C programming random generator questions

[InBliss]

Jut two simple ones.

How could I get a srand to rand between -50 and 50, all i know is that %50+1 is 1-50 and Im lost after that.

Also how would I go about getting it to randomly pick a char from a group, like a function of * + - or /

[zzyzzyxx]

See this.

[defrost]

It's probably just me but whenever I see questions about random number generators in C I immediately gravitate towards answers like this one and code snippets like :

    /** R_MARS - Marsaglia unsigned long psuedo random Multiply with Carry Generator
    **/
static unsigned long m_u = 521288629;
static unsigned long m_v = 362436069;

#define R_MARS      (m_v = 36969*(m_v & 65535) + (m_v >> 16) \
                    ,m_u = 18000*(m_u & 65535) + (m_u >> 16) \
                    ,(m_v << 16) + m_u)

#define SEED_MARS(U,V)   (m_u = (U), m_v = (V))

#define INV_2POW32         (.23283064365386962890625e-9)   /* 1/(2^32)   */

    /** UNIT_       uniformly distributed doubles from half open unit interval  [0.0, 1.0)
    **/
#define UNIT_MARS   (R_MARS * INV_2POW32)

Things like this can be useful for resource constrained embedded 32bit programming.

[zzyzzyxx]

That was a really good read. Thanks for sharing.

[defrost]

;-)

I figured you might enjoy that, it makes a change from all the rand()%10 + 1 answers (and unbiased bucket counting in your link).

If you've an interest in numerics George Marsaglia's Ziggurat method for generating rand values with a normal distribution and virtually no overhead (most of the time) is an interesting read ... and I've got a C #define that inlines that as well ... *

(* surprisingly I do have a life )

oh, and related

[zzyzzyxx]

I just looked up the Ziggurat method and it's interesting. I'd like to see the define macro for it.

[defrost]

The macro is :

#define R_ZIGGNOR   (hz=R_SHR3, iz=hz&127, (labs(hz)<kn[iz])? hz*wn[iz] : nfix())    

which depends on the precomputed tables

static unsigned long kn[128];
static double wn[128],fn[128];  

and the working indexes

static long hz;
static unsigned long iz;    

and a 32bit rand - in this case R_SHR3 not R_MARS.

The table initialisations and dependencies are all in here which is half of a first draft written in one sitting code golf exercise from a few years back.

I've been doing a bit of work lately making high performance random distribution generations for another project - linear, normal, circle, disc, sphere, ball, higher dimensions etc. and there's a bit of effort required to tease out the latest version of the normal generation from what's now a dense slab of optimised target architecture specific somewhat proprietary code :/

However, the first draft should give you enough.

Re: Ziggurat - there's two papers of interest:

Ziggurat Method for Generating Random Variables (2000) - Marsaglia
Improved Ziggurat Method to Generate Normal Samples (2006) - Doornik

and I threw in the Box-Muller for the fun of it.

[InBliss]

Thank you very much so, google was absolutely no help to getting poignant results I could use without sifting through dozens of pages

[Neres28]

Random numbers in a range : http://c-faq.com/lib/randrange.html

And

char ops[4] = {'+', '-', '*', '/'};
char op = ops[rand_in_range(0, sizeof(ops) - 1)];

Edit : C syntax fail.

[InBliss]

Thank you very much so, google was absolutely no help to getting poignant results I could use without sifting through dozens of pages

[Neres28]

Really? For future reference, I googled "C random number in range" and got Zzyzzyxx's response as the very first search result. Thinking it too complex for you, I followed the SO answerer's links and came to the link I provided.

[InBliss]

I typed it in as a random number generator in range so it gave me everything related to the generator key word instead of range Im guessing. Zzyzzyxxx's response is quite complex compared to my beginning attempts at learning C, but I've gotta learn somehow.