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# decimal and trunkating

i want to trunkate 199.999 to 199.99
getcontext.prec = 2 isn't what i'm after either, all that does is E's
the value.
do i really have to use floats to do this?
Jul 19 '05 #1
11 2918
Le Thu, 02 Jun 2005 19:59:08 +1000, Timothy Smith a écrit :
i want to trunkate 199.999 to 199.99
round(199.999, 2) # 2 digits after the decimal point
do i really have to use floats to do this?

19.999 is a float :
type(19.999) is float # ==> True
Jul 19 '05 #2
F. Petitjean wrote:
Le Thu, 02 Jun 2005 19:59:08 +1000, Timothy Smith a écrit :
i want to trunkate 199.999 to 199.99

round(199.999, 2) # 2 digits after the decimal point

Wrong. This will yield 200.00.
do i really have to use floats to do this?

19.999 is a float :
type(19.999) is float # ==> True

He is speaking of Decimals...

d = Decimal("199.999")
d._round(5, decimal.ROUND_DOWN)

Reinhold
Jul 19 '05 #3
Timothy Smith <ti*****@open-networks.net> wrote:
i want to trunkate 199.999 to 199.99
getcontext.prec = 2 isn't what i'm after either, all that does is E's
the value.
do i really have to use floats to do this?

You could try this (from a script I use for my phone bill):

from decimal import Decimal as d

def roundDecimal(num, prec):
return d(num).quantize(d("1e%d" % (-prec)))

where `prec` is the number of places after the decimal point.

I'm sure there is a better solutions and someone will tell it, thereby
teaching us both. ;-)

Jul 19 '05 #4
Timothy Smith wrote:
i want to trunkate 199.999 to 199.99
getcontext.prec = 2 isn't what i'm after either, all that does is E's
the value.
do i really have to use floats to do this?

I think you need a context with appropriate rounding set (e.g.
ROUND_FLOOR?) and then use the quantize() method with an argument with
the appropriate number of decimal places.

For example, this works, though I'm definitely not a Decimal expert and
am confident there's a more elegant approach (which might depend on more
d = decimal.Decimal('199.999')
decimal.getcontext().rounding = decimal.ROUND_FLOOR
d.quantize(decimal.Decimal('1.00'))

Decimal("199.99")

-Peter

(I hope this inspires someone who actually knows what he's doing with
Decimal to post an improved solution.)
Jul 19 '05 #5
Reinhold Birkenfeld wrote:
He is speaking of Decimals...

d = Decimal("199.999")
d._round(5, decimal.ROUND_DOWN)

Is one really supposed to call the underscore methods like that?

-Peter
Jul 19 '05 #6
Peter Hansen wrote:
>>> d = decimal.Decimal('199.999')
>>> decimal.getcontext().rounding = decimal.ROUND_FLOOR
>>> d.quantize(decimal.Decimal('1.00'))

Decimal("199.99")

Or skip changing the context and use the second argument to quantize:

d.quantize(Decimal('1.00'), decimal.ROUND_FLOOR)

-Peter
Jul 19 '05 #7
Peter Hansen wrote:
Reinhold Birkenfeld wrote:
He is speaking of Decimals...

d = Decimal("199.999")
d._round(5, decimal.ROUND_DOWN)

Is one really supposed to call the underscore methods like that?

Umm... no, I think not ;) But I couldn't find something better.

Reinhold
Jul 19 '05 #8
On 6/2/05, Peter Hansen <pe***@engcorp.com> wrote:

>>> d = decimal.Decimal('199.999')
>>> decimal.getcontext().rounding = decimal.ROUND_FLOOR
>>> d.quantize(decimal.Decimal('1.00')) Decimal("199.99")

-Peter

(I hope this inspires someone who actually knows what he's doing with
Decimal to post an improved solution.)

This is the right solution, but take in consideration that normally
you'll use one rounding method and you'll round to always the same
places, so, at the beggining of your program you'll do:
import decimal
decimal.getcontext().rounding = decimal.ROUND_FLOOR
d2 = decimal.Decimal("0.01")
and each time you want to round....
d = decimal.Decimal('199.999')
d.quantize(d2)

Decimal("199.99")

So it's not really that ugly....

.. Facundo

Blog: http://www.taniquetil.com.ar/plog/
PyAr: http://www.python.org/ar/
Jul 19 '05 #9
>> i want to trunkate 199.999 to 199.99
getcontext.prec = 2 isn't what i'm after either, all that does
is E's the value. do i really have to use floats to do this?
The precision is the total number of digits (i.e 199.99 has 5 digit
precision). Either round to that precision level or use the quantize
method to round to a fixed number of places after the decimal point:

Context(prec=5, rounding=ROUND_DOWN).create_decimal('199.999') Decimal("199.99") Decimal('199.999').quantize(Decimal('0.01'), rounding=ROUND_DOWN)

Decimal("199.99")
Raymond Hettinger

Jul 19 '05 #10

"Reinhold Birkenfeld" <re************************@wolke7.net> wrote in
message news:3g************@individual.net...
Peter Hansen wrote:
Reinhold Birkenfeld wrote:
He is speaking of Decimals...

d = Decimal("199.999")
d._round(5, decimal.ROUND_DOWN)

Is one really supposed to call the underscore methods like that?

Umm... no, I think not ;) But I couldn't find something better.

Reinhold

I'm new to Python ... and I've used decimal._round() as above. What's the
deal with using underscore methods? (A link will do if that'll save you some
typing).

Jul 19 '05 #11
chris wrote:
I'm new to Python ... and I've used decimal._round() as above. What's the
deal with using underscore methods? (A link will do if that'll save you some
typing).

Generally the underscore methods provide *internal* functionality that
might be used by other, more externally accessible (i.e. documented!)
methods in the object. While as I've said I'm no expert in Decimal and
can't say how _round() is intended to be used, it is not documented (as
far as I can see) and certainly therefore follows this way of thinking
about underscore methods. Several of us have found at least one
suitable alternative (i.e. quantize()) that don't rely on underscore
methods.

(Think of the underscore as being a non-binding convention that says
"don't use this externally if possible, as it doesn't form part of the
contract guaranteed by this object... it may be removed in the future,
may not work exactly as you wish, may have side effects that aren't
documented or haven't been analyzed fully when used externally, etc.")

-Peter
Jul 19 '05 #12

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