Range with floating point numbers and negative steps

Question

I wrote the following to create a range with negative floating point steps:

def myRange(start, stop, step):
    s = start
    if step < 0:
        while s > stop:
            yield s
            s += step
    if step > 0:
        while s < stop:
            yield s
            s += step

But the conclusion r = myRange(1,0,-0.1)

looks pretty weird

>>> r = myRange(1,0,-0.1)
>>> for n in r: print n
... 
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1.38777878078e-16

where is the last number from? And why is it not 0?

Answer 1

Not all floating point numbers can be represented exactly. For example, this is the result with Python 3.5:

1
0.9
0.8
0.7000000000000001
0.6000000000000001
0.5000000000000001
0.40000000000000013
0.30000000000000016
0.20000000000000015
0.10000000000000014
1.3877787807814457e-16

One solution can be rounded:

def myRange(start, stop, step):
    s = start
    if step < 0:
        while s > stop:
            yield s
            s += step
            s = round(s, 15)
    if step > 0:
        while s < stop:
            yield s
            s += step
            s = round(s, 15)

r = myRange(1,0,-0.1)
for n in r: 
    print(n)

Output:

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0