Re: Math.random()

maya <>
Wed, 05 Mar 2008 14:33:30 -0500
thank you all very much for yr responses.. actually I decided that
instead of displaying the images at random in server (which means every
time user comes back photos would be displayed in a different order, I'm
not sure I want that) I want do do the following:

when photos first come in from memory card they are named thus:

DSC_0001.JPG, DSC_0002.JPG, DSC_0003.JPG, etc...

so instead of dislaying images at random in actual site I want to use
random function when RENAMING images (locally, w/stand-alone class)
before uploading them.. now I have written a class for this, thus:


public class Rename {

     public static void main(String[] args) {

     int posDot;
     String _temp = "";
     String no = "";

     // if no arg passed start naming of imgs @ "1"
     // else start at whatever no. is passed to class...
     if (null == args || args.length < 1) {
       no = "1";
      } else {
        no = args[0];
     int iNo = Integer.parseInt(no);

         File directory = new File(System.getProperty("user.dir"));
         String[] textFileNames = directory.list(new FilenameFilter() {
              public boolean accept(File dir, String name) {
                    return name.endsWith(".JPG");

     for (int j=0; j < textFileNames.length+1; j++) {
     // System.out.print(textFileNames[j] + "\n");

         for (int i = 0; i < textFileNames.length; ++i) {
             File oldFile = new File(textFileNames[i]);
       posDot = textFileNames[i].indexOf(".");
       _temp = textFileNames[i].substring(0,posDot);
     // System.out.print(_temp + "\n");
       File newFile = new File(i+iNo + ".jpg");

(adapted from example I found here:

I would like to use random function to rename imgs at random (i.e., so
instead of 1.jpg, 2.jpg, etc, in same order as orig's ones, I would like
to do, say, name first img a number, 2nd image another number, but at
random.. )
(reason sometimes I'll need to start renaming at @ no. diff from "1" is
that in each dir I have imgs from one mem. card, and thus can have one
img named DSC_0002.JPG in two diff folders b/c they came from diff
cards, so can't just put all imgs from all cards in one folder.. so,
after renaming, will have in 1st folder 1.jpg, etc.. thru, say, 15.jpg,
then in second folder need to start at 16.jpg.. etc.. and if I want to
shuffle among all imgs (not just imgs in one folder) I need to do this..
   I hope this makes sense..)

thank you very much..

Lew wrote:

maya wrote:

oh brother... I think get gist of what you're saying.. will try
it....;) thank you very much.. (boolean array.... hmmm.. not sure if
I've ever even seen code for a boolean array...;)

 package testit;
 import java.util.Random;
 import java.util.Set;
 import java.util.HashSet;

 public class Scrambler
  private final Random rand = new Random();

  public Set <Integer> scrambleEggs( int upper, int count )
    Set <Integer> chosen = new HashSet <Integer> ( count * 4 / 3 + 1 );
    boolean [] selected = new boolean [ upper ];

    while ( count > 0 )
      int indx = rand.nextInt( upper );
      if ( ! selected [ indx ] )
        if ( chosen.add( indx ))
        selected [ indx ] = true;
    return chosen;

Untried, untested. (Pseudo-)Non-deterministic run time.

The bizarre reference to chosen.add()'s return value is a hint that one
doesn't really need the boolean array if one is using a Set, in that Set
guarantees uniqueness of its values with respect to equals(), and add()
tells you if the item was not already in the Set. This could really
help if 'upper' has a large value.

Things get trickier if you want multi-threaded use or a deterministic
number of times through the loop.

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