lame question about rand() :)

[Cymric]

rand() returns a number between 0 and RAND_MAX, so you would want something like

a = 1 + (int)(3.0*rand()/(RAND_MAX+1.0));

for an integer between 1 and 3 inclusive. If you want between 0 and 3, use

a = (int)(4.0*rand()/(RAND_MAX+1.0));

because 4.0*rand() is at most 4*RAND_MAX, so when you divide by RAND_MAX+1.0, you get a number which is slightly less than 4 at most, and thus rounded down to 3.

Don't use code like

a = 1 + (int)(rand() % 3);

because it emphasises a subset of bits of the random number, and thus is inherently less random. (Think of the rules you learned in primary school to determine whether a number is divisible by 2, 3, 4, ... .)

[Cymric]

Odd. The code won't compile without the necessary #include's---you require a few at least (stdio.h, stdlib.h, stddef.h, perhaps unistd.h too, I sort-of-forgot ever since I began using a modular code library  :-P) but if they weren't included...

Try and print out a few rand()'s in a loop, without any multiplications; include RAND_MAX too. Perhaps SAS/C is using a different convention for the random functions---these things are a royal mess, and it wouldn't be the first time something platform-dependent cropped up.

Oh, and what libraries are you linking with?

[Cymric]

Duh. Hadn't realised that as we're not running a loop, but yes, Thomas is correct.

[Cymric]

kas1e wrote:
In other words, i have in my code fucntion, which do some visual effects (here is 4 effect). And want to do always different effect. So, i need to generate always rand numbers between 0 and 3, and put rezult to my effect_do_function as integer value. of course will be good if it will compiller/platform independed. Any ideas ?

I'm not sure I understand your question. You now have a random generator which yields a random number between 0 and 3. But now you want to use that number to select a visual effect, and make sure that you don't get the same effect twice in a row. Is that correct? (In other words, 0-1-2-3-0-1-3-1-2 is okay, 0-1-2-3-2-1-2-2 is not.)

Doesn't sound too complicated (assuming a function my_rand() which returns the random number):

Code: [Select]

void do_visual_effect()
{
    static int old_effect;
    int effect;

    do {
        effect = my_rand();
    } while (effect == old_effect);

    old_effect = effect;

    // rest of code producing visual effect 'effect'...

    return;
}

Hope this helps!

[Cymric]

Well, a single statement

Code: [Select]

srand(time(NULL)); at the beginning of your code (say, the first executable statement of main()) should suffice. Don't go about calling srand() with every instance of rand()---that screws up the random generator.

Also keep in mind that you cannot conclude on the basis of just 10 numbers that the generator isn't random. Don't worry, it works .

[Cymric]

Repeat the loop, say, 10000 times. (Pause between 1 and 2 seconds between each run with Delay().) If every time you run it the sequence starts with a 2, I'll be more inclined to say that the random generator is broken and that you need another one.