I know how to generate a random number between 1 and 2 :
def random
return (1 + rand(2))
end
but let’s say I would like to return 1 in 80% of the calls and 2 in
20% of the call…
which library should I use ? ( I’ve seen a lot at
http://raa.ruby-lang.org/
)
thanks for your suggestions
On 12 March 2011 10:10, Erwin [email protected] wrote:
No doubt there are better ways but something like
(rand(100) + 120)/100
should do it.
Colin
On Mar 12, 2011, at 5:37 AM, Colin L. wrote:
)
No doubt there are better ways but something like
(rand(100) + 120)/100
should do it.Colin
Well, perhaps something that reveals the intention better:
def usually_one
rand < 0.80 ? 1 : 2
end
-Rob
Rob B.
[email protected] http://AgileConsultingLLC.com/
[email protected] http://GaslightSoftware.com/
On 12 March 2011 13:42, Rob B. [email protected]
wrote:
On Mar 12, 2011, at 5:37 AM, Colin L. wrote:
On 12 March 2011 10:10, Erwin [email protected] wrote:
I know how to generate a random number between 1 and 2 :
def random
return (1 + rand(2))
No it doesn’t actually. 1 + rand would be better, though it depends
what you mean by between whether even that is correct.
(rand(100) + 120)/100
should do it.Colin
Well, perhaps something that reveals the intention better:
def usually_one
rand < 0.80 ? 1 : 2
end
Yes, much better. I suppose it is the old programmer in me
instinctively avoiding floating point to save on processor time. A
bit silly when everything has hardware floating point.
Colin
On 12 March 2011 20:47, Michael P. [email protected] wrote:
def usually_one
[1,2,2,2,2][rand(5)]
end
ahem of course… this is the total opposite of what the OP asked
for, and returns 1 in 20% of the calls 
sorry for not paying attention… oops!
On 12 March 2011 14:23, Colin L. [email protected] wrote:
On 12 March 2011 13:42, Rob B. [email protected] wrote:
def usually_one
rand < 0.80 ? 1 : 2
endYes, much better. I suppose it is the old programmer in me
instinctively avoiding floating point to save on processor time. A
bit silly when everything has hardware floating point.
Although I prefer Rob’s suggestion as a better example of self
documented code; this is another alternative…
def usually_one
[1,2,2,2,2][rand(5)]
end
…there’s many ways to skin a cat 
On 12 March 2011 21:07, Colin L. [email protected] wrote:
Ha! Vindicated
I expected to get different times, but to keep the proportions the
same… but I get a little difference in order of speed:
begin
?> t=Time.now
1000000.times{(rand(100) + 120)/100}
puts Time.now-t
end
1.200484
begin
?> t=Time.now
1000000.times{rand < 0.80 ? 1 : 2}
puts Time.now-t
end
1.249109
begin
?> t=Time.now
1000000.times{[1,2,2,2,2][rand(5)]}
puts Time.now-t
end
1.498434
…there’s many ways to skin a cat
But how fast can you do it?
A fair bit faster if I don’t build the array in every loop 
x=[1,2,2,2,2]
=> [1, 2, 2, 2, 2]
begin
?> t=Time.now
1000000.times{x[rand(5)]}
puts Time.now-t
end
1.026202
On 12 March 2011 21:36, Michael P. [email protected] wrote:
end
?> t=Time.nowx=[1,2,2,2,2]
=> [1, 2, 2, 2, 2]
begin
?> t=Time.now
1000000.times{x[rand(5)]}
puts Time.now-t
end
1.026202
OK, I admit defeat. I am using ruby 1.8.7 which may be the reason for
the different ratios you are seeing (assuming you are using 1.9.2).
Interestingly the benefit of your technique is even greater on 1.8.7,
taking only 0.67 secs on my machine.
Colin
On 12 March 2011 20:47, Michael P. [email protected] wrote:
Although I prefer Rob’s suggestion as a better example of self
documented code; this is another alternative…def usually_one
[1,2,2,2,2][rand(5)]
end
Ha! Vindicated
ruby-1.8.7-p302 > begin
ruby-1.8.7-p302 > t=Time.now
ruby-1.8.7-p302 ?> 1000000.times{(rand(100) + 120)/100}
ruby-1.8.7-p302 ?> puts Time.now-t
ruby-1.8.7-p302 ?> end
0.905431
ruby-1.8.7-p302 > begin
ruby-1.8.7-p302 > t=Time.now
ruby-1.8.7-p302 ?> 1000000.times{rand < 0.80 ? 1 : 2}
ruby-1.8.7-p302 ?> puts Time.now-t
ruby-1.8.7-p302 ?> end
1.712769
ruby-1.8.7-p302 > begin
ruby-1.8.7-p302 > t=Time.now
ruby-1.8.7-p302 ?> 1000000.times{ [1,2,2,2,2][rand(5)] }
ruby-1.8.7-p302 ?> puts Time.now-t
ruby-1.8.7-p302 ?> end
1.150481
…there’s many ways to skin a cat
But how fast can you do it?
