decimal and trunkating

[Timothy Smith]

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?

[F. Petitjean]

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

[Reinhold Birkenfeld]

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("199.99 9")
d._round(5, decimal.ROUND_D OWN)

Reinhold

[Marc Christiansen]

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(nu m, 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. ;-)

AdiaÅ*, Marc

[Peter Hansen]

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
information about what you're doing):

d = decimal.Decimal ('199.999')
decimal.getcont ext().rounding = decimal.ROUND_F LOOR
d.quantize(deci mal.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.)

[Peter Hansen]

Reinhold Birkenfeld wrote:

He is speaking of Decimals...

d = Decimal("199.99 9")
d._round(5, decimal.ROUND_D OWN)

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

-Peter

[Peter Hansen]

Peter Hansen wrote:

>>> d = decimal.Decimal ('199.999')
>>> decimal.getcont ext().rounding = decimal.ROUND_F LOOR
>>> d.quantize(deci mal.Decimal('1. 00'))

Decimal("199.99 ")

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

d.quantize(Deci mal('1.00'), decimal.ROUND_F LOOR)

-Peter

[Reinhold Birkenfeld]

Peter Hansen wrote:

Reinhold Birkenfeld wrote:

He is speaking of Decimals...

d = Decimal("199.99 9")
d._round(5, decimal.ROUND_D OWN)

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

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

Reinhold

[Facundo Batista]

On 6/2/05, Peter Hansen <pe***@engcorp. com> wrote:

>>> d = decimal.Decimal ('199.999')
>>> decimal.getcont ext().rounding = decimal.ROUND_F LOOR
>>> d.quantize(deci mal.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.getcont ext().rounding = decimal.ROUND_F LOOR
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/

[Raymond Hettinger]

>> 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_de cimal('199.999' ) Decimal("199.99 ") Decimal('199.99 9').quantize(De cimal('0.01'), rounding=ROUND_ DOWN)

Decimal("199.99 ")
Raymond Hettinger