php.net |  support |  documentation |  report a bug |  advanced search |  search howto |  statistics |  random bug |  login

go to bug id or search bugs for

Bug #28982 rand()%4 is cyclical
Submitted: 2004-07-01 08:05 UTC Modified: 2004-07-14 19:21 UTC
From: chsnick at yahoo dot com Assigned:
Status: Not a bug Package: Math related
PHP Version: 5.0.0RC3 OS: Windows XP
Private report: No CVE-ID: None
View Developer Edit
Welcome back! If you're the original bug submitter, here's where you can edit the bug or add additional notes.
If you forgot your password, you can retrieve your password here.
Password:
Status:
Package:
Bug Type:
Summary:
From: chsnick at yahoo dot com
New email:
PHP Version: OS:

 

 [2004-07-01 08:05 UTC] chsnick at yahoo dot com 
Description:
------------
rand()%4 is cyclical.  It proceeds from 0 to 1 to 2 to 3 to 0 to 1, etc.

Reproduce code:
---------------
srand(5);
for($i = 0; $i < 30; $i++)
	{
		$j = rand();
		$k = $j%4;
		echo "$k:$j<br>\n";
		
	}

Actual result:
--------------
2:14426
3:6795
0:22376
1:6017
2:38
3:21607
0:532
1:10941
2:30898
3:20483
0:11136
1:13753
2:28158
3:26975
0:32684
1:29813
2:27658
3:15483
0:27288
1:25329
2:16086
3:9559
0:6212
1:15661
2:29282
3:16371
0:29872
1:32553
2:21166
3:26703

Patches

Pull Requests

History

AllCommentsChangesGit/SVN commitsRelated reports
 [2004-07-14 19:21 UTC] moriyoshi@php.net 
Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

See: http://www.eskimo.com/~scs/C-faq/q13.18.html

The tip is of C language, but PHP calls the rand() 
library function to generate random values, so it 
applies to PHP as well.

Use mt_rand() and you might get better results.
 
PHP Copyright © 2001-2025 The PHP Group
All rights reserved. 		Last updated: Tue Mar 11 06:01:29 2025 UTC