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From: Avishay Zilka on 20 May 2010 02:25
Hi,
this is my first message here, be nice :)

I've read here some posts about rounding issues, but didn't find the exact thing I'm looking for:
I need to write a function that rounds a number to have d digits in its mantisaa.
For example:
if d=3, then: 123.4 --> 123, 1.567 --> 1.57, -45.81 --> -45.8
I know how to round to d digits after the floating point, but this doesn't work on 123.4 for example, as I need to cut the whole floating part there...

Please help :)
Thanks!
Avishay
From: Greg Heath on 20 May 2010 07:00
On May 20, 2:25 am, "Avishay Zilka" <avis...(a)gmail.com> wrote:
> Hi,
> this is my first message here, be nice :)
>
> I've read here some posts about rounding issues, but didn't find the exact thing I'm looking for:
> I need to write a function that rounds a number to have d digits in its mantisaa.
> For example:
> if d=3, then: 123.4 --> 123, 1.567 --> 1.57, -45.81 --> -45.8
> I know how to round to d digits after the floating point, but this doesn't work on 123.4 for example, as I need to cut the whole floating part there....
>
> Please help :)

a = [123.4 1.567 -45.81]
b = 10.^(2-floor(log10(abs(a))))
c = sign(a).*round(abs(a).*b)./b

Hope this helps.

Greg
From: Walter Roberson on 20 May 2010 10:25
Avishay Zilka wrote:

> I need to write a function that rounds a number to have d digits in its
> mantisaa.
> For example:
> if d=3, then: 123.4 --> 123, 1.567 --> 1.57, -45.81 --> -45.8
> I know how to round to d digits after the floating point, but this
> doesn't work on 123.4 for example, as I need to cut the whole floating
> part there...

Sorry, rounding 1.567 to 1.57 is not possible in any current Matlab
version, not unless you are using the Fixed-point toolbox or the
Symbolic Toolbox.

The problem is that 1.57 is not exactly representable in finite binary
floating point arithmetic, similar to the same way that Pi/2 is not
exactly representable in finite decimal arithmetic.
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