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Re: Hex to Dec Conversion

From: "Donald Kerr" <donald(dot)kerr(at)dkerr(dot)co(dot)uk>
To: "'Josh Kupershmidt'" <schmiddy(at)gmail(dot)com>
Cc: <pgsql-novice(at)postgresql(dot)org>
Subject: Re: Hex to Dec Conversion
Date: 2010-10-20 17:08:30
Message-ID: A178A369C8334C29A1365310C41B58DD@DELLM4500 (view raw or flat)
Thread:
Lists: pgsql-novice
-----Original Message-----
From: pgsql-novice-owner(at)postgresql(dot)org
[mailto:pgsql-novice-owner(at)postgresql(dot)org] On Behalf Of Josh Kupershmidt
Sent: 19 October 2010 01:36
To: Donald Kerr
Cc: pgsql-novice(at)postgresql(dot)org
Subject: Re: [NOVICE] Hex to Dec Conversion


On Mon, Oct 18, 2010 at 5:47 PM, Donald Kerr <donald(dot)kerr(at)dkerr(dot)co(dot)uk>
wrote:
> My first post to the mailing list and I hope I am in the right place!
>
> I am trying to convert from hex to decimal and can do that 
> successfully using the following code:
>
> SELECT x'FF'::integer;
>
> which outputs 255 and is exactly what I want.
>
> I want to substitute the string in the code 'FF' for a column in the 
> database like so:
>
> SELECT x'db_column'::integer FROM db_table;
>
> but no matter the combinations I try, I cannot get it to work.
>
> Thew data colum contains html color codes like "0099FF" and I want to 
> convert these to, in this case, "0 153 255".
>
> The following code behaves well:
>
> SELECT  x'00'::integer || ' ' || x'99'::integer || ' ' ||  
> x'FF'::integer;
>
> resulting in "0 153 255". All correct
>
> I was hopeful that something similar to the following would work but I 
> just cannot get it to work despite trying various combinations.
>
> SELECT x'substring(col,1,2)'::integer || ' ' || 
> x'substring(col,3,2)'::integer || ' ' || 
> x'substring(col,5,2)'::integer
>
> I would much prefer to do this as part of the query rather than having 
> to create a function. There must be a way! :)
>
> Any help would be very greatly apprecaited.

Hrmph. I took a look at this and couldn't figure out how to make it work
without declaring a PL/pgSQL function, so that I could construct a dynamic
query using EXECUTE. I googled around a bit, and found a few people having
roughly your same problem: I think this comes from Postgres not having a
counterpart to its built-in to_hex() function.

Anyway, I adapted the function from here
<http://archives.postgresql.org/pgsql-general/2004-05/msg00923.php> to use
more modern function syntax, and came up with this:

CREATE OR REPLACE FUNCTION hex_to_int(hexval varchar) RETURNS integer AS $$
DECLARE
   result  int;
BEGIN
 EXECUTE 'SELECT x''' || hexval || '''::int' INTO result;  RETURN result;
END; $$ LANGUAGE 'plpgsql' IMMUTABLE STRICT;

which you should be able to use like this:

SELECT hex_to_int(substring(color,1,2)) AS first,
               hex_to_int(substring(color,3,2)) AS second,
               hex_to_int(substring(color, 5,2)) AS third
FROM colors;

 first | second | third
-------+--------+-------
     0 |    153 |   255
(1 row)

I know it's not exactly what you were looking for, but IMO the cleanest way
to do this anyway would be to make a wrapper function like
"html_color_to_int()" that would handle all this conversion for you in one
place.

Josh


-----Original Message-----
From: pgsql-novice-owner(at)postgresql(dot)org
[mailto:pgsql-novice-owner(at)postgresql(dot)org] On Behalf Of Josh Kupershmidt
Sent: 19 October 2010 01:36
To: Donald Kerr
Cc: pgsql-novice(at)postgresql(dot)org
Subject: Re: [NOVICE] Hex to Dec Conversion


On Mon, Oct 18, 2010 at 5:47 PM, Donald Kerr <donald(dot)kerr(at)dkerr(dot)co(dot)uk>
wrote:
> My first post to the mailing list and I hope I am in the right place!
>
> I am trying to convert from hex to decimal and can do that 
> successfully using the following code:
>
> SELECT x'FF'::integer;
>
> which outputs 255 and is exactly what I want.
>
> I want to substitute the string in the code 'FF' for a column in the 
> database like so:
>
> SELECT x'db_column'::integer FROM db_table;
>
> but no matter the combinations I try, I cannot get it to work.
>
> Thew data colum contains html color codes like "0099FF" and I want to 
> convert these to, in this case, "0 153 255".
>
> The following code behaves well:
>
> SELECT  x'00'::integer || ' ' || x'99'::integer || ' ' ||  
> x'FF'::integer;
>
> resulting in "0 153 255". All correct
>
> I was hopeful that something similar to the following would work but I 
> just cannot get it to work despite trying various combinations.
>
> SELECT x'substring(col,1,2)'::integer || ' ' || 
> x'substring(col,3,2)'::integer || ' ' || 
> x'substring(col,5,2)'::integer
>
> I would much prefer to do this as part of the query rather than having 
> to create a function. There must be a way! :)
>
> Any help would be very greatly apprecaited.

Hrmph. I took a look at this and couldn't figure out how to make it work
without declaring a PL/pgSQL function, so that I could construct a dynamic
query using EXECUTE. I googled around a bit, and found a few people having
roughly your same problem: I think this comes from Postgres not having a
counterpart to its built-in to_hex() function.

Anyway, I adapted the function from here
<http://archives.postgresql.org/pgsql-general/2004-05/msg00923.php> to use
more modern function syntax, and came up with this:

CREATE OR REPLACE FUNCTION hex_to_int(hexval varchar) RETURNS integer AS $$
DECLARE
   result  int;
BEGIN
 EXECUTE 'SELECT x''' || hexval || '''::int' INTO result;  RETURN result;
END; $$ LANGUAGE 'plpgsql' IMMUTABLE STRICT;

which you should be able to use like this:

SELECT hex_to_int(substring(color,1,2)) AS first,
               hex_to_int(substring(color,3,2)) AS second,
               hex_to_int(substring(color, 5,2)) AS third
FROM colors;

 first | second | third
-------+--------+-------
     0 |    153 |   255
(1 row)

I know it's not exactly what you were looking for, but IMO the cleanest way
to do this anyway would be to make a wrapper function like
"html_color_to_int()" that would handle all this conversion for you in one
place.

Josh

------------------------------------------------------

Thank you for your reply, Josh.

I was rather hoping not to have to call a function but it seems like it
might be the only way. I am actually using the query in a MapServer map file
and I think I may only be able to use one line of code i.e. I have to get
everything I need from the single query. I am assuming that I can add a
function to PostgreSQL so that it is available globally to any query and I
will have to do a wee bit of research to see if that is the case.

If anyone knows how to make a suggestion for inclusion in future PostgreSQL
releases then I think that a Hex_to_Int function such as
x'db_column'::integer, where db_column can be replaced by a variable, would
be a very good idea.

Many thanks once again.

Regards,

Donald


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