How to generate random numbers in C

[pereges]

I need to generate two uniform random numbers between 0 and 1 in C ?
How to do it ?

I looked into rand function where you need to #define RAND_MAX as 1
but will this rand function give me uniformly distributed and unique
numbers ?

[Ben Bacarisse]

"Malcolm McLean" <re*******@btin ternet.comwrite s:

"pereges" <Br*****@gmail. comwrote in message news:

>>I need to generate two uniform random numbers between 0 and 1 in C ?
How to do it ?

I looked into rand function where you need to #define RAND_MAX as 1
but will this rand function give me uniformly distributed and unique
numbers ?

No, defining RAND_MAX as something else won't help. All it will do is
make the identifier useless in the rest of your program, without
touching the random number generator. Strictly it is UB in case
RAND_MAX appears in some standard library macro or other.

This is the generally accpeted way to do it
#define uniform() (rand() / (RAND_MAX + 1.0))
#define rnd(x) ( (x) * uniform() )

we add 1.0 to make rand() never return exactly unity, so rnd() is
always in the range 0 to x-1.

rnd(m) can be much closer to m than m-1. Did you mean:

#define rnd(x) ((int)((x) * uniform()))

perhaps? The OP wanted numbers between 0 and 1 so your uniform() is
sufficient if they mean doubles in [0,1).

--
Ben.

[Gordon Burditt]

If you try to offer casino gambling games (e.g. craps, blackjack,

roulette, etc.) for real money using pseudo-random numbers, you're
going to lose.

There is no mathematical basis for this general statement, however,
there have been cases where people caught on to the sequence coming
from a poorly- or non-reseeded pseudo-random number generator
in a casino game and won hundreds of $thousands before the
casino realized their error...

My statement is more based on economics and psychology than
mathematics. If there's enough to gain, people will go to great
lengths to break your scheme, including stealing a machine (or
BUYING one) and taking it apart, bribing employees or ex-employees
to get source code and procedures, dedicating huge amounts of donated
CPU time under the guise of searching for aliens or a cure for
cancer, etc.

A really clever crack will not take so much in winnings that the
casino realizes it immediately, but will take less but over a longer
period of time.

[Malcolm McLean]

"Ben Bacarisse" <be********@bsb .me.ukwrote in message
news:87******** ****@bsb.me.uk. ..

"Malcolm McLean" <re*******@btin ternet.comwrite s:

>"pereges" <Br*****@gmail. comwrote in message news:

>>>I need to generate two uniform random numbers between 0 and 1 in C ?
How to do it ?

I looked into rand function where you need to #define RAND_MAX as 1
but will this rand function give me uniformly distributed and unique
numbers ?

No, defining RAND_MAX as something else won't help. All it will do is
make the identifier useless in the rest of your program, without
touching the random number generator. Strictly it is UB in case
RAND_MAX appears in some standard library macro or other.

This is the generally accpeted way to do it
#define uniform() (rand() / (RAND_MAX + 1.0))
#define rnd(x) ( (x) * uniform() )

we add 1.0 to make rand() never return exactly unity, so rnd() is
always in the range 0 to x-1.

rnd(m) can be much closer to m than m-1. Did you mean:

#define rnd(x) ((int)((x) * uniform()))

perhaps? The OP wanted numbers between 0 and 1 so your uniform() is
sufficient if they mean doubles in [0,1).

That would be fine if int was 64 bits :-(

--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm

[Richard Tobin]

In article <IA************ ********@bgtnsc 04-news.ops.worldn et.att.net>,
Bill Reid <ho********@hap pyhealthy.netwr ote:

>In a fair game, the player with the biggest house money volume wins.

>Oh, a "fair game"...who the hell offers a "fair game"? In any event,
it's irrelevant, because the actual most important factor (to the extent
that we indulge in the pointless semantics of pronouncing a "most
important factor") is the "expectatio n" of the game.

Actually, you can reliably win against poor odds if you have unlimited
resources and your opponent is required to accept your bets. Decide
how much you want to win. Bet enough to make that if you win. If you
do, quit. If you don't, bet enough to win back your losses plus your
desired win. If you can continue making bets in this manner, you will
eventually win, no matter how poor the odds.

In practice, you can't do this, because your resources are limited
compared to the Casino's. If they weren't, they would eventually
refuse your bet.

-- Richard
--
:wq

[Bill Reid]

Gordon Burditt <go***********@ burditt.orgwrot e in message
news:T7ydnSMQJK BSnJzVnZ2dnUVZ_ sytnZ2d@interne tamerica...

If you try to offer casino gambling games (e.g. craps, blackjack,
roulette, etc.) for real money using pseudo-random numbers, you're
going to lose.

There is no mathematical basis for this general statement, however,
there have been cases where people caught on to the sequence coming
from a poorly- or non-reseeded pseudo-random number generator
in a casino game and won hundreds of $thousands before the
casino realized their error...

My statement is more based on economics and psychology than
mathematics. If there's enough to gain, people will go to great
lengths to break your scheme,

There's a limit to greed; just because somebody offers $1trillion
to run a mile in one second doesn't mean anybody is ever going
to be able to do it, no matter how motivated they are and how
hard they try.

including stealing a machine (or
BUYING one) and taking it apart, bribing employees or ex-employees
to get source code and procedures, dedicating huge amounts of donated
CPU time under the guise of searching for aliens or a cure for
cancer, etc.

