Very Computer

Hi:

        I am using random()&01 to get a binary random
number. Turns out if I generate more than 50 numbers,
the last 30 or more are all 1s or 0s, i.e., my random
number generator is not random!

        Worse, if I generate 500 numbers, 497 are 1s.
The fraction of consistent numbers increases as the number
of trials increases.

        Any clues? Anybody got a pointer to an FAQ where
I can get a random number generator that is random?

Thanks in advance,

-Badri

Maybe you could call srand(clock()).  That'll randomize the seed, but it
doesn't really sound like that's the problem you're having.  Something to
try...

The FAQ for comp.lang.c has a section on just that.
Basically the standard random number generator is good
at generating random numbers in general, but if you extract
just the low bit the result is, as you found out, less than
random. You may try to look at a different "bit" from
the result.

--
=======================================================

=======================================================

[snip...]

Did you initialize the seed using srand() function? I normally
do
  srand (gettimeofday());
before I actually call random(). Seems to work for me...

K.
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:>Hi:
:>
:>   I am using random()&01 to get a binary random
:>number. Turns out if I generate more than 50 numbers,
:>the last 30 or more are all 1s or 0s, i.e., my random
:>number generator is not random!
:
:[snip...]
:
:Did you initialize the seed using srand() function? I normally
:do
:  srand (gettimeofday());
:before I actually call random(). Seems to work for me...

It must only _seem_ to.  Gettimeofday(), when called correctly, always
returns 0 (at least on the UNIX I'm using).  Read the man page.

Rand() is typically a terrible random number generator, often yielding
alternating even and odd numbers continuously. Rand()&1 doesn't do too
well then.  I suggest investigating the rand48() family of functions.
Do "man -k rand48".  Also look at "Numerical Recipes in C"; it has a
whole chapter (with C source) on generating pseudo-random numbers.
(No, the code is not available for FTP, just on disk when you buy the book.)

Ron
--
A just machine to make big decisions                  | Ron Charlton

We'll be clean when their work is done                 ----------------------
We'll be eternally free yes and eternally young -- Donald Fagen, I.G.Y.

should've been

gettimeofday (tp, tzp);

Use these values to calculate the seed, i.e. make the seed for the random
generator to be a function of the current time and then do

srand (seed);

Thats's what I meant. Never post an article after 8 hours of
no-lunch-virtual-reality-session. *smile*

K.
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<a href="http://www.ucs.usl.edu/~kxg5947/me.html>My Home page</a>|what I was g
I am not my *Sysadmin* I don't do emacs. I am student and I use  |oing to do w
vi. That's what I am and that's what I do - Peter Kaine (??)     |ith this \s

|> >      I am using random()&01 to get a binary random
|> > number. Turns out if I generate more than 50 numbers,
|> > the last 30 or more are all 1s or 0s, i.e., my random
|> > number generator is not random!
|>
|> The FAQ for comp.lang.c has a section on just that.
|> Basically the standard random number generator is good
|> at generating random numbers in general, but if you extract
|> just the low bit the result is, as you found out, less than
|> random. You may try to look at a different "bit" from
|> the result.

Most people replying to the original article seem to think that badri
was using rand when in fact he is using random. The man page for random
says:

                                                The difference is
     that rand(3V) produces a much  less  random  sequence  -  in
     fact,  the  low  dozen  bits  generated by rand go through a
     cyclic pattern.  All the  bits  generated  by  random()  are
     usable.  For example,

          random()&01

     will produce a random binary value.

Are you saying that this statement is fertilizer?

--

Michael Salmon

#include        <standard.disclaimer>
#include        <witty.saying>
#include        <fancy.pseudo.graphics>

Ericsson Telecom AB
Stockholm

: Hi:

:       I am using random()&01 to get a binary random
: number. Turns out if I generate more than 50 numbers,
: the last 30 or more are all 1s or 0s, i.e., my random
: number generator is not random!

:       Worse, if I generate 500 numbers, 497 are 1s.
: The fraction of consistent numbers increases as the number
: of trials increases.

:       Any clues? Anybody got a pointer to an FAQ where
: I can get a random number generator that is random?

Post your code segement that makes the call.

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