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
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