Sure, this all happens now, and has always happened. Back
when I used to consult for the electronic slot machine and lottery
equipment makers, a popular method of getting money from the
machines was to zap them with about a zillion volts, hopefully
sometimes causing the CPU to fritz out and activate the hopper
dump solenoid and dump the coins in the hopper into the "loud bowl".
Then they'd scoop up as many as they could and try to hit the
doors before security locked them (they were in most cases
already detected and being tracked by security cameras
the moment the coins dropped).

A slightly more sophisticated method involved an operation
using a small drill and a procedure not unlike othroscopic
surgery to trigger a hopper dump; in any event, they still
couldn't fake the internal sequence of microprocessor events
that had to occur and were always recorded that were evidence
of a genuine jackpot, along with those omnipresent security
cameras...

Guys who REALLY knew the machines could even have
a whole set of fake EEPROMs ready to install in seconds
after they jimmied open the door; but seriously, wouldn't
it just be easier to get a friggin' job?

A really clever crack will not take so much in winnings that the
casino realizes it immediately, but will take less but over a longer
period of time.

Oh, they'll figure it out eventually, but the problem will still
be to predict a series of pseudo-random numbers seeded from
some split-second event at pseudo-random times...this would
be tough enough under the best of circumstances, and in
a casino environment, purt near impossible, even WITH
"inside information"...

---
William Ernest Reid

[Richard Tobin]

In article <Hh************ *********@bgtns c04-news.ops.worldn et.att.net>,
Bill Reid <ho********@hap pyhealthy.netwr ote:

>And no, not even with "unlimited resources", because "unlimited
resources" mean infinity bets and thus, INFINITY LOSSES,

I suggest you learn some mathematics before you rant in future.
All the bets involved are finite.

-- Richard
--
:wq

[Chad]

On Apr 11, 12:20*pm, gordonb.k5...@b urditt.org (Gordon Burditt) wrote:

I need to generate two uniform random numbers between 0 and 1 in C ?

Computers do not generate truly random numbers without hardware support.
Some techniques include the detection of radioactive decay and thermal
noise from a reverse-biased diode. *There is some belief that there
is randomness in the timing (say, down to picoseconds) of keystrokes,
although I don't think anyone has managed to tie human typing to quantum
effects yet. *Some CPU chips have hardware for random number generation
on them.

You may want pseudo-random numbers. *In cryptography, random numbers
are very important and the difference between pseudo-random numbers
and real random numbers used in encryption may get you killed as a
spy.

If you try to offer casino gambling games (e.g. craps, blackjack,
roulette, etc.) for real money using pseudo-random numbers, you're
going to lose.

rand() returns the same sequence of numbers each time the program
starts up unless you call srand() with a different seed value from
the last time. *Seed values are commonly derived from the time and/or
process ID (but NOT in applications where real random numbers are needed,
like gambling or cryptography: *32 bits of randomness isn't enough, and
a poor seed can cripple a good random number generator).

How to do it ?

There are better pseudo-random number generators than rand().
These have the disadvantage that they are not included in all C
libraries.

I looked into rand function where you need to #define RAND_MAX as 1

You do not get to redefine RAND_MAX. *Also, the return type of rand()
is int, so don't expect any values where 0 < rand() < 1 . *If RAND_MAX
were 1 (not allowed by the standard) all you would get is 0 and 1.

but will this rand function give me *uniformly distributed and unique
numbers ?

Consider using algebra. *rand() returns [0, RAND_MAX]. *You want
numbers between 0.0 and 1.0 including both endpoints. *Or is that just
including 0.0 but not 1.0? *Consider what these might give you:

* * * * ((double)rand() )/RAND_MAX
or
* * * * ((double)rand() )/(RAND_MAX+1)

What' the difference between random numbers and pseudo-random numbers?

[Bill Reid]

Richard Tobin <ri*****@cogsci .ed.ac.ukwrote in message
news:ft******** ***@pc-news.cogsci.ed. ac.uk...

In article <Hh************ *********@bgtns c04-news.ops.worldn et.att.net>,
Bill Reid <ho********@hap pyhealthy.netwr ote:

And no, not even with "unlimited resources", because "unlimited
resources" mean infinity bets and thus, INFINITY LOSSES,

I suggest you learn some mathematics before you rant in future.

"The ironing is delicious" - Bart Simpson

All the bets involved are finite.

Which, according to you, require "unlimited resources"...we
can put Las Vegas out of business, as long as we make a "finite"
series of bets with an infinite amount of money...you're a genius!!!

But it's like I've said before...you guys have GOT to stick to
the return value of main() because everything else on planet
Earth is just too complicated for you...

---
William Ernest Reid

[Flash Gordon]

Chad wrote, On 13/04/08 18:22:

<snip>

What' the difference between random numbers and pseudo-random numbers?

psueudo-random numbers are numbers that look random (for some definition
of look random) but are really following some sequence (and this, by the
definition of C, includes all implementations of rand()) where as random
numbers really are random.

A quick search on Google will give you a lot more information on pseudo
random numbers.
--
Flash Gordon

[Chris Torek]

In article <66************ **@mid.individu al.net>
Ian Collins <ia******@hotma il.comwrote:

>Why would you attempt to redefine RAND_MAX? ...

Not everyone starts out with the right mental model of RAND_MAX.

It may help to think of RAND_MAX as if it were a highway sign,
telling you how far away the next town or exit is. If you stop
your car, walk up to the sign, paint out the original number, and
then paint in a "1", will the next town or exit move to just 1 mile
(or 1 kilometer) away? :-)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: gmail (figure it out) http://web.torek.net/torek/index.html