Re: Rounding Problems?

elbriga wrote:
> Hi,
> I have this pl function:
> CREATE OR REPLACE FUNCTION ceilDecimal(num float) RETURNS float AS
> $BODY$
> BEGIN
> RETURN CEIL(num * 100) / 100;
> END
> $BODY$
> LANGUAGE 'plpgsql';
>
> It is supposed to do a "decimail ceil" for 2 decimal places.
> But when I do "SELECT ceilDecimal(0.07)"
> It will return
> ceildecimal
> -------------
> 0.08
>
> For other numbers the result is as expected:
> SELECT ceilDecimal(0.17);
> ceildecimal
> -------------
> 0.17
>
> WHY? rsrsr

Let's do your calculation step by step.

100 is (implicitly) an integer value.

When a "real" and an "integer" are multiplied, PostgreSQL casts them to
"double precision" before the operation. That would be necessary anyway,
because "ceil()" only operates on "double precision" (or "numeric").

test=> SELECT CAST (REAL '0.07' AS double precision);
float8
--------------------
0.0700000002980232
(1 row)

The weird digits are because 0.07 can never represented exactly
as a floating point number (with base 2).
They become visible because "double precision" has greater precision.

test=> SELECT REAL '0.07' * 100;
?column?
------------------
7.00000002980232
(1 row)

test=> SELECT ceil(REAL '0.07' * 100);
ceil
------
8
(1 row)

The value is rounded up correctly, because it is greater than 7.

For some "real" values, the representation will be slightly less
then the correct value:

test=> SELECT CAST (REAL '0.47' AS double precision);
float8
-------------------
0.469999998807907
(1 row)

For such values, your function will work as you expect.

You could work around the problem by subtracting a small delta
from the value after converting it to "double precision".

Yours,
Laurenz Albe