using list operator "in" with floating point values

Question

I have a list with floats, each number with 3 decimals (eg. 474.259). If I verify the number in the list like this:

if 474.259 in list_sample:
    print "something!"

Then the message is shown, but if I take the number from another list and I round it:

number = other_list[10]
number = round(number, 3)
if number == 474.259:
    print "the numbers are same!"
if number in list_sample:
    print "something!"

The second message is not shown.

Answer 1

Comparing floating point numbers for exact equality usually won't do what you want. This is because floating point numbers in computers have a representation (storage format) which is inherently inaccurate for many real numbers.

I suggest reading about it here: http://floating-point-gui.de/ and doing something like a "fuzzy compare" using an "epsilon" tolerance value to consider the numbers equal so long as they differ by less than x% or whatever.