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

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Thanks for the detailed answer!

Changing the function sinature seams to have solved the problem:
CREATE OR REPLACE FUNCTION ceilDecimal(num numeric) RETURNS float AS
$BODY$
BEGIN
RETURN CEIL(num * 100) / 100;
END
$BODY$
LANGUAGE 'plpgsql';

SELECT ceilDecimal(0.07);
ceildecimal
-------------
0.07

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elbriga wrote:

Thanks for the detailed answer!

Changing the function sinature seams to have solved the problem:
CREATE OR REPLACE FUNCTION ceilDecimal(num numeric) RETURNS float AS
$BODY$
BEGIN
RETURN CEIL(num * 100) / 100;
END
$BODY$
LANGUAGE 'plpgsql';

SELECT ceilDecimal(0.07);
ceildecimal
-------------
0.07

Yes, because "numeric" stores the exact number, so it will be
exactly 0.07 after conversion.

Yours,
Laurenz Albe

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