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From: mbrennan@swift.cs.tcd.ie
Newsgroups: comp.lang.c
Subject: Re: I'm confused about this
Message-ID: <1990Nov22.173858.7530@swift.cs.tcd.ie>
Date: 22 Nov 90 17:38:57 GMT
References: <3945@vela.acs.oakland.edu>
Organization: Computer Science Department, Trinity College Dublin
Lines: 35

In article <3945@vela.acs.oakland.edu>, jmwojtal@vela.acs.oakland.edu (Wojo) writes:
> I got this C program and this part I don't understand:
> 
> int   getpid();
> long  now;
> 
> now = time(&now) / rand();
> srand(getpid() + (int)((now >> 16) + now + time(&now)));
> 
> What does "srand" do to be exact?
It is a function which 'seeds' the pseudo randon number generator rand().
Look up the manual under rand() if you couldnt find it under srand().  You
did RTFM didn't you :-)

A simple view of how pseudo random number generator operates:
- internally it has a circular list of numbers 
- successive calls to it yields the next number from this circular list
- if you start at the same point in the list for two series of
  numbers, then both series will be the same
- to avoid this problem we 'seed' it, this seed can be considered as
  the position in the circular list to start yielding successive numbers.
- we try and make the seed as unpredictable (random) as possible.  In
  the above fragment the seed is calulated by:
        getpid() + (int)((now >> 16) + now + time(&now)
  which is a mixture of the process id, and the current time 

> It doesn't bring a value back?
No it doesn't. 
> "getpid()" is declared, but the function is not in the program.
Oh yes it is.  It's  used for calculating the seed (the parameter to srand())

-- 
 ,   ,  ,      ,              ,       ,          ,          , ,      ,
Micheal O Braonain     Roinn Riomheolaiochta, Colaiste Na Trinoide, BAC 2.
Email                  mbrennan@cs.tcd.ie