Thread: evaluating equation stored in a string

I have equations stored in strings and I would need to evaluate them in
pgsql - get the result.
Maybe there is an eval() function but I couldn't find it.

The only way I found was to use something like this:

calcstring_arg:= 'SELECT ' || calcstring_arg || ' ::numeric AS
outparam';
FOR tmp IN EXECUTE calcstring_arg LOOP END LOOP;
RETURN tmp.outparam;

However I have a problem with this cast. I don't know how to handle
brackets in the equation, since

SELECT 35/124 ::numeric
--0.28....

while

SELECT (35/124) ::numeric
-- 0

so I should do something like

SELECT (35::numeric/124::numeric)
--0.28....

But than I would need to start to look inside the string and do
manipulation to it that I would like to avoid. How could I evaluate
these strings independently from whether there are brackets in them or
not?

Thanks for the help.
Balázs

Hi :

 How about creating a view that converts integer into
numeric?

create view <name> as
select <col1>::numeric as <col1>,
select <col2>::numeric as <col2>
...

and then query the view instead. This way your formula
doesn't need to use cast anymore.

ludwig lim


--- SunWuKung <Balazs.Klein@t-online.hu> wrote:

> I have equations stored in strings and I would need to
> evaluate them in
> pgsql - get the result.
> Maybe there is an eval() function but I couldn't find it.
>
> The only way I found was to use something like this:
>
> calcstring_arg:= 'SELECT ' || calcstring_arg || '
> ::numeric AS
> outparam';
> FOR tmp IN EXECUTE calcstring_arg LOOP END LOOP;
> RETURN tmp.outparam;
>
> However I have a problem with this cast. I don't know how
> to handle
> brackets in the equation, since
>
> SELECT 35/124 ::numeric
> --0.28....
>
> while
>
> SELECT (35/124) ::numeric
> -- 0
>
> so I should do something like
>
> SELECT (35::numeric/124::numeric)
> --0.28....
>
> But than I would need to start to look inside the string
> and do
> manipulation to it that I would like to avoid. How could
> I evaluate
> these strings independently from whether there are
> brackets in them or
> not?
>
> Thanks for the help.
> Balázs
>
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
>
>                http://www.postgresql.org/docs/faq
>


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SunWuKung wrote:
> However I have a problem with this cast. I don't know how to handle
> brackets in the equation, since
>
> SELECT 35/124 ::numeric
> --0.28....
>
> while
>
> SELECT (35/124) ::numeric
> -- 0

The brackets aren't your problem, your numbers aren't as accurate as the
result you require. An eval() function would give the same results.
You'll want this instead:

SQL> select (35.0/124.0)::numeric;
         numeric
------------------------
  0.28225806451612903226


Regards,
--
Alban Hertroys
alban@magproductions.nl

magproductions b.v.

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