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# decimal and trunkating

 P: n/a 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
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 P: n/a 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

 P: n/a 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

 P: n/a 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? 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. ;-) AdiaÅ*, Marc Jul 19 '05 #4

 P: n/a 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.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

 P: n/a 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

 P: n/a 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

 P: n/a 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

 P: n/a On 6/2/05, Peter Hansen 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

 P: n/a >> 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

 P: n/a "Reinhold Birkenfeld" 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

 P: n/a 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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