Random numbers

[Anxiety]

Hey its me again. Now this time, how can i get a random number between 100 and 700?

Sorry for multiple posts, but i dont think the mouse and random numbers are related to eachother.

[nevon]

Use the math module.

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--First we set the seed.
math.setrandomseed(os.time)

--If you're using Windows, the first random numbers will be quite uniform.
--Fix that by just discarding a couple of random values
math.random(); math.random(); math.random();

--And now we generate a number between 100 and 700
math.random(100,700)

More info here: http://lua-users.org/wiki/MathLibraryTutorial

[Anxiety]

Okay thanks.

[vrld]

What nevon said, except: use math.randomseed() and os.time() instead of os.time to set the seed:

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math.randomseed(os.time())

Also:

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--If you're using Windows, the first random numbers will be quite uniform.
--Fix that by just discarding a couple of random values
math.random(); math.random(); math.random();

All numbers returned by math.random() will be distributed uniformly - that's actually a good thing. You can transform the uniform distribution to any other one (e.g. normal) though.
Calling math.random() multiple times doesn't make the numbers "more random". What does "more random" mean anyway? Coding advice that suggests to do so stems from the old days(tm), where a random generator could be coded in a way that returned the seed as initial random value. If you had a program that uses os.time() (a number that increases each second) as seed, and start that program with one second delay in between, you'd see the first value increase by one for each program instance. You'd prevent that by calling rand() one time after setting the seed. Nowadays (and with programs that don't run one instance per second), this isn't the case anymore.
Other advice on how to use C's rand() is probably not applicable in Lua, as it already uses rand in the correct way.[/size]

[nevon]

What nevon said, except: use math.randomseed() and os.time() instead of os.time to set the seed:

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math.randomseed(os.time())

Whoops!

[Robin]

Also:

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--If you're using Windows, the first random numbers will be quite uniform.
--Fix that by just discarding a couple of random values
math.random(); math.random(); math.random();

All numbers returned by math.random() will be distributed uniformly - that's actually a good thing. You can transform the uniform distribution to any other one (e.g. normal) though.
Calling math.random() multiple times doesn't make the numbers "more random". What does "more random" mean anyway? Coding advice that suggests to do so stems from the old days(tm), where a random generator could be coded in a way that returned the seed as initial random value. If you had a program that uses os.time() (a number that increases each second) as seed, and start that program with one second delay in between, you'd see the first value increase by one for each program instance. You'd prevent that by calling rand() one time after setting the seed. Nowadays (and with programs that don't run one instance per second), this isn't the case anymore.
Other advice on how to use C's rand() is probably not applicable in Lua, as it already uses rand in the correct way.[/size]

I don't think nevon meant "uniform". Thing is, people have complained about the first few values all being alike, and all have said that calling math.random() a few times fixes that.

[bartbes]

Yeah, it's all very nice how you say modern RNGs don't have this problem anymore, well, they do.

[vrld]

Thing is, people have complained about the first few values all being alike, and all have said that calling math.random() a few times fixes that.

Reminds me of this:

Yeah, it's all very nice how you say modern RNGs don't have this problem anymore, well, they do.

Yeah, it's all very nice how you say they do have this problem, well, they don't.

But let's actually have a look at that before making such claims:

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math.randomseed(os.time()) for i = 1, 5 do print(math.random()) end

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Gentoo             SUSE                 WinXP             Win7
----------------   -------------------  ----------------  ----------------
0.34058737584417   0.13838563446812     0.7230750450148   0.73024689474166
0.84193615887404   0.47572395413915     0.51796014282662  0.13504440443129
0.87171956797676   0.7024961345375      0.24887844477676  0.5012054811243
0.48991742380425   0.0038691856916385   0.55269020661031  0.4261909848323
0.22074050839094   0.47359758311584     0.12158574175237  0.78545487838374

But I'm sure the majority of all computers out there behave differently.

C99 gives a naive example implementation of rand():

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static unsigned long int next = 1; 

int rand(void) // RAND_MAX assumed to be 32767 
{ 
      next = next * 1103515245 + 12345; 
      return (unsigned int)(next/65536) % 32768; 
} 

void srand(unsigned int seed) 
{ 
      next = seed; 
}

Even with that I don't get numbers that look too much alike (seed 1): 16838, 5758, 10113, 17515, 31051, 5627, 23010, 7419, 16212, 4086

Obligatory XKCD:

[sharpobject]

What kind of RNG does lua expose? Is it just a linear congruential generator?

[Robin]

What kind of RNG does lua expose? Is it just a linear congruential generator?

IIRC, Lua just uses the C function, so that would be whatever that is. (I think it depends on the platform.)