Nik Nik - 5 months ago 166
Python Question

Python: single colon vs double colon

What is the difference between single and double colon in this situation?

data[0:,4]
vs
data[0::,4]


women_only_stats = data[0::,4] == "female"

men_only_stats = data[0::,4] != "female"


I tried to replace
data[0::,4]
with
data[0:,4]
and I see no difference. Is there any difference in this or another case?

data
is 2-dimensional array with rows like
['1' '0' '3' 'Braund, Mr. Owen Harris' 'male' '22' '1' '0' 'A/5 21171' '7.25' '' 'S']

Answer

No, there is no difference.

See the Python documentation for slice:

From the docs: a[start:stop:step]

The start and step arguments default to None. Slice objects have read-only data attributes start, stop and step which merely return the argument values (or their default).

In this case, you are including an empty step parameter.

>>> a = [1,2,3,4]
>>> a[2:]
[3,4]
>>> a[2::]
[3,4]
>>> a[2:] == a[2::]
True

And to understand what the step parameter actually does:

>>> b = [1,2,3,4,5,6,7,8,9,10]
>>> b[0::5]
[1, 6]
>>> b[1::5]
[2, 7]

So by leaving it to be implicitly None (i.e., by either a[2:] or a[2::]), you are not going to change the output of your code in any way.

Hope this helps.