comparing nan "number"

[Guest]

Hello !

I need to know if the result of math formula is nan (Not a number).
How can I do that?

Thanks,

B.

 

[Georgy Pruss]

Hello !

I need to know if the result of math formula is nan (Not a number).
How can I do that?

Thanks,

B.

I used repr() as a workaround:
.... rx = repr(float(x))
.... return rx==repr(NAN) or rx==repr(-NAN)
....True

You can also use binary representation of numbers:
.... xs = struct.pack('d',float(x))
.... return xs==NAN_POS or xs==NAN_NEG
....


HTH

 

[Alexander Schmolck]

Hello !

I need to know if the result of math formula is nan (Not a number).
How can I do that?

The only non-hacky way I know of is installing scipy and using scipy.isnan.

'as

 

[John J. Lee]

I need to know if the result of math formula is nan (Not a number).
How can I do that?

I suppose:

NAN = any_calculation_that_returns_nan()

if some_other_calculation() is NAN:
...


John

 

[Michael Hudson]

I suppose:

NAN = any_calculation_that_returns_nan()

if some_other_calculation() is NAN:
...

I wouldn't expect that to work. Unfortunately, I don't think there is
a reliable way.

result = some_calculation()
if result != result:
# it's a nan

*might* work, depending probably on compiler, Python version, platform
and phase of the moon.

Cheers,
mwh

 

[Alexander Schmolck]

result = some_calculation()
if result != result:
# it's a nan

*might* work, depending probably on compiler, Python version, platform
and phase of the moon.

Since this gives different results for two recent python versions installed on
my machine, I'd rather recommend against it if you value your sanity. IMO it's
either the repr hack or scipy, unlike you like to live dangerously.

[In a similar vain, I'd also recommend resisting the temptation of putting
nans (or richly comparing types) into containers like lists etc. because
that's just asking for undefined behavior. E.g. ``x in [x,x,x]`` will quite
possibly be `False` and {x:"some value}[x] might well raise an exception if
`x` is a nan (or indeed one of infinitely many Numeric arrays with
non-symmetric `==`) . Somehow all this suggests to me that too many things are
folded into python's __eq__.]


'as

 

[John J. Lee]

(e-mail address removed) (John J. Lee) writes: [...]

I suppose:

NAN = any_calculation_that_returns_nan()

if some_other_calculation() is NAN:
...

I wouldn't expect that to work. Unfortunately, I don't think there is
a reliable way.

[...]

Yeah, I forgot nan is a floating point value, not a special Python
thingy like None, duh (thanks Alexander).


John

 

[John J. Lee]

possibly be `False` and {x:"some value}[x] might well raise an exception if
`x` is a nan (or indeed one of infinitely many Numeric arrays with

[...]

Don't you mean SomeMapping((x, "some value"))[x]? A dict lookup
doesn't use __eq__.


John

 

[Michael Hudson]

possibly be `False` and {x:"some value}[x] might well raise an exception if
`x` is a nan (or indeed one of infinitely many Numeric arrays with

[...]

Don't you mean SomeMapping((x, "some value"))[x]? A dict lookup
doesn't use __eq__.

Huh? Yes it does (*after* __hash__, of course, but...).

Cheers,
mwh

 

[John J. Lee]

possibly be `False` and {x:"some value}[x] might well raise an exception if
`x` is a nan (or indeed one of infinitely many Numeric arrays with

[...]

Don't you mean SomeMapping((x, "some value"))[x]? A dict lookup
doesn't use __eq__.

Huh? Yes it does (*after* __hash__, of course, but...).

Oh, collisions, right?


John

 

[Michael Hudson]

[...]
possibly be `False` and {x:"some value}[x] might well raise an exception if
`x` is a nan (or indeed one of infinitely many Numeric arrays with
[...]

Don't you mean SomeMapping((x, "some value"))[x]? A dict lookup
doesn't use __eq__.

Huh? Yes it does (*after* __hash__, of course, but...).

Oh, collisions, right?

Yes. You can't know a collision has (or has not) happened until after
you've tried __eq__...

Cheers,
mwh