Float value is equal -1.#IND

Question

A function returns a list which contains of float values. If I plot this list, I see that some of the float values are equal -1.#IND. I also checked the type of those -1.#IND values. And they are also of float type. But how can I understand this -1.#IND values? What do they represent or stand for?

Answer 1

-1.#IND means indefinite, the result of a floating point equation that doesn't have a solution. On other platforms, you'd get NaN instead, meaning 'not a number', -1.#IND is specific to Windows. On Python 2.5 on Linux I get:

>>> 1e300 * 1e300 * 0
-nan

You'll only find this on python versions 2.5 and before, on Windows platforms. The float() code was improved in python 2.6 and consistently uses float('nan') for such results; mostly because there was no way to turn 1.#INF and -1.#IND back into an actual float() instance again:

>>> repr(inf)
'1.#INF'
>>> float(_)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for float(): 1.#INF
>>> repr(nan)
'-1.#IND'
>>> float(_)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for float(): -1.#IND

On versions 2.6 and newer this has all been cleaned up and made consistent:

>>> 1e300 * 1e300 * 0
nan
>>> 1e300 * 1e300
inf
>>> 1e300 * 1e300 * -1
-inf