CODEWITHSUNDEEP
randomluanumbers
Leviathan
Leviathan

Reputation: 83

Lua 5.2.1 - Random Numbers

In Lua 5.2.1, I tried to generate a random number with

num = math.random(9)

However, every time I run my program:

num = math.random(9)
print("The generated number is "..num..".")

I get the same number.

brendan@osiris:~$ lua number 
The generated number is 8.
brendan@osiris:~$ lua number 
The generated number is 8.
brendan@osiris:~$ lua number 
The generated number is 8.

This is frustrating, because everytime I try to generate a new number and restart the program, I get the same sequence.

Is there a different way of generating a number?

Also, I've looked into 

math.randomseed(os.time())

but I don't really get it. If this is indeed the solution could you explain how it works, what it does, and what number I'd get?

Thx,

Upvotes: 1

Views: 2303

Answers (3)

grmmhp
grmmhp

Reputation: 131

First, you have to call 'math.randomseed()'

'Why?'

Because Lua generates pseudo random numbers.

--One of the best seeds for 'math.randomseed()' is the time.

So, you'll first write:

math.randomseed(os.time())

After this,

num = math.random(9)
print("The generated number is "..num..".")

However, there is a bug on Windows. Then if you just write 'num = math.random(9)' the generated number will be the same for 1 hour, i think.

'So how can I resolve this?'

It's easy, you need to do a for loop.

for n = 0, 5 do
    num = math.random(9)
end

So, in Windows, the final code would be:

math.randomseed(os.time())

for n = 0, 5 do
    num = math.random(9)
end

print("The generated number is "..num..".")

OBS: If 'for n = 0, 5 do' does not work perfectly, then replace 5 with 10.

Upvotes: 1

Henrik Karlsson
Henrik Karlsson

Reputation: 5723

In Lua this is the expected output. You are not guranteed to get different sequences across different sessions.

However, any subsequent calls to math.random will generate a new number:

>> lua
> =math.random(9)
1

>> lua
> =math.random(9)
1

>> lua
> =math.random(9)
1
> =math.random(9)
6
> =math.random(9)
2

math.randomseed() will change which sequence is replayed. If you set math.randomseed(3) for example, you will always get the same sequence, just like above:

>> lua
> math.randomseed(3)
> =math.random(9)
1
> =math.random(9)
2
> =math.random(9)
3

>> lua
> math.randomseed(3)
> =math.random(9)
1
> =math.random(9)
2
> =math.random(9)
3

If you however set math.randomseed() to a unique value each run, for example os.time(), you will ofcourse get an unique sequence each time.

Upvotes: 2

lhf
lhf

Reputation: 72362

This is not particular to Lua. Pseudorandom generators usually work like that: they need a seed to start and the sequence they generate is not really random, but actually deterministic given a seed. This is a good thing for debugging but for production you need to alter the seed in a "random" way. An easy and typical way of doing that is to use the time to set the seed once at the start of the program.

Upvotes: 5

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