rand()

[salvadoravi]

Hi again,

I know that if I want to get different random values I should use srand() before using rand(), my questions are:

1) Why using srand() inside a loop will produce the same results?

for (i=0;i<20;++i)
{
	srand(time(NULL));
	printf("%d\n",rand()%((10)+1));
}

2) Why rand() (without srand) always return the same value, what seed does it use??


Thank you very much

[laserlight]

1) Why using srand() inside a loop will produce the same results?

Probably because you are seeding with the same seed and then taking the first pseudorandom number associated with the sequence generated by that seed, thus it is always the same (but may not be, it depends on your luck).

Read Prelude's articles on random numbers and using rand().

[Prelude]

>Why using srand() inside a loop will produce the same results?
The time function only guarantees second resolution. If your loop runs in less than one second, the seed will always be the same.

>Why rand() (without srand) always return the same value, what seed does it use??
The default seed for rand is the same as if you used srand(1).

[salvadoravi]

Guys when I use the following loop

for (i=0;i<20;++i)
{
	srand(time(NULL));
	printf("&#37;d\n",rand()%((10)+1));
}

I get results lile :
5,5,5,5,5,5,5,5,5.....
or
8,8,8,8,8,8,8,8,8.....
or
n,n,n,n,n,n,n,,n,n
But they all the same number!


?? Why is that?

[Mr_Miguel]

When you call srand() with the same value, the first call to rand() will always return the same number. That's basically what you're doing.

[tabstop]

?? Why is that?

Because you reset the "random" number sequence inside the loop by calling srand again, and again, and again, and again, and.... That loop takes way less than a second to run, so time returns the same number, so you're continually re-seeding the RNG with the same number.

(Of course, all this was mentioned in the post right above your question, but I thought we'd try again.)

[Prelude]

>?? Why is that?
Did you even bother to read my answer to your question before asking the same thing again?

[Salem]

> srand(time(NULL));
Unless your loop takes substantially more than a second per iteration, time() is effectively returning a constant.

So yes, you're always resetting the seed to the same value, and thus always getting the same answer from rand().

Call srand() exactly ONCE at the start of the program. Calling it multiple times doesn't add anything.

[salvadoravi]

Sorry for the stupied question, it makes alot of sense and I should figure it out from Prelude reply . (too much computer for one day....)

Thanks again