help with rand function...

[coryw]

Hi there,

Disclaimer: This is my 3rd day learning PHP, so the following is likely going to sound silly:

Problem: The book I am using to teach myself (PHP5/MySQL for the absolute beginner by Andy Harris) has "challenges" at the end of each chapter, and I am stuck on a simple one...

"Write a 'loaded dice' program that half the time generates the value 1 and half the time generates some other value"

The basic die rolling looks like this:

<?
$roll = rand(1,6);
print "You rolled a $roll";
?>

When the html page containing the code is refreshed, it regenerates with a new random roll number.

My first idea was to make if statements for half of the possible numbers rolled (1-3 or 4-6, for a 6 sided die etc.) and then just have PHP print "You rolled a one" - but that won't really be "half of the time" because, well, it's random...

Am I missing some really simple here? Or is there another function I could add in to help in delivering the "1" value 50% of the time?

TIA for any help.

[teknomage1]

why not write something like this

<?php
if( rand(0,1) ) {
     $roll = rand(2,6);
     print "You rolled a $roll";
  }
  else {
     print "You rolled a 1";
  }
?>

That way you're mathematically garunteed to roll 1 fifty percent of the time, thanks to the first if statement. But taking 1-3 out of a six sided roll is also 50%.

[redhead]

But taking 1-3 out of a six sided roll is also 50%.

Then it wouldn't be an evenly displaying of the numbers 2 - 6 appart from the fact, that 1 is reprecented 50% of the rolls.

[teknomage1]

Originally Posted by redhead

Then it wouldn't be an evenly displaying of the numbers 2 - 6 appart from the fact, that 1 is reprecented 50% of the rolls.

True but the objective is just to display 1 fifty percent of the time and something else the other half of the time.

[coryw]

Thanks for the input!

I'm not exactly sure how to test this...

In theory, when we land on the page, it should show us a one, and when we refresh, it should show another number (between 2 and 6), or, the opposite (first a number between 2-6, followed by a 1).

So, in a successive number of even rolls I would expect to see half of the rolls delivering a 1 (for example over 12 rolls, I should see 6 ones). But its not quite doing that.

The page and code is up at (ignore the banners, it's a free host...)

http://corwhi.tollfreepage.com/test_...loadedDice.php

Am I missing something? Sorry to be obtuse here...

[DJMaze]

rand(0,1) isn't accurate enough.
This is because 50% of the time:
2 rolls = 1x1
4 rolls = 2x1
6 rolls = 3x1

There's no guarantee that if you use rand(0,1) 2 times it returns 1x0 and 1x1
To avoid this you should use a cookie or session.

Inside the cookie/session you count how much times the dice is rolled and how much times it was '1'

like:

PHP Code:

<?php
session_start();
if (!isset($_SESSION['count'])) {
  $_SESSION['count'] = 0;
  $_SESSION['one'] = 0;
}
if ($_SESSION['count']%2 != 0) {
    if ($_SESSION['one']*2 < $_SESSION['count']) {
        $val = 1;
    } else {
        $val = rand(2,6);
    }
} else {
    $val = rand(1,6);
}
++$_SESSION['count'];
if ($val == 1) ++$_SESSION['one'];

echo $val;

This is the most accurate but it does things like: 15411614411513

[coryw]

Thanks DJMaze,

As I am just getting started with PHP (I only know HTML at this point), this is a bit over my head, but I'll hang onto this and try to use it once I have a better understanding of cookie(s)/session(s).

Since the "problem/assignment" is from the beginning (chapter 3: conditions and fucntions) of a "beginners" book, I wonder if there is a more simple solution I am just missing somehow...

[coryw]

Um... I just realized I was equating alternating "1" and "other values" with 1 showing up 50% of the time - which is not the same thing... I was making this more complicated than it needed to be!

I suppose if I really want to test if 1 and other values (between 2-6) are showing up 50 % of the time, I would want to write something that kept running the page over and over, and tracked the results... But that's more than the assignment asked.

D'oh!

Thanks all.

[sde]

yeah i think although djmaze has a good point about the question in that you would have to maintain state in order to guarantee 1 showing up 50% of the time, .. but being that it's a beginner's book, i think that technomage has the right idea.

i would inturprit this problem as "make loaded six sided dice that has a 50% chance of rolling a 1.

and welcome to the site

[DJMaze]

sorry i went a bit to sophisticated coryw

I hope you learn a lot

[coryw]

No problem. It's fun stuff! Hopefully, in a while this book I'm using will seem like child's play.

Thanks to everyone for the input.