask and returning information such as network, broadcast address, and host range.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

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

Suggested Solutions

Title # Comments Views Activity
Problem to Alipay 10 44
Crystal Reports Licensing Questions 4 36
Delphi Firemonkey: if the Sms contain special characters it won't send it 3 44
What browser will run Java? 7 64
Introduction This article is the second of three articles that explain why and how the Experts Exchange QA Team does test automation for our web site. This article covers the basic installation and configuration of the test automation tools used by…
Java functions are among the best things for programmers to work with as Java sites can be very easy to read and prepare. Java especially simplifies many processes in the coding industry as it helps integrate many forms of technology and different d…
Viewers will learn one way to get user input in Java. Introduce the Scanner object: Declare the variable that stores the user input: An example prompting the user for input: Methods you need to invoke in order to properly get  user input:
In this fifth video of the Xpdf series, we discuss and demonstrate the PDFdetach utility, which is able to list and, more importantly, extract attachments that are embedded in PDF files. It does this via a command line interface, making it suitable …

832 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