Colin
On 12 March 2011 22:16, Michael P. [email protected] wrote:
On 12 March 2011 22:01, Colin L. [email protected] wrote:
OK, I admit defeat. the benefit of your technique is even greater on 1.8.7,
taking only 0.67 secs on my machine.Nah! Who cares if it takes half-a-second or a second to select a
million random numbers… any of the methods are perfect for
selecting one number.
The OP might be doing some statistical simulation, so might need
millions of values, but probably not.
Colin
On Sat, Mar 12, 2011 at 1:07 PM, Colin L. [email protected]
wrote:
bit silly when everything has hardware floating point.
ruby-1.8.7-p302 ?> puts Time.now-t
ruby-1.8.7-p302 ?> end
1.712769ruby-1.8.7-p302 > begin
ruby-1.8.7-p302 > t=Time.now
ruby-1.8.7-p302 ?> 1000000.times{ [1,2,2,2,2][rand(5)] }
ruby-1.8.7-p302 ?> puts Time.now-t
ruby-1.8.7-p302 ?> end
1.150481
Here’s my number from within and outside IRB from Mac OS 10.6.6:
ruby-1.9.2-head :001 > begin
ruby-1.9.2-head :002 > t=Time.now
ruby-1.9.2-head :003?> 1000000.times{(rand(100) + 120)/100}
ruby-1.9.2-head :004?> puts Time.now-t
ruby-1.9.2-head :005?> end
from-irb: 0.16657 from-ruby-script: 0.159452
ruby-1.9.2-head :006 > begin
ruby-1.9.2-head :007 > t=Time.now
ruby-1.9.2-head :008?> 1000000.times{rand < 0.80 ? 1 : 2}
ruby-1.9.2-head :009?> puts Time.now-t
ruby-1.9.2-head :010?> end
from-irb: 0.280634 from-ruby-script: 0.150247
ruby-1.9.2-head :011 > begin
ruby-1.9.2-head :012 > t=Time.now
ruby-1.9.2-head :013?> 1000000.times{[1,2,2,2,2][rand(5)]}
ruby-1.9.2-head :014?> puts Time.now-t
ruby-1.9.2-head :015?> end
from-irb: 0.521992 from-ruby-script: 0.35944
-Conrad
On 12 March 2011 23:29, Conrad T. [email protected] wrote:
Yes, much better. I suppose it is the old programmer in me
ruby-1.8.7-p302 > t=Time.now
1.150481Here’s my number from within and outside IRB from Mac OS 10.6.6:
ruby-1.9.2-head :001 > begin
ruby-1.9.2-head :002 > t=Time.now
ruby-1.9.2-head :003?> 1000000.times{(rand(100) + 120)/100}
ruby-1.9.2-head :004?> puts Time.now-t
ruby-1.9.2-head :005?> end
from-irb: 0.16657 from-ruby-script: 0.159452
I think I need a new laptop.
Colin
On 12 March 2011 22:01, Colin L. [email protected] wrote:
OK, I admit defeat. the benefit of your technique is even greater on 1.8.7,
taking only 0.67 secs on my machine.
Nah! Who cares if it takes half-a-second or a second to select a
million random numbers… any of the methods are perfect for
selecting one number.
Was a nice distraction from the recent run of posts…
I use this often:
class Uuid
def self.uuid
return rand( 18446744073709551615 ).to_s( 36 ).rjust( 13, ‘0’ )
end
end
On 15 March 2011 13:37, Martin S. [email protected]
wrote:
I use this often:
class Uuid
def self.uuid
return rand( 18446744073709551615 ).to_s( 36 ).rjust( 13, ‘0’ )
end
end
Nice, simple process (assuming it works… I’ve not tried it), but I
don’t think it has much to do with the topic of this thread, and it’s
not likely to create a real UUID (RFC4122) is it?
This old thread discusses it a bit:
http://www.ruby-forum.com/topic/164078
Colin L. wrote in post #987114:
But how fast can you do it?
Ooh, that’s an evil tweak… (a minor diversion which is much more fun
than what I was doing)
Lenovo T61p, Core2Duo, T9300, 2.5GHz, ruby 1.9.2-p0
= 0.253025
And for curiosity, my Ubuntu 10.10 VM on top of the same hardware:
= 0.323791
I don’t think it makes a true UUID, but I’ve run it into the billions
and never got a repeat. So, effectively, it acts like UUID.
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