Question

I am using the arange function to define my for loop iterations and getting unexpected results.

i = arange(7.8, 8.4, 0.05)
print i

yeilds the following:

[ 7.8   7.85  7.9   7.95  8.    8.05  8.1   8.15  8.2   8.25  8.3   8.35 8.4 ]

yet using the stop value of 8.35 as follows

i = arange(7.8, 8.35, 0.05)

yields the following

[ 7.8   7.85  7.9   7.95  8.    8.05  8.1   8.15  8.2   8.25  8.3 ]

But I want my range to end at 8.35! I know I can use the stop value of > 8.35 and < 8.4 to achieve my result, but why is it different and in my mind, inconsistent?

Edit: I am using Python 2.7

Answer 1

Perhaps it has to do with limitations on floating point numbers. Due to machine precision, it is not possible to store every conceivable value perfectly as a floating point. For example:

>>> 8.4
8.4000000000000004
>>> 8.35
8.3499999999999996

So, 8.4 as a floating point is slightly greater than the actual value of 8.4, while 8.35 as a floating point is a tiny bit less.