python numpy arange unexpected results

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 version 2.7

Answer 1

I'm guessing that you're seeing the effects of floating point rounding.

numpy.arange does the same thing as python's range: It doesn't include the "endpoint". (e.g. range(0, 4, 2) will yield [0,2] instead of [0,2,4])

However, for floating point steps, the rounding errors are accumulate, and occasionally the last value will actually include the endpoint.

As noted in the documentation for arange:

When using a non-integer step, such as 0.1, the results will often not be consistent. It is better to use linspace for these cases.

numpy.linspace generates a specified number of points between a starting and ending point. Incidentally, it does include the endpoints by default.