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Re: Floating point calculation problem

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On Sun, Feb 3, 2013 at 2:48 AM, Schizoid Man <schiz_man@21stcentury.com> wrote:
> def Numer(s, k):
>    return math.log(s / k)
>
> s = float(input("Enter s:   "))
> k = float(input("Enter k:   "))
> print("Result: ", Numer(s, k))
>
> For the v2.7 version, the only difference is the input lines:
>
> s = input("Enter s:   ")
> k = input("Enter k:   ")
>
> So for values of s=60 and k=50, the first code returns 0.1823215567939546
> (on the PC), whereas the second returns 0.0 (on the Mac). This this
> expression is evaluated in the numerator, it never returns a divide by zero
> error, and the result of 0 is treated as legitimate.

So on your Python 2 install, you're working with integers, dividing
one by another, and getting back a value of 1 - and log(1) is 0. The
problem is your division operator, which can be fixed as Steven
suggests, with the future directive.

> However, if I cast the v2.7 inputs as float() then it does indeed return the
> right result. But what threw me was that no cast didn't result in a runtime
> error in 2.7, but did in 3.3.
>
> Also, if the cast is necessary, then now exactly does the dynamic typing
> work?

It's not quite a cast. Python's type system is broadly this: Every
object (value) has a type, every name (variable, sort of) doesn't. So
I can do this:

spam = "hello, world" # spam is a string
spam = 5 # spam is now an int
spam = (1,2,3) # spam is now a tuple
spam = object() # you get the idea

I strongly recommend you either go with Python 3 or raw_input, but NOT
with float(input()) in Py2. Go with float(raw_input()) and you're
doing exactly one translation, and the correct one.

ChrisA

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Thread

Floating point calculation problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-02 10:27 +0000
  Re: Floating point calculation problem Chris Angelico <rosuav@gmail.com> - 2013-02-02 21:34 +1100
    Re: Floating point calculation problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-02 11:14 +0000
      Re: Floating point calculation problem Chris Angelico <rosuav@gmail.com> - 2013-02-02 22:20 +1100
        Re: Floating point calculation problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-02 11:34 +0000
          Re: Floating point calculation problem Chris Angelico <rosuav@gmail.com> - 2013-02-02 22:38 +1100
            Re: Floating point calculation problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-02 11:51 +0000
              Re: Floating point calculation problem Chris Angelico <rosuav@gmail.com> - 2013-02-02 22:58 +1100
                Re: Floating point calculation problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-02 15:48 +0000
                Re: Floating point calculation problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-02 15:54 +0000
                Re: Floating point calculation problem Chris Angelico <rosuav@gmail.com> - 2013-02-03 03:00 +1100
                Re: Floating point calculation problem Michael Torrie <torriem@gmail.com> - 2013-02-02 11:19 -0700
                Re: Floating point calculation problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-03 12:25 +1100
                Re: Floating point calculation problem Dave Angel <d@davea.name> - 2013-02-02 21:20 -0500
                Re: Floating point calculation problem Michael Torrie <torriem@gmail.com> - 2013-02-02 21:22 -0700
              Re: Floating point calculation problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-02 23:11 +1100
      Re: Floating point calculation problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-02 22:45 +1100
        Re: Floating point calculation problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-02 11:50 +0000
  Re: Floating point calculation problem Chris Rebert <clp2@rebertia.com> - 2013-02-02 02:47 -0800
  Re: Floating point calculation problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-02 23:05 +1100

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