how do i get a random number between 0 and 1?

[vunterslaush]

such as 0.423, 0.0001, 0.123214, etc
thanks. don't has to be insanely random or something, for a simple game.

[Duncan C]

Originally Posted by vunterslaush

such as 0.423, 0.0001, 0.123214, etc
thanks. don't has to be insanely random or something, for a simple game.

The code below should return a double precision float from 0.0 to 1.0:

double random = arc4random() / ((double) (((long long)2<<31) -1));

[vunterslaush]

Originally Posted by Duncan C

The function arc4random() returns a number from 0 to (2^32) -1

I'm installing a new version of Xcode right now so I can't test it, but you should be able to generate a random number from zero to 1 with code like this:

double random = arc4random() / ((double) 2<<32 - 1);

thanks for your response, i get "Invalid operands to binary expression ('double' to 'int')"

this is the code that i wrote:

-(double) rand{
    return arc4random() / ((double) 2<<32 - 1);
}

[Duncan C]

Originally Posted by vunterslaush

thanks for your response, i get "Invalid operands to binary expression ('double' to 'int')"

this is the code that i wrote:

-(double) rand{
    return arc4random() / ((double) 2<<32 - 1);
}

I tested, edited and fixed my post. Try the updated version.

[sneaky]

Sorry, Duncan, what is the

2<<32 - 1

And what is the benefit over

arc4random() % 1

[cujo30227]

Here's a simple random number generator

float random = rand()/(float)RAND_MAX;

gives you a number between 0 and 1.

[vunterslaush]

Originally Posted by cujo30227

Here's a simple random number generator

float random = rand()/(float)RAND_MAX;

gives you a number between 0 and 1.

thanks a lot that worked perfectly!

[baja_yu]

Keep in mind that rand() needs to be seeded.

[Duncan C]

Originally Posted by sneaky

Sorry, Duncan, what is the

2<<32 - 1

And what is the benefit over

arc4random() % 1

The corrected line is

double random = arc4random() / ((double) (((long long)2<<31) -1));

The "(((long long)2<<31) -1)" bit is just a way to get a constant thats the size of the maximum random number arc4random() will generate. There may be a constant, but I couldn't find it. arc4random generates a larger range of numbers than rand(). It also seeds itself, and generates much better random numbers.

You'll find that unless you seed rand(), it generates exactly the same sequence of "random" numbers every time you run your program.


You can't use "arc4random() % 1" because "%" is the remainder operator. Dividing by 1 doesn't do anything. You need to divide your random number by the largest possible value it can give you.

[vunterslaush]

sorry for being a noob, how do i seed? i just noticed now that the numbers are repeating
as i said i don't need to it be insanely random i'm just playing around so rand is enough for me, and also i think it's good that i can replay the same values if needed for debug. so how do i seed? :P

[Duncan C]

Originally Posted by vunterslaush

sorry for being a noob, how do i seed? i just noticed now that the numbers are repeating
as i said i don't need to it be insanely random i'm just playing around so rand is enough for me, and also i think it's good that i can replay the same values if needed for debug. so how do i seed? :P

I haven't used rand() in so long that I can't remember the call to seed it, and the docs are really bad. Baja?

[baja_yu]

You can do it by using srand() which takes an unsigned integer as a parameter. People usually pass it the time/date or some other changing number.

[Duncan C]

Originally Posted by Duncan C

I haven't used rand() in so long that I can't remember the call to seed it, and the docs are really bad. Baja?

Note that the docs recommend that you not use rand. The lower 12 bits or so of the numbers it generates have a repeating (non-random) pattern.

These bits are taken from the docs:

rand, rand_r, srand, sranddev -- bad random number generator

These interfaces are obsoleted by random(3).

You're much better off using random() or arc4random.

random() also has a seed method, srandom, but produces much better random numbers than rand().

arc4random is the best, and produces very good random numbers.

[baja_yu]

The only thing about arc4random is that I've read somewhere that it was slower than random or rand, but I haven't tested it nor was I in a situation where I needed to call a random number generator 1000 times per second or more, so I've always used arc4random.

[Duncan C]

Originally Posted by baja_yu

The only thing about arc4random is that I've read somewhere that it was slower than random or rand, but I haven't tested it nor was I in a situation where I needed to call a random number generator 1000 times per second or more, so I've always used arc4random.

The only BAD thing I've heard about arc4random is that it's a little slow. It generates MUCH, MUCH better random numbers than either rand() or random().

I just did a little test, and on my iPhone 3GS, which is slow by today's standards, here's what I got:

arc4random()
0.527984

rand()
0.091877

random()
0.077830

So rand() is a little over 5 times faster than arc4rand; Interestingly, random() is the fastest of all.

Clearly, if you care about speed, use random(). It is a better random number generator than rand (which stinks and shouldn't be used.) and also faster.

arc4random generates the best random numbers, and is self-seeding, but it is significantly slower than the others.

[sandromandro]

i want to understand your code but it doesn't work in my project.
is it possible to create random numbers exactly between 0.035 and 0.009?

thank you

[Duncan C]

Originally Posted by sandromandro

i want to understand your code but it doesn't work in my project.
is it possible to create random numbers exactly between 0.035 and 0.009?

thank you

This code will generate a random number from 0 to 1:

double random = arc4random() / ((double) (((long long)2<<31) -1));

So, you need to divide your result by (largest-smallest), and add the smallest value to it.

- (double) randomFromMinimum: (double) min toMax: (double) max;
{
  double result;
  double random = arc4random() / ((double) (((long long)2<<31) -1));
  result = random / (max-min) + min;
  return result;
}

That is off the top of my head, without testing, while I'm deep in some nasty video rendering OpenGL code. It will need to be tested and debugged.