Short-circuit Logic
Ahmed Abdulshafy
abdulshafy at gmail.com
Mon May 27 16:11:28 EDT 2013
On Sunday, May 26, 2013 2:13:47 PM UTC+2, Steven D'Aprano wrote:
> On Sun, 26 May 2013 04:11:56 -0700, Ahmed Abdulshafy wrote:
>
>
>
> > Hi,
>
> > I'm having a hard time wrapping my head around short-circuit logic
>
> > that's used by Python, coming from a C/C++ background; so I don't
>
> > understand why the following condition is written this way!
>
> >
>
> > if not allow_zero and abs(x) < sys.float_info.epsilon:
>
> > print("zero is not allowed")
>
>
>
> Follow the logic.
>
>
>
> If allow_zero is a true value, then "not allow_zero" is False, and the
>
> "and" clause cannot evaluate to true. (False and X is always False.) So
>
> print is not called.
>
>
>
> If allow_zero is a false value, then "not allow_zero" is True, and the
>
> "and" clause depends on the second argument. (True and X is always X.) So
>
> abs(x) < sys.float_info.epsilon is tested, and if that is True, print is
>
> called.
>
>
>
> By the way, I don't think much of this logic. Values smaller than epsilon
>
> are not necessarily zero:
>
>
>
> py> import sys
>
> py> epsilon = sys.float_info.epsilon
>
> py> x = epsilon/10000
>
> py> x == 0
>
> False
>
> py> x * 3 == 0
>
> False
>
> py> x + epsilon == 0
>
> False
>
> py> x + epsilon == epsilon
>
> False
>
>
>
> The above logic throws away many perfectly good numbers and treats them
>
> as zero even though they aren't.
>
>
>
>
>
> > The purpose of this snippet is to print the given line when allow_zero
>
> > is False and x is 0.
>
>
>
> Then the snippet utterly fails at that, since it prints the line for many
>
> values of x which can be distinguished from zero. The way to test whether
>
> x equals zero is:
>
>
>
> x == 0
>
>
>
> What the above actually tests for is whether x is so small that (1.0+x)
>
> cannot be distinguished from 1.0, which is not the same thing. It is also
>
> quite arbitrary. Why 1.0? Why not (0.0001+x)? Or (0.00000001+x)? Or
>
> (10000.0+x)?
>
>
>
>
>
>
>
> --
>
> Steven
That may be true for integers, but for floats, testing for equality is not always precise
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