Bizarre arithmetic results
in [Python]
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From: Terrence Cole on 10 Feb 2010 19:44 Can someone explain to me what python is doing here? Python 3.1.1 (r311:74480, Feb 3 2010, 13:36:47) [GCC 4.3.4] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> -0.1 ** 0.1 -0.7943282347242815 >>> a = -0.1; b = 0.1 >>> a ** b (0.7554510437117542+0.2454609236416552j) >>> -abs(a ** b) -0.7943282347242815 Why does the literal version return the signed magnitude and the variable version return a complex? Cheers, Terrence
From: Mark Dickinson on 11 Feb 2010 07:27 On Feb 11, 12:44 am, Terrence Cole <list- s...(a)trainedmonkeystudios.org> wrote: > Can someone explain to me what python is doing here? > >>> -0.1 ** 0.1 > -0.7943282347242815 Here you're computing -(0.1 ** 0.1). The exponentiation operator binds more strongly than the negation operator. > >>> a = -0.1; b = 0.1 > >>> a ** b > (0.7554510437117542+0.2454609236416552j) Here you're computing (-0.1) ** 0.1. -- Mark
From: Andre Engels on 11 Feb 2010 07:26 On Thu, Feb 11, 2010 at 1:44 AM, Terrence Cole <list-sink(a)trainedmonkeystudios.org> wrote: > Can someone explain to me what python is doing here? > > Python 3.1.1 (r311:74480, Feb  3 2010, 13:36:47) > [GCC 4.3.4] on linux2 > Type "help", "copyright", "credits" or "license" for more information. >>>> -0.1 ** 0.1 > -0.7943282347242815 >>>> a = -0.1; b = 0.1 >>>> a ** b > (0.7554510437117542+0.2454609236416552j) >>>> -abs(a ** b) > -0.7943282347242815 > > Why does the literal version return the signed magnitude and the > variable version return a complex? It's an issue of precedence of operators: -0.1 ** 0.1 is interpreted as -(0.1 ** 0.1) and not as (-0.1) ** 0.1 -- André Engels, andreengels(a)gmail.com
From: Peter Otten on 11 Feb 2010 07:25 Terrence Cole wrote: > Can someone explain to me what python is doing here? > > Python 3.1.1 (r311:74480, Feb 3 2010, 13:36:47) > [GCC 4.3.4] on linux2 > Type "help", "copyright", "credits" or "license" for more information. >>>> -0.1 ** 0.1 > -0.7943282347242815 >>>> a = -0.1; b = 0.1 >>>> a ** b > (0.7554510437117542+0.2454609236416552j) >>>> -abs(a ** b) > -0.7943282347242815 > > Why does the literal version return the signed magnitude and the > variable version return a complex? Operator precedence. >>> -0.1**0.1 -0.7943282347242815 >>> (-0.1)**0.1 (0.7554510437117542+0.2454609236416552j) Quoting http://docs.python.org/3.1/reference/expressions.html: """ The power operator binds more tightly than unary operators on its left; it binds less tightly than unary operators on its right. """ Peter
From: Christian Heimes on 11 Feb 2010 07:33
Terrence Cole wrote: >>>> -0.1 ** 0.1 > -0.7943282347242815 >>>> a = -0.1; b = 0.1 >>>> a ** b > (0.7554510437117542+0.2454609236416552j) >>>> -abs(a ** b) > -0.7943282347242815 > > Why does the literal version return the signed magnitude and the > variable version return a complex? The binary power operator has a higher precedence than the unary negative operator. -0.1 ** 0.1 is equal to -(0.1**0.1) Christian |