I'm new to Python, the following output I'm getting from a simple list slice operation confused the jebuse out of me.
Here is the code.
>>> a = [1,2,3,4];
>>> a[1:3]
[2, 3]
>>> a[3]
4
shouldn't a[1:3] returns [2,3,4] instead of [2,3]?
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4 Answers
a[1:3] specifies a half-closed interval, which means it includes the values starting at the 1st specified index up to, but not including, at the 2nd index.
So in this case a[1:3] means the slice includes a[1] and a[2], but not a[3]
You see the same in the use of the range() function. For instance
range(1, 5)
will generate a list from 1 to 4, but will not include 5.
This is pretty consistent with how things are done in many programming languages.
3 Comments
darkphoenix
thanks for editing it to make to make it clearer, though i don't know what you mean by 'a half interval'
Levon
@darkphoenix half-closed interval means that the range includes only one of the values, and not the other. In math notation we'd write this as
[1, 5) to mean 1, 2, 3, 4, whereas [1, 5] is 1, 2, 3, 4, 5 (if I remember my math notation correctly .. it's been a while :) .. see also this: mathworld.wolfram.com/Half-ClosedInterval.html
ddzialak
For details: en.wikipedia.org/wiki/…
Slicing returns up to (but not including) the second slice index.
Comments
The docs for slice may help.
slice([start], stop[, step])
Return a slice object representing the set of indices specified byrange(start, stop, step).
The slice format that most of are familiar with is just a shorthand:
a[start:stop:step]
Comments
You've got some good answers about how it works, here's one with why:
a = '0123456789' a[2:4] '23' a[4:6] '45' a[6:8] '67'
IOW, it makes it easy to step through a list or string or tuple n characters at a time, without wasting time on the +1 / -1 stuff needed in most languages.
