NAN by IEEE-744 standard is not equal to any other float value, including other NAN, infinities etc. import math print(math.nan == math.nan) # False To check if a floating point value is NAN, you can use the isnan function: import math import numpy print(math.isnan(math.nan)) # True print(numpy.isnan(numpy.nan)) # True More on reddit.com

>>> np.full(3, np.nan) But the bigger question is why would you want to? Edit: as an explanation, your example does not work because you initialized an array of ints. ints have no "NaN" value, only floats do. So your method would work if you initialized an array of floats: >>> x = np.array([0.0,0.0,0.0]) >>> x.fill(np.nan) >>> x array([ nan, nan, nan]) Or converted the ints to floats: >>> x = np.array([0,0,0], dtype=np.float) >>> x.fill(np.nan) >>> x array([ nan, nan, nan]) But the np.full() method is much better. More on reddit.com

That's not what I get, which is TypeError: 'float' object is not subscriptable on the line where you assign to r. The result of m.e**(2*x) is a number, not a list and you can't use [] with a number. y[x+h] is nonsense. It looks like you're thinking of these assignment statements as if they were equations, but they're not. (That's not an uncommon mistake, because programming often borrows concepts and notation from mathematics, but not in this case.) For example, if you wrote y = x**4, that means "take the current value of x, raise is to the fourth power, and assign the result to y." It does not mean that y is now a function that can be called to raise any x to the fourth power, as would be the case if you were in a mathematics class and wrote y(x) = x4 on the board. If you want that, you need to explicitly define y as a function (i.e. with def or a lambda.) More on reddit.com

That's very interesting, because what you describe is the opposite of the expected behavior. >>> float('nan') == float('nan') False >>> math.isnan(float('nan')) True Show your code, please use a code block if posting on reddit. https://www.reddit.com/r/learnpython/wiki/faq#wiki_how_do_i_format_code.3F Or paste in pastebin.com and give a link here. More on reddit.com