Some strange behavior

6 * 0.2 CTRL-P 1.2
1.2 - 1 CTRL-P 0.2
6 * 0.2 - 1 CTRL-P 0.2000000000000002

Squeak 4.1 #9957

bug or not?

Mateusz Grotek pisze:
> 6 * 0.2 CTRL-P 1.2
> 1.2 - 1 CTRL-P 0.2
> 6 * 0.2 - 1 CTRL-P 0.2000000000000002
>
> Squeak 4.1 #9957
>
> bug or not?
>
>
oops, ALT-P of course...

On 28.10.2010, at 21:59, Mateusz Grotek wrote:

> 6 * 0.2 CTRL-P 1.2
> 1.2 - 1 CTRL-P 0.2
> 6 * 0.2 - 1 CTRL-P 0.2000000000000002
>
> Squeak 4.1 #9957
>
> bug or not?

Not.

- Bert -

On Thu, 28 Oct 2010, Bert Freudenberg wrote:

You can find the explanation here: http://floating-point-gui.de/ . If you
really need exact results, then you can use Fractions:

6 * (1/5) - 1 "===> (1/5)"


Levente

>
> - Bert -
>
>
>

Am 28.10.2010 21:59, schrieb Mateusz Grotek: Not a bug. Welcome to the wonders of binary floating point arithmetic.
Binary float representation of decimals are inexact. The bit pattern you
get for 0.2 is different from the one that you get for 6 * 0.2 - 1.
You can look at the bit patterns by using the message #hex
(6 * 0.2) hex
(1.2 - 1) hex
(6 * 0.2 - 1) hex

This has nothing to do with Squeak, see the following C code for a
similar  effect:

#include <stdio.h>

main() {
    printf("%1.16f\n", 6 * 0.2);
    printf("%1.16f\n", 1.2 - 1);
    printf("%1.16f\n", 6 * 0.2 - 1);
}

The fact that Squeak prints 6 * 0.2 as 1.2 and not as 1.2000000000000002
is a result of the particular algorithm that is being used for printing
floats as decimal numbers.

Cheers,
Hans-Martin

On 28.10.2010, at 22:30, Hans-Martin Mosner wrote:

> The fact that Squeak prints 6 * 0.2 as 1.2 and not as 1.2000000000000002
> is a result of the particular algorithm that is being used for printing
> floats as decimal numbers.

(6 * 0.2) storeString

==> '1.2000000000000002'

- Bert -

> The fact that Squeak prints 6 * 0.2 as 1.2 and not as 1.2000000000000002
> is a result of the particular algorithm that is being used for printing
> floats as decimal numbers.
>
> Cheers,
> Hans-Martin
>
>
Yes, that was something I wondered. Thanks.

> (6 * 0.2) storeString
>
> ==> '1.2000000000000002'
>
> - Bert -
>

Yup, I should have check it first. Thanks.

On 10/28/10 2:46 PM, Mateusz Grotek wrote: The moral is: don't use floating point math to do accounting...


Lawson

Hi,

On 29 October 2010 02:57, Lawson English <[hidden email]> wrote:
> The moral is: don't use floating point math to do accounting...

believe it or not, I once received the most hilarious invoice. The
relevant part is attached. Note that this is a snippet taken out of a
real invoice I got in a box with some article I had ordered. *This is
real.* (Need to emphasise.)

See attachment. The text at the bottom says "amount remaining to be
paid", and the negative number suggests they owe me. ;-)

Well, I sent them a friendly e-mail, thanking them for their kindness
- after all they actually admitted they would take care of me and my
family for some time. Very generous. They didn't respond to my
question whether they would rather pay all at once or by installments.
Huh.

Best,

Michael

2010/10/28 Bert Freudenberg <[hidden email]>:
I would add that Squeak4.1 now gives your some insights about what is going on.
You can try to evaluate these expressions:
   (1/5) = 0.2.
   (1/5) asFloat = 0.2.
   0.2 asTrueFraction.

Nicolas