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Thread: Random numbers

Random numbers

From
Karel Riveron Escobar
Date:
Hello list,

I want to generate random numbers in Pl/pgSQL. How can I do this?

To be more specific, I have to generate random numbers among 0 and 5.

Thanks in advance.

Regards, Karel Riverón
Student Scientific Council
Informatics Science University



Re: Random numbers

From
bricklen
Date:

On Sat, May 25, 2013 at 8:45 AM, Karel Riveron Escobar <kescobar@estudiantes.uci.cu> wrote:
I want to generate random numbers in Pl/pgSQL. How can I do this?
To be more specific, I have to generate random numbers among 0 and 5.

One way:
select n from unnest(ARRAY[0,1,2,3,4,5]) n order by random() limit 1;

Re: Random numbers

From
Ian Lawrence Barwick
Date:
2013/5/26 bricklen <bricklen@gmail.com>:
>
> On Sat, May 25, 2013 at 8:45 AM, Karel Riveron Escobar
> <kescobar@estudiantes.uci.cu> wrote:
>>
>> I want to generate random numbers in Pl/pgSQL. How can I do this?
>> To be more specific, I have to generate random numbers among 0 and 5.
>
>
> One way:
> select n from unnest(ARRAY[0,1,2,3,4,5]) n order by random() limit 1;

somewhat shorter:
 SELECT (random() * 5)::INT

Ian Barwick

Re: Random numbers

From
Karel Riveron Escobar
Date:
Thanks bricklen.

I found another way in PostgreSQL 9.1.0 Documentation.

SELECT * INTO random_number FROM random(); -- random function gives me a random value among 0.0                                                                                                -- and 1.0

SELECT * INTO rounded_number FROM round(random_number * 5); -- this line is completly obviously.

Saludos, Karel Riverón
Consejo Científico Estudiantil
Universidad de las Ciencias Informáticas



From: "bricklen" <bricklen@gmail.com>
To: "Karel Riveron Escobar" <kescobar@estudiantes.uci.cu>
Cc: pgsql-general@postgresql.org
Sent: Saturday, May 25, 2013 11:50:03 AM
Subject: Re: [GENERAL] Random numbers


On Sat, May 25, 2013 at 8:45 AM, Karel Riveron Escobar <kescobar@estudiantes.uci.cu> wrote:
I want to generate random numbers in Pl/pgSQL. How can I do this?
To be more specific, I have to generate random numbers among 0 and 5.

One way:
select n from unnest(ARRAY[0,1,2,3,4,5]) n order by random() limit 1;




Re: Random numbers

From
Szymon Guz
Date:
On 25 May 2013 17:56, Ian Lawrence Barwick <barwick@gmail.com> wrote:
2013/5/26 bricklen <bricklen@gmail.com>:
>
> On Sat, May 25, 2013 at 8:45 AM, Karel Riveron Escobar
> <kescobar@estudiantes.uci.cu> wrote:
>>
>> I want to generate random numbers in Pl/pgSQL. How can I do this?
>> To be more specific, I have to generate random numbers among 0 and 5.
>
>
> One way:
> select n from unnest(ARRAY[0,1,2,3,4,5]) n order by random() limit 1;

somewhat shorter:
 SELECT (random() * 5)::INT


I'd rather use something like:

SELECT floor(random()*6)::INT

as this gives uniform results distribution.

Compare the two below queries, in the first you have twice less results for 0 and 5 than for the rest numbers.

WITH x AS (SELECT (random()*5)::INT r FROM generate_series(1,1000*1000)) SELECT r, count(*) FROM x GROUP BY r ORDER BY r ; 

WITH x AS (SELECT floor(random()*6)::INT r FROM generate_series(1,1000*1000)) SELECT r, count(*) FROM x GROUP BY r ORDER BY r ; 

regards
Szymon

Re: Random numbers

From
Karel Riveron Escobar
Date:
How would be if I would want to generate values among 3 and 5?

Regards, Karel Riverón
Student Scientific Council
Informatics Science University

From: "Szymon Guz" <mabewlun@gmail.com>
To: "Ian Lawrence Barwick" <barwick@gmail.com>
Cc: "bricklen" <bricklen@gmail.com>, "Karel Riveron Escobar" <kescobar@estudiantes.uci.cu>, pgsql-general@postgresql.org
Sent: Saturday, May 25, 2013 12:10:52 PM
Subject: Re: [GENERAL] Random numbers

On 25 May 2013 17:56, Ian Lawrence Barwick <barwick@gmail.com> wrote:
2013/5/26 bricklen <bricklen@gmail.com>:
>
> On Sat, May 25, 2013 at 8:45 AM, Karel Riveron Escobar
> <kescobar@estudiantes.uci.cu> wrote:
>>
>> I want to generate random numbers in Pl/pgSQL. How can I do this?
>> To be more specific, I have to generate random numbers among 0 and 5.
>
>
> One way:
> select n from unnest(ARRAY[0,1,2,3,4,5]) n order by random() limit 1;

somewhat shorter:
 SELECT (random() * 5)::INT


I'd rather use something like:

SELECT floor(random()*6)::INT

as this gives uniform results distribution.

Compare the two below queries, in the first you have twice less results for 0 and 5 than for the rest numbers.

WITH x AS (SELECT (random()*5)::INT r FROM generate_series(1,1000*1000)) SELECT r, count(*) FROM x GROUP BY r ORDER BY r ; 

WITH x AS (SELECT floor(random()*6)::INT r FROM generate_series(1,1000*1000)) SELECT r, count(*) FROM x GROUP BY r ORDER BY r ; 

regards
Szymon



Re: Random numbers

From
Szymon Guz
Date:



On 25 May 2013 18:14, Karel Riveron Escobar <kescobar@estudiantes.uci.cu> wrote:
How would be if I would want to generate values among 3 and 5?



Hi Karel,
try something like:

SELECT floor(3 + random()*(5-3+1))::INT

Or generally:

CREATE OR REPLACE FUNCTION
random_range(INTEGER, INTEGER) RETURNS INTEGER
AS $$   SELECT floor(($1 + ($2 - $1 + 1) * random()))::INTEGER;
$$ LANGUAGE SQL;
and then:
SELECT random_range(3,5);

regards
Szymon