Re: 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